Small Waters

Female Bryocamptus minutus, from here.

For this week's semi-random post topic, I drew the copepod genus Bryocamptus. Copepods have made an appearance on this site before (see here, here and here), seeing as these minute crustaceans inhabit almost all the world's waters. Bryocamptus belongs within the harpacticoids, one of the three main groups of free-living copepods (the others are the calanoids and cyclopoids), and like other harpacticoids members of this genus have a more-or-less parallel-sided, somewhat wormlike form, though Bryocamptus species are shorter than some. Within the harpacticoids, this genus belongs to the family Canthocamptidae, members of which have the first segment of the body bearing swimming legs fused to the cephalothorax (Caramujo & Boavida 2009).

There are over 100 recognised species of Bryocamptus, found in a wide range of fresh-watery habitats (Lee & Chang 2006). They may be found in mountain streams, in springs and temporary pools, or in subterranean groundwaters. Some may even be found 'terrestrially', living in the water film around leaf-litter, mosses or within the soil (Fiers 2013). One type of habitat that I haven't found reference to Bryocamptus living in is larger water bodies such as lakes. This is not particularly unusual: nutrients and micro-organisms tend to accumulate along boundaries, so habitats with a high proportion of edges tend to attract a higher diversity than the relative deserts that are larger water bodies.

Sometimes these habitats can be very small indeed. Groundwater species, for instance, may be restricted to the cracks within formations only some tens of metres in extent. Cottarelli et al. (2012) described Bryocamptus stillae from Conza Cave near Palermo in Sicily. This species was found in seasonal rimstone pools within the cave: temporary pools that would be filled by water dripping from the ceiling during the winter, only to dry up in the summer. However, the copepods are unable to survive out of water, and canthocamptids do not have a resistant phase in their life cycle that could survive the ppols drying out. Cottarelli et al. therefore inferred that the pools were not the copepods' primary habitat; rather, the copepods normally lived in the epikarst, the layer of limestone above the cave. Despite being only a few metres thick, this limestone layer retained enough pockets of moisture to provide a home for the copepods. During the rainy season, when water was more actively flowing through the epikarst, some of the more unfortunate copepods would be carried by the water as it dripped through the cave ceiling into the pools below. They would survive (and even breed) so long as the pools remained wet but they would be doomed to die off over the summer, with the following year's copepods representing an entirely new batch. Interestingly, though, Cottarelli et al. found B. stillae in only one group of pools in the cave. In a second group of pools, only about ten or fifteen metres away, an entirely different copepod species was found. Cottarelli et al. collected in the cave over three separate seasons, and each time the same species was found in the same pools. The evidence indicated that, even though these pools were so close, the water dripping into them came from separate, isolated epikarst formations, each one home to its own species of highly localised copepods.

REFERENCES

Caramujo, M.-J., & M.-J. Boavida. 2009. The practical identification of harpacticoids (Copepoda, Harpacticoida) in inland waters of central Portugal for applied studies. Crustaceana 82 (4): 385–409.

Cottarelli, V., M. C. Bruno, M. T. Spena & R. Grasso. 2012. Studies on subterranean copepods from Italy, with descriptions of two new epikarstic species from a cave in Sicily. Zoological Studies 51 (4): 556–582.

Fiers, F. 2013. Bryocamptus (Bryocamptus) gauthieri (Roy, 1924): a Mediterranean edaphic specialist (Crustacea: Copepoda: Harpacticoida). Revue Suisse de Zoologie 120 (3): 357–371.

Lee, J. M., & C. Y. Chang. 2006. Taxonomy on freshwater canthocamptid harpacticoids from South Korea V. Genus Bryocamptus. Korean J. Syst. Zool. 22 (2): 195–208.

Salpidobolus

The photo above (copyright Dmitry Telnov) shows a millipede of the genus Salpidobolus, photographed in West Papua. Salpidobolus is a genus of the family Rhinocricidae (in the order Spirobolida) that is found over a range from the Philippines, Sulawesi and Lombok in the west to Fiji in the east and Queensland in the south. There are also a handful of species that have been described from northern South America as part of Polyconoceras, a genus now regarded as synonymous with Salpidobolus, but Hoffman (1974) expressed the expectation on biogeographical grounds that future revision will show these species to be misplaced. Salpidobolus species are scavengers of vegetable matter and most active at night. When threatened, they can release a caustic spray from glands on the body segments that can cause irritation if it contacts mucous membranes such as around the eyes (Hudson & Parsons 1997). There are also reports (albeit unconfirmed) of production of bioluminescence by Salpidobolus (see here); observations on other millipedes suggest such bioluminescence could be related to the aforementioned caustic spray.

As has been mentioned in an earlier post, most millipedes tend not to be extravagant in their external variation, and spirobolidan millipedes look about as millipede-y as you can get. Notable features of the spirobolids as a whole include the presence of only a single pair of legs on each of the first five body rings, and modification of the eight and ninth pairs of legs into the gonopods (Milli-PEET). The Rhinocricidae are characterised by a broad collum (the first segment behind the head) with a rounded ventrolateral margin, and the anterior gonopods forming a single, more or less triangular, transverse plate. Sensory pits called scobinae are often present on the dorsal segments (Marek et al. 2003). Below the family level, as with other millipedes, it all comes down to genitalia. In Salpidobolus, the distal section of the posterior gonopods is flagellate and divided into two branches, one branch carrying the seminal channel (Hoffman 1974).

Gonopods of Salpidobolus meyeri, from Hoffman (1974).

The status of Salpidobolus was most recently reviewed by Hoffman (1974). The majority of species now included in the genus had previously been placed in the separate genera Dinematocricus or Polyconoceras. Salpidobolus was initially restricted to the type species, S. meyeri from Sulawesi, which differs from other species in the presence on the first three pairs of legs of distinct processes on some of the leg segments. Dinematocricus and Polyconoceras were supposed to differ on the basis of the number of sensilla at the end of each antenna: four in Dinematocricus, more than four in Polyconoceras. Hoffman felt that none of these differences warranted generic separation in light of the consistency of gonopod structure between the three 'genera', and united them all under the oldest available name.

REFERENCES

Hoffman, R. L. 1974. Studies on spiroboloid millipeds. X. Commentary on the status of Salpidobolus and some related rhinocricid genera. Revue Suisse de Zoologie 81(1): 189–203.

Hudson, B. J., & G. A. Parsons. 1997. Giant millipede ‘burns’ and the eye. Transactions of the Royal Society of Tropical Medicine and Hygiene 91: 183–185.

Marek, P. E., J. E. Bond & P. Sierwald. 2003. Rhinocricidae systematics II: a species catalog of the Rhinocricidae (Diplopoda: Spirobolida) with synonymies. Zootaxa 308: 1–108.

Amphiascus: Can a Copepod be a Friend of Mine?

Amphiascus sp., copyright Alexandra.

The animal shown in the image above is a member of Amphiascus, a cosmopolitan genus of about thirty known species of benthic harpacticoid copepods. Amphiascus is a genus of the family Miraciidae; in older texts, you will find it referred to the Diosaccidae, but this family is now regarded as a synonym of the former. Miraciids are somewhat elongate harpacticoids generally with a fusiform body shape and females with paired egg sacs; as with other copepod taxa, their specific characterisation depends on fairly fine characters of the appendage setation (Willen 2002). Wells et al. (1982) placed Amphiascus in association with a group of related genera in the miraciid family tree on the basis of its retention of a fairly extensive setation on the pereiopods, two inner setae on the endopod of pereiopod II in females, and two articulated claws on that segment in males. However, the proposed phylogeny of Wells et al. provides no apomorphies for Amphiascus itself, implying that it is characterised only by plesiomorphies relative to related genera.

