The Lonely Life of the Cave Collembolan

For a few weeks last year, I had the job of sorting and identifying a collection of Collembola, springtails. Prior to doing this work, I had only the vaguest of understandings of springtail diversity: I knew that there were the round blobby ones, the long thin ones, and the ones that look a bit like sausages, but that was about as far as it went. Needless to say, there's a bit more to it than that.

Pseudosinella immaculata, copyright Andy Murray.

Pseudosinella is the largest genus of Collembola currently recognised, with over 280 described species. The greater number of those species are in Europe and North America, but various Pseudosinella have also been described from other regions of the world (there don't appear to be any from South America, but then I don't know how thoroughly anyone's looked). Pseudosinella species are mostly associated with subterranean habitats, from soil and litter to deep caves, with the highest diversity in the latter. According to a key at collembola.org, Pseudosinella are distinguished from related genera by having reduced eyes (with six or fewer ommatidia, as opposed to the eight ommatidia of other genera), and a bidentate mucro lacking a projecting lamella (the mucro is the claw-like structure at the end of the furcula, the posteroventral prong that forms a springtail's 'spring'). The key also distinguishes Pseudosinella from the similar genus Rambutsinella by it's not having the fourth antennal segment swollen as in the latter, but Bernard et al. (2015) described the species Pseudosinella hahoteana as also having the fourth antennal segment swollen so I'm not sure how reliable that feature is. Pseudosinella is very similar to another genus Lepidocyrtus, the main difference between the two being Pseudosinella's reduced eyes, and more than one author has raised the possibility that Pseudosinella may be a polyphyletic assemblage derived from Lepidocyrtus adapted for life underground.

As well as the reduced eyes, Pseudosinella tend to show a number of other features commonly associated with a subterranean lifestyle, such as a pale coloration and relatively elongate appendages. The claws of the feet also tend to become modified, with the larger of the two becoming longer and progressively narrower (Christiansen 1988). This latter feature is probably an adaptation to movement on the wet surfaces that predominate in caves. At a moderate length, the claws dig into the substrate surface more than those of surface-dwelling forms, allowing greater grip. At longer lengths, the claws are suited to allow the springtail to walk over the surface of the water itself (most springtails float on water surfaces due to their small size and low density, but not all can move with purpose in this position).

Pseudosinella hahoteana, from Bernard et al. (2015). Scale bar = 200 µm.

The aforementioned Pseudosinella hahoteana is worthy of extra attention, as it is one of a half-dozen springtail species endemic to caves on Rapa Nui, the landmass previously known as Easter Island. Many of you will be aware of the ecological catastrophe that beset Rapa Nui following human settlement, as its entire forest covering was cleared away. As a result of this clearing, the native fauna was also all but wiped out; no vertebrates survive, and of about 400 arthropods known from the island only about twenty are indigenous (Bernard et al. 2015). As such, the handful of minute animals clinging to survival in patches of ferns and moss at the entrance to caves represent a significant proportion of Rapa Nui's surviving native fauna.

REFERENCES

Bernard, E. C., F. N. Soto-Adames & J. J. Wynne. 2015. Collembola of Rapa Nui (Easter Island) with descriptions of five endemic cave-restricted species. Zootaxa 3949 (2): 239–267.

Christiansen, K. 1988. Pseudosinella revisited (Collembola, Entomobryinae). Int. J. Speleol. 17: 1–29.

Parastenocaris

Parastenocaris brevipes, copyright A. Hobaek.

It's time for another consideration of the overwhelming diversity of stygofaunal copepods. Parastenocaris is a genus of copepods found on almost all the landmasses of the world except, presumably, Antarctica (New Zealand also stands out as an intriguing void in the genus' distribution). The majority of species in this genus are insterstitial, mostly found in soils saturated with fresh water; a small number of species are found in brackish habitats such as estuaries. A few species have also been found above ground, particularly in the tropics (Galasi & Laurentiis 2004). The type species of the genus, P. brevipes, has been found in sphagnum bogs (Karanovic 2005).

As commonly recognised, Parastenocaris is a pretty huge genus, with well over 200 species having been assigned to it over the years. However, the genus has been poorly defined and many authors have questioned its integrity. Galasi & Laurentiis (2004) suggested that Parastenocaris should be restricted to those species most closely related to the type species, P. brevipes. Such a group would still be pretty cosmopolitan; indeed, P. brevipes itself has a Holarctic distribution and is known from both Europe and North America (this stands in pretty stark contrast to the super-short ranges of some stygofaunal copepods). Distinctive features of this restricted P. brevipes group include a characteristic endopodal complex on leg 4 of the male, with the endopod hyaline and with one or two large claws. In contrast, the leg IV endopod in females is long and distally serrate.