The title of this post refers to the circumstances surrounding the discovery of a relatively recently described Amphiascus species, A. kawamurai Ueda & Nagai 2005. In the cultivation in Japan of nori, the edible alga used (among other things) in wrapping sushi rolls, the conchocelis phase of the life cycle is grown on oyster shells in outdoor tanks of seawater (like many algae, nori goes through an alternation of generations, with its life cycle including two very distinct forms; as well as the familiar large flat alga, the life cycle of nori includes a small filamentous shell-boring stage, initially mistaken for a distinct organism and called Conchocelis). Unfortunately, the oyster shells may also become overgrown with diatoms, retarding the growth of conchocelis. As a result, nori growers may be required to laboriously scrub the shells of diatoms several times over the conchocelis growth period. However, it was noticed in Ariake Bay in Kyushu that some form of copepod would sometimes appear in the nori tanks, presumably brought in with seawater from the bay. When this copepod was present, it would graze on the diatoms, reducing the need for other controls. Study of the nori-tank copepod revealed it to be a previously undescribed species, revealing once more that even the species we are not aware of have the potential to directly improve our lives.

REFERENCES

Ueda, H., & H. Nagai. 2005. Amphiascus kawamurai, a new harpacticoid copepod (Crustacea: Harpacticoida: Miraciidae) from nori cultivation tanks in Japan, with a redescription of the closely related A. parvus. Species Diversity 10: 249–258.

Wells, J. B. J., G. R. F. Hicks & B. C. Coull. 1982. Common harpacticoid copepods from New Zealand harbours and estuaries. New Zealand Journal of Zoology 9 (2): 151–184.

Willen, E. 2002. Notes on the systematic position of the Stenheliinae (Copepoda, Harpacticoida) within the Thalestridimorpha and description of two new species from Motupore Island, Papua New Guinea. Cah. Biol. Mar. 43: 27–42.

Hadromeros: A Trilobite Survivor

Reconstruction of Hadromeros subulatus, from Kielan-Jaworowska et al. (1991).

Trilobites of the genus Hadromeros were widespread in the Late Ordovician and Early Silurian of Eurasia and North America. They are classified in the Cheiruridae, the same family that includes another trilobite genus that has been featured on this site, Sphaerexochus. However, Hadromeros differs from Sphaerexochus in that its glabella (its 'nose') is not as large. Whereas in my earlier post I suggested that Sphaerexochus may have been a predator, Hadromeros was probably a less aggressive feeder. It was possibly a detritivore, picking bits of nutritious material out of the sand and mud. This interpretation is supported by the known leg morphology of a closely related genus, Ceraurus, in which the legs are fairly generalised and show little adaptation for food processing (Bergström 1973).

Hadromeros and Ceraurus are placed in the Cheirurinae, a distinct subfamily from the Sphaerexochinae that includes Sphaerexochus. One characteristic feature of many members of the Cheirurinae is that the bases of the pleura, the plates that run down each side of the thorax, are swollen in dorsal view. The purpose of these swellings remains unknown. You might expect that, trilobites being as abundant in the fossil record as they are, we would know a great deal about them, and in many respects that is quite true. However, in other respects our knowledge is also frustratingly incomplete. Trilobites have an extensive fossil record because their dorsal exoskeleton was mineralised, and it is this that is usually preserved. The ventral section of the body, on the other hand, was not mineralised, and is only preserved under exceptional circumstances. This includes such significant features as the legs and mouthparts (as indicated above, we have some knowledge of the leg morphology of Cerarurus, but no direct evidence for Hadromeros). It is possible that the cavities underneath the cheirurine pleural bases housed some modification of the gills, if the gills in trilobites were comparable to those of living crustaceans. But how or to what purpose the gills were modified can only be speculated upon.

Morphologically, Hadromeros was a fairly unremarkable trilobite, but it stands out from the other genera of the Cheirurinae in one important respect (that has, indeed, already been alluded to in this post). The end of the Ordovician saw a mass extinction in marine life, by some measures the second largest to have ever occurred. Few groups of animals made it through unscathed, and the cheirurids were no exception. Of the eight subfamilies of Cheiruridae recognised by Přibyl et al. (1985), only three made it through to the Silurian: the Cheirurinae, Sphaerexochinae and Deiphoninae. Within the Cheirurinae, Hadromeros is the only genus currently known from both sides of the Ordovician-Silurian boundary, and may have been ancestral to all other post-Ordovician cheirurines. While other genera were whisked away, Hadromeros became the Trilobite that Lived.

REFERENCES

Bergström, J. 1983. Palaeoecologic aspects of an Ordovician Tretaspis fauna. Acta Geologica Polonica 23 (2): 179-206.

Kielan-Jaworowska, Z., J. Bergström & P. Ahlberg. 1991. Cheirurina (Trilobita) from the Upper Ordovician of Västergötland and other regions of Sweden. Geologiska Föreningen i Stockholm Förhandlingar 113 (2-3): 219-244.

Přibyl, A., J. Vaněk & I. Pek. 1985. Phylogeny and taxonomy of family Cheiruridae (Trilobita). Acta Universitatis Palackianae Olomucensis Facultas Rerum Naturalium Geographica-Geologica XXIV 83: 107-193.

Horny-Arsed Trilobites

Reconstruction of Ceratopyge, from here.

Just a short post for today. The Ceratopygidae are a family of trilobites known from the Late Cambrian and Early Ordovician. The name of the type genus, Ceratopyge, means 'horned rump', and one of the features that has classically defined the family is the presence of one or two pairs of spines on either side of the pygidium, the plate the makes up that hind end of a trilobite. These spines appear to be derived from lateral extensions of one of the anterior segments incorporated into the pygidium. However, there are also some genera without pygidial spines that share other features with the family (such as a narrow rim to the cheeks) and so have also been recognised as ceratopygids. Ceratopygids also possessed narrow spines extending back from the posterior corners of the head. The number of segments between head and pygidium varied between genera: early genera have nine segments, but some later genera have only six (Fortey & Chatterton 1988) (offhand, the drawing above looks to have one too many segments).

Proceratopyge gamaesilensis, from here.

Otherwise, ceratopygids seem to have been fairly generalised trilobites. The eyes were present but not large, and there don't appear to be any features suggesting they were swimmers. The features of the underside of the head are poorly known in ceratopygids overally, but where known, the hypostome (the plate on the underside of the head that would have sat in front of the mouth) is firmly attached to the anterior margin of the head. Trilobites with this arrangement are believed to have been scavengers or predators on small invertebrates (Fortey & Owens 1999). In some later genera, such as Ceratopyge, the glabella in the midline of the cephalon expanded forward, with a corresponding reduction in the width of the anterior margin. As the glabella would have contained the trilobite's stomach, its enlargement may indicate that these later ceratopygids were taking larger prey.

REFERENCES

Fortey, R. A., & B. D. E. Chatterton. 1988. Classification of the trilobite suborder Asaphina. Palaeontology 31 (1): 165-222.

Fortey, R. A., & R. M. Owens. 1999. Feeding habits in trilobites. Palaeontology 42 (3): 429-465.

Stunning Central American Millipedes

Blue cloud forest millipede Pararhachistes potosinus, copyright Luis Stevens.

For my semi-random selection of taxon to write about this week, I drew the millipede family Rhachodesmidae. Rhachodesmids are members of the millipede group called the Polydesmida, characterised by the presence of lateral keels on each segment of the body. The presence of these keels had lead to platydesmidans sometimes being referred to as 'flat-backed millipedes' though depending on how strong the keels are, not all species are necessarily 'flat-backed'.

In an earlier post on millipedes, I stressed the importance of genitalia in characterising millipedes, and the Rhachodesmidae are no exception. In polydesmidans, it is the front pair of legs on the seventh segment that is modified into the gonopods in males (with one notable exception that I may refer to later). Gonopods of rhachodesmids lack the solenite or coxal spur found in many other polydesmidans, and the inner side of the gonopod has a distinct elongate or oval concavity that is densely setose. Other noteworthy features of rhachodesmids are that they are often relatively large, with a conical terminal segment and more or less thickened rims to the lateral keels (Loomis 1964).

An unidentified rhachodesmid, copyright Sergio Niebla.

Beyond that, rhachodesmids become a little more difficult to characterise. Though they are not a widespread group, being restricted to Mexico and Central America, they are very diverse in appearance. Loomis & Hoffman (1962) commented that, "Rhachodesmoids collectively are members of a group notable for great variability and the development of bizarre features. Among their ranks we find millipeds which are bright blue, green, orange, and even pure white as adults; here the gonopod structure ranges from the normal polydesmoid appearance down to monoarticular fused remnants. Body form varies from a slender juliform shape to broad, flat, limaciform contour. Within the limits of this so-called single family occurs more variation than in all of the remaining polydesmoids." They also noted that the group was in need of review, something that apparently remains undone to this day (though there is someone working on it). If the photographs I've commandeered in this post are any indication, this is definitely a group that deserves more love.

Paratype of Tridontomus procerus, from Loomis & Hoffman (1962).

Loomis & Hoffman (1962) made their comments in comparing the Rhachodesmidae to another Central American polydesmidan family they were then describing as new, the Tridontomidae, and if I'm referring to the rhachodesmids then I should probably give a shout-out to these remarkable beasts as well. So far as I've found, this family is still only known from two species, Tridontomus procerus and Aenigmopus alatus, from Guatemala. Not only is the appearance of tridontomids striking, with long spinose processes on either side of the body, but the genital morphology of one species, A. alatus, is especially bizarre: it doesn't have any where it should. Where males of other polydesmidans have the legs of the seventh segment modified into gonopods, those of A. alatus have a perfectly ordinary pair of walking legs. In normal polydesmidans, the gonopods are used to transfer sperm from seminal processes on the coxae of the second pair of legs to the female's genital opening (more details are available here), but obviously Aenigmopus must do things differently. The seminal processes are still present, and the second legs themselves are thickened compared to other millipedes; it is possible that they are somehow used to transfer sperm directly from process to female without the use of gonopods. However it does it, there is no question that Aenigmopus is unique in the world of polydesmidans.

REFERENCES

Loomis, H. F. 1964. The millipeds of Panama (Diplopoda). Fieldiana: Zoology 47 (1): 1-136.

Loomis, H. F., & R. L. Hoffman. 1962. A remarkable new family of spined polydesmoid Diplopoda, including a species lacking gonopods in the male sex. Proceedings of the Biological Society of Washington 75: 145-158.

Bobble-Nosed Trilobites

Cranidium and part of thorax of Onchonotellus sp., from Bao & Jago (2000).

While most fossil invertebrates manage to completely fly under the radar when it comes to popular culture (it's not as if we're drowning under cartoon depictions of euthycarcinoids or eldoniids), one group that will often get a passing nod is the trilobites. Any depiction of early animal life worth its salt is going to feature a couple of these crunchy bugs scurrying about. Nevertheless, the range of varieties of trilobite shown will generally be low, and will usually be something similar to Olenellus or Elrathia. Seeing as trilobites persisted for hundreds of millions of years, it should be no surprise that their actual diversity was much higher.

The fossil shown at the top of this post is a representative of the trilobite genus Onchonotellus. Remains have been assigned to this genus from the late Cambrian and the early Ordovician, though Adrain (2013) expressed some reserve about the genus' monophyly. It has been assigned to the Catillicephalidae, a mostly Cambrian group of trilobites, but again the coherence of this total group is uncertain.

Most known fossils of Onchonotellus are represented by isolated cranidia, the plates that in life covered the trilobites' head. Onchonotellus and other catillicephalids are characterised by an inflation of the glabella, the middle lobe of the cranidium. Generally, the glabella of Onchonotellus is barrel-shaped. In some Onchonotellus specimens, the glabella may almost look spherical in side-view, making this trilobite look like a Bubble O'Bill. In many trilobites, the glabella will bear a series of furrows along the sides, but in Onchonotellus these disappear so that the glabella surface is smooth. In other catillicephalids, the glabella extends right to the front margin of the head, but Onchonotellus does retain a distinct rim around its front. The cheeks on either side of the glabella are relatively broad (Öpik 1967; Shergold 1980). In their time, members of the genus were found around the world, and some species have been highlighted as index fossils, useful in determining the age of rock strata.

So what was the significance of the large glabella? Most researchers have suggested that it probably held some sort of expansion of the digestive system, such as a crop for storing food. Some trilobites in which the glabella became large enough that it actually overhung the front margin have been suggested to be predatory (Fortey & Owens 1999), with the glabella containing a large oesophagus that allowed the trilobite to swallow larger food items. Onchonotellus probably didn't take things that far: not only was its glabella just that little bit smaller, but I haven't found any indication of it possessing the enlarged eyes also found in the predatory forms. The furrows on the glabella of most trilobites may have marked the attachment positions for muscles associated with the oesophagus/crop/whatever, so did the reduction of these furrows in Onchonotellus indicate a correspondingly less muscular pharynx? Perhaps Onchonotellus was a detritus feeder, with an expanded crop allowing it to take in mouthfuls of sediment from which to sieve out tasty organic morsels. Or perhaps it was a scavenger, breaking off lumps from the carcasses of other animals. Whatever it was doing, it was something that involved a big nose that was in actuality a big mouth.

REFERENCES

Adrain, J. M. 2013. A synopsis of Ordovician trilobite distribution and diversity. Geological Society, London, Memoirs 38: 297-336.

Bao, J.-S., & J. B. Jago. 2000. Late Late Cambrian trilobites from near Birch Inlet, south-western Tasmania. Palaeontology 43 (5): 881-917.

Fortey, R. A., & R. M. Owens. 1999. Feeding habits in trilobites. Palaeontology 42 (3): 429-465.

Öpik, A. A. 1967. The Mindyallan fauna of northwestern Queensland. Commonwealth of Australia, Department of National Development, Bureau of Mineral Resources, Geology and Geophysics—Bulletin 74, vol. 1: 404 pp., vol. 2: 166 pp., 67 pls.

Shergold, J. H. 1980. Late Cambrian trilobites from the Chatsworth Limestone, western Queensland. Bureau of Mineral Resources, Geology and Geophysics—Bulletin 186: 1-111.

Giant Centipedes (That Aren't All Giants)

Scolopendra morsitans, copyright Jiri Lochman/Lochman Transparencies.

It was a dark night, but not stormy (nights tend to be dark, as a rule). We were out collecting for our regular survey when we encountered a large centipede (the same species as in the photo above) crossing the road, and decided to add it to our collection. Taking out the large 20-cm forceps that we had on hand for dealing with venomous animals, one of us used them to grab the centipede.

The response was electric. Rather than trying to escape its attacker, the centipede instantly whipped back and lashed itself around the forceps, doing its best to bite into them. Had the actual wielder of the forceps been within its reach, they would have been in for a world of pain. When dealing with scolopendrid centipedes, you should always remember three things: they are big, they are fast, and they are mean.

Scolopendridae are unmistakeable. They include the giants of the centipede world, with the largest species (the South American Scolopendra gigantea) reaching up to a foot in length. Even the smaller species are relatively robust compared to other centipedes. Like all centipedes, the first pair of legs is modified into a robust pair of 'fangs' used for delivering venom (when I referred above to a centipede 'biting', this is what I was properly referring to). Large scolopendrids have the most dangerous centipede stings, potentially causing intense pain, though fatalities are very rare (Bush et al., 2001, noted that no centipede fatalities were known from the US, though they did refer to a single known child fatality in the Philippines). Their hunting prowess is amply demonstrated in this video of a large scolopendrid hunting bats by quite literally snatching them out of the air:


But lest you think that scolopendrids are all venom and viciousness, let me point out that they also have their endearing qualities. Female scolopendrids make devoted mothers, coiling around their egg clutches and regularly cleaning them to prevent fungal attack. Even after the eggs hatch, the female continues to coddle and groom her young. Brunhuber (1970) recorded that females of Cormocephalus anceps spent at least three months (from late September to late December) caring for young before they struck out on their own. Even after becoming independent, the young do not reach sexual maturity until they are at least two years old. Other scolopendrids may mature more quickly, at about one year (Lewis 1972). Individual centipedes may live for several years.

Female Scolopendra morsitans cleaning her eggs, copyright H. J. B..

The Scolopendridae belong to a larger centipede group called the Scolopendromorpha. Most scolopendromorphs have bodies with 21 or 23 leg-bearing segments, except for one remarkable scolopendrid species from central Brazil that has 39 or 43 leg-bearing segments (Chagas-Junior et al. 2008). Non-scolopendrid species are often much smaller than the Scolopendridae, with some being only about 10 mm in length. These smaller scolopendromorphs also differ in eye morphology: Scolopendridae have a patch of four ocelli on either side of the head, but other scolopendromorphs are mostly blind and lack ocelli. In the past the blind scolopendromorphs have been treated as a single family Cryptopidae, but recent authors have mostly recognised three separate families Cryptopidae, Scolopocryptopidae and Plutoniumidae in light of uncertainty about the monophyly of a broader Cryptopidae. Nevertheless, a recent phylogenetic analysis by Vahtera et al. (2012) combining both morphological and molecular data did support a single blind clade. Unfortunately, no phylogenetic analysis to date has been able to include Mimops orientalis, an odd scolopendromorph known from a single specimen collected in China in 1903 and placed by Lewis (2006) in its own distinct family. Mimops possesses but a single ocellus on either side of the head, potentially making it very intriguing for the question of whether blindness has evolved in scolopendromorphs more than once.

The blind scolopendromorph Scolopocryptops sexspinosus, copyright Troy Bartlett.

REFERENCES

Brunhuber, B. S. 1970. Egg laying, maternal care and development of young in the scolopendromorph centipede, Cormocephalus anceps anceps Porat. Zoological Journal of the Linnean Society 49 (3): 225-234.

Bush, S. P., B. O. King, R. L. Norris & S. A. Stockwell. 2001. Centipede envenomation. Wilderness and Environmental Medicine 12 (2): 93-99.

Chagas-Junior, A., G. D. Edgecombe & A. Minelli. 2008. Variability in trunk segmentation in the centipede order Scolopendromorpha: a remarkable new species of Scolopendropsis Brandt (Chilopoda: Scolopendridae) from Brazil. Zootaxa 1888: 36-46.

Lewis, J. G. E. 1972. The life histories and distribution of the centipedes Rhysida nuda togoensis and Ethmostigmus trigonopodus (Scolopendromorpha: Scolopendridae) in Nigeria. Journal of Zoology 167 (4): 399-414.

Lewis, J. G. E. 2006. On the scolopendromorph centipede genus Mimops Kraepelin, 1903, with a description of a new family (Chilopoda: Scolopendromorpha). Journal of Natural History 40 (19-20): 1231-1239.

Vahtera, V., G. D. Edgecombe & G. Giribet. 2012. Evolution of blindness in scolopendromorph centipedes (Chilopoda: Scolopendromorpha): insight from an expanded sampling of molecular data. Cladistics 28: 4-20.

The Importance of Genitalia

Take a look at the figure above (taken from Mauriès 2003). What you're looking at is the intimate business of a male millipede, in this case a Bosnian millipede called Fagina silvatica. And if you ever had the pleasure of finding yourself working on millipede taxonomy, you'd be looking at a lot of these.

Fagina silvatica belongs to a superfamily of millipedes called the Neoatractosomatoidea, which is in turn part of the order Chordeumatida in the clade Helminthomorpha. Helminthomorph millipedes (as indicated by their name, which means 'worm-like') all cleave pretty closely to the classic image of their kind, with an elongate body bearing large numbers of relatively short legs. Chordeumatida are characterised by having silk-spinning glands on the telson, the very end segment of the body, and three pairs of strong bristles on the top of each body segment. Male chordeumatidans also have the eighth and ninth pairs of legs modified into the gonopods, the copulatory structures. Because millipedes are generally not extravagant animals in overall appearance, it is the gonopods that have become the primary structures for identifying them, and many millipede species cannot be reliably distinguished without examining them. In the Neoatractosomatoidea, the eighth pair of legs forms the gonopods proper that deliver the male's sperm to the female's vulvae, while the ninth pair form protective structures called paragonopods. The gonopods proper are divided into two branches that fold around each other, usually to guide a whip-like flagellum or other extended structure passing between them (one genus, Guizhousoma, lacks the flagellum—Mauriès 2005). One neoatractosomatoid genus, Osellasoma, also has the seventh pair of legs modified into protective structures (Mauriès 2003). Neoatractosomatoids have 28 or 30 body segments. Some neoatractosomatoids have the sides of the body extended into flattened processes called paraterga; others have the body more or less cylindrical. And no, I haven't been able to find a single photograph or illustration showing a neoatractosomatoid in its entirety. You'll have to content yourself with looking at their genitals (Wikipedia has photos of other Chordeumatida).

As defined by Mauriès (2003, 2005), the Neoatractosomatoidea only includes about 25 known species, mostly found in southern Europe. A single species, the aforementioned Guizhousoma latellai, is known from caves in China. Mauriès (2003) separated three families previously placed in the Neoatractosomatoidea into a separate superfamily Mastigophorophylloidea; if the mastigophorophylloids are included with the neoatractosomatoids, then the group includes further species found in northern Asia. Mauriès separated the two superfamilies on the basis that mastigophorophylloids possessed a flagellum on both the gonopods and the paragonopods, instead of only on the gonopods. The subsequent discovery of the entirely flagellum-less Guizhousoma could raise questions about the significance of this character, and the flagellum appears much reduced if not entirely absent on the paragonopods of at least one putative mastigophorophylloid, Kirkayakus pallidus, as illustrated by Mikhaljova (2004)*. However, I have to admit to having absolutely zero experience with interpreting millipede gonopods, so I am hardly one to be voicing an opinion.

*Mikhaljova (2004) illustrates this species under the name of Altajella pallida, but it has since been renamed by Özdikmen (2008) (yes, that Özdikmen) due to the original genus being preoccupied).

REFERENCES

Mauriès, J.-P. 2003. Schizmohetera olympica sp.n. from Greece, with a reclassification of the superfamily Neoatractosomatoidea (Diplopoda: Chordeumatida). Arthropoda Selecta 12 (1): 9-16.

Mauriès, J.-P. 2005. Guizhousoma latellai gen.n., sp.n., de Chine continentale, type d'une nouvelle famille de la superfamille des Neoatractosomatoidea (Diplopoda: Chordeumatida). Arthropoda Selecta 14 (1): 11-17.

Mikhaljova, E. V. 2004. The Millipedes (Diplopoda) of the Asian Part of Russia. Pensoft: Sofia.

Özdikmen, H. 2008. New family and genus names, Kirkayakidae nom. nov. and Kirkayakus nom. nov., for the millipedes (Diplopoda: Chordeumatida). Munis Entomology & Zoology 3 (1): 342-344.

Arthropods in the Precambrian?

The Ediacaran animal Spriggina floundersi, from here.

The Ediacaran biota has been touted as one of the great mysteries of palaeontology. Comprising the latest part of the Precambrian era, the Ediacaran is generally believed to have given us the earliest known animal fossils. However, palaeontologists have disagreed on just how the Ediacaran fossils relate to modern animals (see McCall 2006 for an exhaustively detailed review). Some see the Ediacarans as including the ancestors of groups that remain with us today: jellyfish, corals, comb jellies, sponges. Others see Ediacarans as outside the modern lineages: ancient animal groups that were swept aside by more modern animals at the beginning of the Cambrian. And some have even questioned whether the Ediacarans were even animals at all, suggesting links instead to fungi or Foraminifera, or even that they were an entirely independent lineage unrelated to any modern multicellular organisms.

In 1996, Benjamin Waggoner proposed the name 'Cephalata' for a clade uniting the arthropods with two groups of Ediacaran organisms: the Sprigginidae and the Vendiamorpha. These are among the most undeniably animal-like of the Ediacarans. The sprigginids (including Spriggina shown at the top of the post) have an undivided 'head' followed by a long segmented body. The vendiamorphs are shield-like organisms that also show evidence for segment-like divisions behind the 'head', such as branching internal structures that may represent side-branches of an internal gut.

The vendiamorph Vendia sokolovi, from Ivantsov (2004).

It is difficult to see these taxa as anything other than mobile animals. One supporter of non-animalian affinities for the Ediacarans, Adolf Seilacher, did suggest that Spriggina was a sessile organism, maintaining that the 'head' was in fact a holdfast while the 'body' extended upwards like the frond of a sea pen (I have seen a memorable reconstruction, though unfortunately I can't recall where, showing an individual of mobile Spriggina crawling past a cluster of sessile Spriggina). However, the numerous Spriggina specimens that have been found in Australia and Russia are invariably preserved lying flat, while sessile organisms from the same locations are preserved with the holdfast below the level of the body. Vendiamorphs, on the other hand, are simply not shaped in a way that allows them to be seen as anything other than lying flat. An immobile sprigginid or vendiamorph lying flat below the water would have been vulnerable to being buried by sediment, without any way of digging itself back out.

But if sprigginids and vendiamorphs were definitely animals, what kind of animals were they? It is at this point that things get a bit more vague. Their segmented appearance immediately suggests arthropods (and onychophorans) or annelids, but there is not a great deal to suggest one or the other. The differentiated head of sprigginids suggests the head of an arthropod, while vendiamorphs have been compared to the larvae of arthropods such as trilobites. However, it is unclear whether the Ediacaran taxa possessed anything like the limbs of arthropods and related taxa. The segments of sprigginids may be separated at the edges, and some have argued that folds in vendiamorph fossils are suggestive of limbs underneath a dorsal shield, but there is nothing that one would call unequivocal. Lateral outgrowths of sprigginids may correlate to annelid parapodia instead of arthropod limbs, and folds in the bodies of vendiamorphs may be nothing more than that. We recognise relationships between fossil and extant animals on the basis of whether they have features in common, but our assessment of what features they have may be coloured by what features we expect to see.

Another possible vendiamorph, Parvancorina minchami, from here. Note the fine parallel lines on the body, which some have interpreted as the outlines of limbs.

Some authors have drawn attention to a feature of both vendiamorphs and sprigginids that is visible in the image of Vendia above: their so-called 'glide reflectional symmetry'. Though their bodies appear segmented, the segments do not go straight across the body as one might expect. Instead, the left and right sides of the body are slightly offset from each other. For this reason, some authors have claimed that these animals do not show true bilateral symmetry and hence argued for placing them outside the Bilateria crown group, along its stem. However, others have suggested that the offset between sides may be an artefact of preservation. Even if it was indeed a feature of the living animal, glide reflectional symmetry may not necessarily force the sprigginids outside the Bilateria: a number of living bilaterians also show a certain degree of symmetry offset either as adults or during development, including basal chordates (Waggoner 1996).

During the period of the Cambrian, directly after the Ediacaran, we have access to beautifully preserved fossil deposits that have allowed us to characterise many animals from that period in exquisite detail. No such fossils exist for the Ediacaran; instead, Ediacaran animals are mostly preserved in coarse sediments that preserve only relatively broad features of the fauna. This can turn the Ediacarans into tantalising shadows, and what we see in them can say more about our assumptions than the animals themselves.

REFERENCES

Ivantsov, A. Yu. 2004. New Proarticulata from the Vendian of the Arkhangel’sk region. Paleontologicheskii Zhurnal 2004 (3): 21–26 (transl. Paleontological Journal 38 (3): 247–253.

McCall, G. J. H. 2006. The Vendian (Ediacaran) in the geological record: enigmas in geology's prelude to the Cambrian explosion. Earth-Science Reviews 77: 1-229.

Waggoner, B. M. 1996. Phylogenetic hypotheses of the relationships of arthropods to Precambrian and Cambrian problematic fossil taxa. Systematic Biology 45 (2): 190-222.

Sphaerexochus: A Possibly Predatory Trilobite

Sphaerexochus brittanicus, from Museum Victoria.

The fossil in the image above belongs to a genus of trilobites that lived from the Mid-Ordovician to the end of the Silurian. Distinguishing characters of Sphaerexochus include the massive inflation of the glabella, the central section of the trilobite head, which became almost spherical. The cheeks on either side of the glabella, in contrast, remained fairly small. The inflated glabella was marked on each side in the posterior part by a deep furrow that run in a curve from the side to the posterior margin. In the photo above, this furrow marks out the circular section that looks a bit like a large eye; the actual eye can just be made out in the photo (I think) as a crescent-shaped structure on the cheek below the glabella. The rear outer corners of the cheeks were more or less blunt in adult individuals, generally lacking the prominent cheek-spines of many other trilobites (small spines were present in juveniles). The spines on the trilobite's rear end (the pygidium) were also blunt and stout.

Přibyl et al. (1985) recognised four subgenera within Sphaerexochus, and this classification was largely supported by Congreve & Lieberman (2011) in a phylogenetic analysis (one of the subgenera, Onukia, is not known from well-preserved material and hence could not be analysed by Congreve & Lieberman). One of the subgenera, Korolevium, retained short cheek-spines as adults; they were absent in other subgenera. The remaining two subgenera, Sphaerexochus and Parvixochus, were distinguished by features of the hypostome, a plate on the underside of the head that covered the mouth in life. Korolevium and Parvixochus were both extinct by the end of the Middle Ordovician, but the Sphaerexochus subgenus sailed on and passed through the mass extinction event at the end of the Ordovician apparently unscathed (Congreve & Lieberman 2011).

Another view of Sphaerexochus brittanicus, from the Carnegie Institution.

Sphaerexochus was restricted to warmer waters in the Palaeozoic ocean (fortunately for Sphaerexochus, during the Silurian this was just about everywhere). Fortey & Owens (1999) interpreted its large glabellar furrows as providing an attachment site for the muscles of a correspondingly well-developed pair of limbs. Features of the hypostome indicate that Sphaerexochus had a relatively large oral cavity, and the combination of large anterior limbs and a big mouth lead Fortey & Owens to suggest that it may have been a raptorial predator. Přibyl et al. (1985) noted that Sphaerexochus specimens have often been found with the main body and head bent at an angle from each other, and suggested that they may have spent a lot of time with the body buried in the sediment with only the head above the surface. These observations add together to suggest Sphaerexochus living as an ambush predator, lurking in wait whilst largely hidden in the sand for any smaller animal unwary enough to get too close.

REFERENCES

Congreve, C. R., & B. S. Lieberman. 2011. Phylogenetic and biogeographic analysis of sphaerexochine trilobites. PLoS ONE 6(6): e21304. doi:10.1371/journal.pone.0021304.

Fortey, R. A., & R. M. Owens. 1999. Feeding habits in trilobites. Palaeontology 42 (3): 429-465.

Přibyl, A., J. Vaněk & I. Pek. 1985. Phylogeny and taxonomy of family Cheiruridae (Trilobita). Acta Universitatis Palackianae Olomucensis Facultas Rerum Naturalium Geographica-Geologica XXIV 83: 107-193.

Brine Fairies

The once-ubiquitous 'sea monkey' advertisement. Take a very good look at the words in the lower margin.

Readers of a certain age (or readers who have perused the comic books once belonging to readers of a certain age) will instantly recognise the image above. It appeared on almost every comic book published between 1962 and 1975, and offered a something truly mind-blowing. For a couple of bucks, you could receive a small packet in the post that, when its contents were added to water, grew into minute fish-tailed humanoids that would create their own minute society, all in one goldfish bowl sitting in your bedroom!

As Robin Ince summed up the sea monkey experience in his Bad Book Club: 'This was a lie'. You did receive a small packet in the post, the contents of the packet did hatch out in water, but you did not get the pictured anthropomorphs. What you actually got were these:

The North American brine shrimp Artemia franciscana, photographed by Jean-François Cart.

The 'sea monkeys' became labelled one of childhood's great disappointments, which I call an utter shame. Because I personally would describe them as some of the most elegant crustaceans that I've ever seen.

Brine shrimp and their relatives belong to a group called the Anostraca. The Anostraca, sometimes referred to as fairy shrimps, are a group of a little under three hundred described species. They are generally less than an inch long, though the larger species can grow to several inches. The taxon name basically means 'without a carapace', and this is one of the distinctive features of the group. The body is elongate and, behind the head, is divided into a thorax bearing feathery swimming legs and an abdomen lacking appendages except a terminal pair of uropods. Most species of Anostraca have eleven pairs of swimming legs, though the species Polyartemiella hazeni and Polyartemia forcipata have, respectively, seventeen and nineteen pairs (Weekers et al. 2002). Anostracans have a distinctive slow swimming style, lying on their back. They are found living in ephemeral or hypersaline waters where predatory fish are few or absent; in order to persist in such environments, they produce resistant eggs that are able to survive drying out, hatching when the temporary pool is refilled by the rain.

Conservancy fairy shrimp Branchinecta conservatio, from here.

The phylogeny of Anostraca was investigated by Weekers et al. (2002), who found that they could be divided between two lineages: one including the genera Artemia and Parartemia, which are found in hypersaline waters, and the other containing the remaining freshwater genera. Most members of both lineages are filter-feeders, but some larger members of the freshwater lineage in the genus Branchinecta have become predators. The most favoured prey of these large Branchinecta? Why, smaller Branchinecta! Studied specimens of the predatory Branchinecta raptor would only deign to take other invertebrate prey if their preferred B. mackini was unavailable (Rogers et al. 2006). These predatory Branchinecta are found living in turbid, sediment-filled waters with low visibility, and mostly found their prey by coming into contact with it whilst swimming in the water column. Squeezing water out of a pipette near one would incite it to try and attack the pipette. If unable to find swimming prey, B. raptor would swim down to the sediment bed and stir it up, then attempt to find invertebrates flushed out of hiding.

Streptocephalus torvicornis, photographed by J.R. Casaña & Manolo Ambou Terradez.

The two hypersaline genera have complementary distributions: Parartemia is endemic to Australia while Artemia is found on the remaining continents (though Artemia is now present in some localities in Australia as an introduced taxon). In the past, all Artemia around the world were often treated as a single species, A. salina. However, the existence of a number of geographically distinct lineages has now been established, with these treated as separate species (A. salina proper is found in Europe). Both sexually and parthogenetically reproducing forms of Artemia exist. The parthenogenetic forms are treated as a single species, A. parthenogenetica, and derive from a single Eurasian origin, but are themselves genetically diverse, including diploid, triploid, tetraploid and pentaploid individuals (Triantaphyllidis et al. 1998). Sadly, this new-found taxonomic complexity of Artemia is in some danger of re-simplifying: the international trade in brine shrimp, used mostly as food for fish, is almost entirely based on eggs derived from the Great Salt Lake in Utah. As a result of this trade, the North American species A. franciscana has become introduced, both accidentally and deliberately, to saline waters around the world, and has been found in many localities to be replacing the native brine shrimp.

REFERENCES

Rogers, D. C., D. L. Quinney, J. Weaver & J. Olesen. 2006. A new giant species of predatory fairy shrimp from Idaho, USA (Branchiopoda: Anostraca). Journal of Crustacean Biology 26 (1): 1-12.

Triantaphyllidis, G. V., T. J. Abatzopoulos & P. Sorgeloos. 1998. Review of the biogeography of the genus Artemia (Crustacea, Anostraca). Journal of Biogeography 25: 213-226.

Weekers, P. H. H., G. Murugan,J. R. Vanfleteren, D. Belk, & H. J. Dumont. 2002. Phylogenetic analysis of anostracans (Branchiopoda: Anostraca) inferred from nuclear 18S ribosomal DNA (18S rDNA) sequences. Molecular Phylogenetics and Evolution 25: 535-544.

The August History of Filter-Feeding Ostracods

Today's post subject, the Cavellinidae, were a family of ostracods that were around from the Middle Silurian period to the Middle Triassic (Adamczak 2003a). And for those of you unfamiliar with ostracods: you lucky, lucky bastards. They're horrible.

I exaggerate slightly. Ostracods are a group of crustaceans that spend their lives enclosed in a pair of shells, superficially a bit like a bivalve, However, what they primarily are is very, very small (often less than a millimetre in total length), which makes them very difficult to work with as identification often requires dissecting out the (even smaller, needless to say) appendages hidden within the shells. Fortunately, I've personally managed so far to avoid being caught in ostracod purgatory, but many of my acquaintances have not been so fortunate. In the case of fossil ostracods like today's subjects, it is generally only the valves themselves and not any of the internal parts that are preserved to be of concern, but they're still small enough overall to hardly be counted as simple to work with.

External dorsal and lateral views of the carapace of each sex of the Silurian Gotlandella martinssoni, from Adamczak (2003a) (adr = admarginal ridge, mr = marginal ridge).

Cavellinids belong to a group of ostracods called the Platycopina, so-called because of their relatively flat sides. Among the modern fauna, platycopines are represented only by the genus Cytherella, whose distant ancestors were almost certainly among the species assigned to the Cavellinidae (Adamczak 2003a), so the 'extinction' of the cavellinids in the Triassic is really a pseudo-extinction as they were replaced by the descendant cytherellids. As befits its phylogenetic isolation from other living ostracods, Cytherella is an oddity in the modern fauna, being one of the few ostracod lineages to make a living as filter-feeders. The rear part of the carapace is expanded on the inside to form a brood chamber in which the eggs are nursed. It has been suggested that the evolution of filter-feeding and of the brood chamber were connected (Adamczak 2003b): as water is drawn in by the process of filter-feeding, it circulates around the brood chamber to keep the contents, whether eggs or newly hatched larvae, oxygenated. The constant flow of water also brings more oxygen to the adult's own gills than it would receive passively, so Cytherella are able to live in places with less dissolved oxygen than other ostracods (Lethiers & Whatley 1994).

Internal view of right valve of the Middle Devonian Birdsallella eifeliensis, from Adamczak (2003a). Abbreviations: cg = contact groove (where the left valve is nestled); li = limen (the inner partition separating off the probable brood chamber).

Though preserved appendages have not yet been recorded from any cavellinid, their valve morphology is very similar to that of Cytherella: closely sized valves (the right is only slightly larger and slightly overlapping the left), with the line of contact between the valves is fairly straight along the underside, and the valves gaping open slightly at the front but tightly closed towards the back. A constriction on the inside of the valve also indicates the presence of a Cytherella-like brood chamber, and like Cytherella the outer surface of the carapace is fairly smooth. In fact, the only really marked difference between cavellinids and cytherellids is the arrangement of the muscle scars indicating where the valves where held together: in Cytherella and fossil members of the Cytherellidae, the scars are arranged in a double row, while members of the Cavellinidae have the scars in a random cluster. Because of the similarities between cavellinids and Cytherella, it is inferred that cavellinids were also filter feeders. Filter-feeding ostracods seem to have been more diverse in the Palaeozoic than in the present, leading Lethiers & Whatley (1994) to suggest that the Palaeozoic marine environment may have contained lower oxygen levels in many places than the modern environment. However, this line of reasoning was dismissed by Becker (2005), who felt that there was no reason to assume that fossil filter-feeders would necessarily show the same preference for low-oxygen environments as modern Cytherella. Instead, Becker has argued that strongly calcified ostracods like cavellinids are indicative of relatively high energy environments in shallow coastal waters (Adamczak 2003a).

REFERENCES

Adamczak, F. J. 2003a. The platycopine dynasty 2. Family Cavellinidae Egorov, 1950. Authentic platycopines. N. Jb. Geol. Paläont. Abh. 229 (3): 375-391.

Adamczak, F. J. 2003b. The early platycopine dynasty (Ostracoda; Palaeozoic). Senckenbergiana Lethaea 83 (1-2): 53-59.

Becker, G. 2005. Functional morphology of Palaeozoic ostracods: phylogenetic implications. Hydrobiologia 538: 23-53.

Lethiers, F., & R. Whatley. 1994. The use of Ostracoda to reconstruct the oxygen levels of Late Palaeozoic oceans. Marine Micropaleontology 24 (1): 57-69.

Life in Sand

Paramesochra mielkei, from Huys (1987).

Paramesochra is a genus of minute marine copepods found around the world. Over twenty species are currently assigned to the genus, but it is likely that many more await description. The extremely small size of paramesochrids (most are less than half a millimetre in length) reflects the interstitial habitat of most species described to date, i. e. they live among the grains of sand beneath the surface of their substrate. Also related to their choice of habitat is their vermiform (worm-like) shape and reduced setation compared to other copepods. These features also mean that they would be poor swimmers so they probably do not often emerge above the substrate surface. Most of the species described so far are from shallower waters, but this possibly reflects a lack of study of deep-sea species rather than reflecting true diversity. For instance, a survey of deep-sea Paramesochridae in the southern Atlantic and Antarctic Oceans by Gheerardyn & Veit-Köhler (2009) identified four species of Paramesochra, none of which corresponded to previously described species. These species probably do not have the same lifestyles as the shallow-water interstitial species due to the deep-sea substrate being fine mud rather than sand. Vasconcelos et al. (2009) suggested that another deep-sea paramesochrid, Kliopsyllus minor, might burrow in fluid mud or live in the 'organic fluff layer' (wonderful words) on top of the sediment. Deep-sea Paramesochra would probably be similar.

For the most part, genera of copepods have generally been distinguished mechanistically—different genera have different combinations of key features (usually related to the number of setae or segments on appendages)—without an explicit consideration of how those characters relate to phylogeny. However, Huys (1987) did propose a phylogenetic arrangement for the genera of Paramesochridae in which he suggested that Paramesochra formed a clade with the genera Kliopsyllus and Kunzia on the basis of their possessing single-segmented exopodites on the antennae and mandibles. However, while his tree shows Paramesochra as a monophyletic sister group to a clade of the other two genera, he did not identify any synapomorphies for Paramesochra. Instead, the features distinguishing it from the other two genera (two-segmented endopodites on the second to fourth legs, four setae on the distal exopodite segment of the first leg and two setae on the distal exopodite segment of the fourth leg) are resolved as plesiomorphies relative to the other clade. So if any of you feel inspired to spend your time dissecting and examining the legs of animals about 0.3 of a millimetre in total length, I know a potential research project going begging...

REFERENCES

Gheerardyn, H., & G. Veit-Köhler. 2009. Diversity and large-scale biogeography of Paramesochridae (Copepoda, Harpacticoida) in South Atlantic Abyssal Plains and the deep Southern Ocean. Deep-Sea Research I 56: 1804-1815.

Huys, R. 1987. Paramesochra T. Scott, 1892 (Copepoda, Harpacticoida): a revised key, including a new species from the SW Dutch coast and some remarks on the phylogeny of the Paramesochridae. Hydrobiologia 144: 193-210.

Vasconcelos, D. M., G. Veit-Köhler, J. Drewes & P. J. Parreira dos Santos. 2009. First record of the genus Kliopsyllus Kunz, 1962 (Copepoda Harpacticoida, Paramesochridae) from Northeastern Brazil with description of the deep-sea species Kliopsyllus minor sp. nov. Zootaxa 2096: 327-337.

A Quick Primer on Arthropod Growth

Successive instars of a generalised bug (Heteroptera). Image from here.

One of the trickiest things to wrap one's head around about insects and other arthropods* is also one of the most basic - how they grow. We tend to forget just how different arthropod growth is from our own - I've even known people who work with arthropods regularly to have it slip their mind.

*Other than that a scorpion's anus is at the very end of its tail next to the sting, not under the base of the tail as we chordates might tend to imagine.

For us as vertebrates, growth to maturity is fairly continuous. We start out small, we get steadily bigger. Take a balloon, blow it up, and you've got a fairly good representation of how we grow (yes, I'm massively simplifying things, but bear with me for a moment). Arthropod growth, on the other hand, is more like a series of balloons of different sizes all one inside the other, with the smallest balloon on the outside and the largest balloon at the centre. Start blowing up the balloons, and you'll only be able to blow it up to the size of the smallest balloon. If that smallest balloon breaks open (like an insect moulting its skin), then the balloons can inflate to the size of the second-smallest balloon. And so on and so forth, until you reach the largest size. Instead of growing in size continuously like we do, arthropods grow in steps - an extended period of no obvious increase in size, then a moult followed by a near-instantaneous increase as the animal swells up to fill its new skin, then another period without obvious growth. The change between moults can be drastic, as most obviously shown by the holometabolous insects with their radically different larval and adult stages. Even if the differences in morphology are not so drastic, separate instars (life cycle stages) may occupy distinctly different size ranges, with little or no overlap, and may have very distinct ecologies.


Internal pupal development of Rhagoletis pomonella (apple maggot fly), from a newly developed pupa on the left, to a pharate adult (fully developed adult still enclosed within the pupa) on the right. Photos by John Fuller, via here.

It's not that the arthropod is not growing at all between moults. A new layer of cuticle is being grown inside the old layer, albeit sort of crinkled up so that it can fit. Once the new cuticle has finished growing, the animal enters the pharate ("cloaked") state until the old cuticle is shed to reveal the new. Sometimes, the pharate period will be minimal, and the old cuticle will be shed pretty much as soon as the new one is ready. At other times, though, the pharate period will last for a considerable time. If conditions aren't right for the arthropod to move on to the next stage in its life, its growth may be effectively put on hold. Desert spiders may remain as subadults almost indefinitely, waiting for the rains to come before they moult into mature adults (and if the rains don't come one year, they can wait as subadults until the next). Caterpillars may moult into pupae at the beginning of autumn, but not emerge as butterflies until some time in the next spring when the flowers they feed on are beginning to bloom. If environmental conditions suddenly deteriorate, vertebrates are forced into the awkward position of having to maintain growth despite their reduced food supply. Arthropods, on the other hand, can afford to wait things out.


Supermajor worker of the ant Pheidologeton affinis, surrounded by minor workers. Both sizes are fully adult - the small ants will not grow into the big ones. Photo by Alex Wild.

On the other hand, vertebrates have some liberties that arthropods do not. Most arthropods have a set number of moults in their life cycle, and as a rule they do not reach maturity until the very last moult. The flipside, of course, is that once an arthropod does reach maturity, that's it. They are unable to resume growth should the occasion arise (this is not necessarily a problem because many, if not most, arthropods do not live long as mature adults). [Update: A couple of readers have pointed out that some arthropods do continue to grow and moult after maturity, but this is not the norm. Arthropods being such a mind-bogglingly enormous group, any attempt to make generalisations leaves one bound to make an idiot of oneself.] Contrary to what the cartoons may suggest, little ants do not grow into big ants. Both are fully adult, both are as big as they're going to get. In those ant species that have different sized castes, it's easy to imagine otherwise, but that's simply not the case. Big ants hatched out from their pupa as big ants, little ants hatched out as little ants. Similarly, if the queen of an ant colony were to die, it would not be possible for one of the workers to develop a functional reproductive system and take her place - sterility is a one-way trip.

Blinding Me with Science

Today's edition of Science, it turns out, is packed so chock-full of goodies that I hardly know where to turn. The discussion of how to distinguish species of bacteria? The beetles with male trimorphism? Blue butterfly larvae mimicking the sounds made by queen ants in order to be tended by the deluded worker ants? All of them very cool, and well worth discussion. But lets look at option four.



This little beastie (just under ten centimetres long) is called Schinderhannes bartelsi*, and its fossil remains are described in a paper by Kühl et al. (2009) (from whence comes the above reconstruction). Some of you may immediately recognise the similarity to the famed larger animals Anomalocaris and Laggania of the Cambrian Burgess Shale. However, Schinderhannes bears a few significant differences from those taxa: (1) it has that bizarre pair of 'wings' attached to the back of the head; (2) certain details of its anatomy suggest that it is more closely related to living arthropods than is Anomalocaris, showing that arthropods are descended from an 'anomalocarid' grade; and (3) it doesn't come from the Burgess Shale, but the German Hunsrück Slate, which is from the Lower Devonian, and shows that 'anomalocarid'-type animals were around for some 100 million years longer than we previously knew. I hate to repeat the old cliché about it being like discovering a Tyrannosaurus alive today, and in fact it's not like that, because the amount of time separating Tyrannosaurus from the present is considerably less than 100 million years.

*The name Schinderhannes is apparently derived from that of an 18th century bandit in the area from which it was found. Neat name, but it hints frustratingly at a back story that we are sadly denied in the paper.

Schinderhannes resembles anomalocarids in its radial mouth, and the large pair of spiny pre-oral appendages. However, certain of its features are more like true arthropods - it has a dorsum divided into distinct, sclerotised tergite plates, and it has biramous (two-branched) appendages like crustaceans. The combination of the large 'wings' and 'flukes' on either side of the tail spine suggest that it was an active swimmer.



Large raptorial pre-oral appendages (dubbed 'great appendages') have also been found in a number of Cambrian arthropods such as Leanchoilia and Yohoia. The phylogenetic position of such 'great-appendage' arthropods has been hotly debated. Budd (2002) suggested that they were a stem grade to the arthropod crown clade, but Cotton & Brady (2004) placed them within the crown clade, in the stem group for chelicerates. Researchers have also debated whether the great appendages of these arthropods are homologous to those of anomalocarids, and whether the great appendages are homologous to the chelicerae of modern chelicerates. The (admittedly pretty rudimentary) phylogenetic analysis of Schinderhannes by Kühl et al. (2009), the results of which are shown above, supports a position of great-appendage arthropods as stem chelicerates (despite the great appendages of these arthropods being a priori coded as homologous to those of anomalocarid-grade animals), which supports the comparison between great appendages and chelicerae. It also suggests that trilobites are closer to crustaceans than chelicerates, contrary to the idea of a trilobite + chelicerate "Arachnomorpha" clade. In some regards, this would make sense - trilobites, like crustaceans and insects, have lost the plesiomorphic state of grasping pre-oral appendages as found in chelicerates and have filamentous antennae instead. However, the position of trilobites in the tree above seems to be primarily due to the presence of antennae, so I don't know if it can be considered well-supported.

REFERENCES

Budd, G. E. 2002. A palaeontological solution to the arthropod head problem. Nature 417: 271-275.

Cotton, T. J., & S. J. Braddy. 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth Sciences 94: 169-193.

Kühl, G., D. E. G. Briggs & J. Rust. 2009. A great-appendage arthropod with a radial mouth from the Lower Devonian Hunsrück Slate, Germany. Science 323: 771-773.

Another Case of Mistaken Identity

Just the other day, Adam Yates showed us a couple of photos of a fossil that had been identified as dinosaurian, but actually belonged to a fish. Identifying isolated pieces of things can be a hazardous activity, and a mistaken identification can become something of a self-fulfilling prophecy - once the idea of a certain identity for your specimen has developed, you will tend to find "characters" that support your identification. Palaeontology, of course, presents researchers with no shortage of fragmentary remains, and it is not entirely surprising that a few snafus have occured. Adam referred to the case of Aachenosaurus multidens, a "hadrosaur" described in 1888 that was soon reidentified as a piece of petrified wood. A similar fate befell the "sauropod jaw" Succinodon putzeri (making the first four letters of the species name even more apropos). But while the most famous (and most dramatic) examples of such misidentifications involve fossils, studies of recent organisms have not been entirely free of impostors.



The figure above from Huys (2001) shows two views of the paratype of Megallecto thirioti, described by Gotto in 1986. The two specimens originally assigned to this species came from a plankton haul off the coast of Mauretania. Gotto identified them as parasitic copepods belonging to the family Splanchnotrophidae, and suggested that their hosts might be pteropods from the same haul.

Parasitic copepods can certainly be very strange creatures. While free-living males (and larvae of both sexes) may look like fairly ordinary copepods, the parasitic females may have highly derived morphologies that barely resemble crustaceans, let alone copepods. Consider the female of another splanchnotrophid, Arthurius elysiae (also from Huys, 2001):



When Huys (2001) revised the Splanchnotrophidae, however, he discovered that Gotto's Megallecto was (A) not a splanchnotrophid, and (B) not even a copepod. In fact:



'Megallecto' was nothing but a large chunk of the detached head of Phrosina semilunata, a pelagic amphipod. Phrosina belongs to a group of amphipods known as Hyperiidea. Most hyperiids feed on gelatinous plankton such as jellyfish or salps. They may or may not feed on pteropods.

REFERENCES

Huys, R. 2001. Splanchnotrophid systematics: A case of polyphyly and taxonomic myopia. Journal of Crustacean Biology 21 (1): 106-156.