Parastenocaris lacustris, from here

Members of the Parastenocaris brevipes groups are found closer to the soil surface than many other members of their family (Karanovic 2005). They are also relatively large, reaching the absolutely monstrous size (I'm sure) of half a millimetre or more. Karanovic (2005) suggested that this larger size could reflect the larger size of the sand grains they live among closer to the surface, or it could simply reflect their access to more reliable food sources that are available to their more deeply buried relatives.

REFERENCES

Galasi, D. M. P., & P. de Laurentiis. 2004. Towards a revision of the genus Parastenocaris Kessler, 1913: establishment of Simplicaris gen. nov. from groundwaters in central Italy and review of the P. brevipes-group (Copepoda, Harpacticoida, Parastenocarididae). Zoological Journal of the Linnean Society 140: 417–436.

Karanovic, T. 2005. Two new subterranean Parastenocarididae (Crustacea, Copepoda, Harpacticoida) from Western Australia. Records of the Western Australian Museum 22: 353–374.

Crabs in Rivers, Crabs in Trees

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Freshwater crab Potamon ibericum, copyright Philipp Weigell.

Crabs are among the most recognisable animals one can find at the sea shore; any child who spends time at the beach will soon come to recognise their brandished pincers and sideways walk. But, as has been discussed by this site before, crabs are not only a coastal phenomenon. In warmer parts of the world, it may be possible to find crabs some distance inland.

Interestingly enough, there is at least circumstantial evidence that crabs made their way into fresh water relatively recently. The Old and New Worlds are each inhabited by a completely independent lineage of freshwater crabs that presumably originated after these continents went their separate ways. In the tropical Americas, rivers and streams are home to the Trichodactylidae, close relatives of the marine swimming crabs of the Portunidae. In the Old World, comparable habitats shelter a distinctly freshwater lineage comprising the superfamilies Gecarcinucoidea and Potamoidea.

Freshwater purple crab Insulamon palawanense, copyright Jolly Ibanez.

The classification of Old World freshwater crabs has (as with almost every other taxonomic group on this planet) shifted around a bit over the years. Many older references will combine all the Old World freshwater crab families into the Potamoidea but some more recent authors have tended to restrict this latter group to a single family, the Potamidae. There are other families, such as the African Potamonautidae and Deckeniidae, whose position superfamily-wise appears to be debated. The differences between the superfamilies Potamoidea and Gecarcinucoidea are primarily expressed in the structure of the males' second gonopods, the modified legs that the crabs use in transferring sperm during mating (Brandis & Sharma 2005). In the Gecarcinucoidea, a basal projection of the second gonopod surrounds the main body like a funnel for much of its length, while the tendril-like distal projection past this funnel is grooved and open on one side. In the Potamidae, the covering projection is restricted to the dorsal side only, and the distal part of the gonopod forms a closed tube.

Socotra limestone crab Socotra pseudocardisoma, copyright Gaëtan Rochez.

The Potamidae are most diverse in the Oriental bioregion with over seventy of the nearly eighty recognised genera being found there (Yeo et al. 2008). A couple of genera are found in the Afrotropical region. Only one genus, Potamon, makes it to Europe with modern species found in Italy and the Baltic peninsula, though the fossil record indicates a broader distribution on this continent in the past (Klaus & Gross 2009). Potamids are found in all types of water bodies, from fast-flowing streams and rivers to calm lakes and ponds, though they are inhabitants of the littoral zone rather than deep waters. The distinctive species Socotra pseudocardisoma is found on semi-arid limestone uplands of (surprisingly enough) the island of Socotra. Crabs of this species spend most of their time sheltered within cracks and crevices in the rocks that remain reasonably cool and damp year-round; they only emerge to the surface to forage during the rainy season while the surface briefly holds pools of standing water (Cumberlidge & Wranik 2002).

Another unusual lifestyle is found in a recently discovered species of the family Potamonautidae. This species from the Usambara Mountains of Tanzania specialises in living in phytotelmata, pools of water that accumulate in hollows in trees (Bayliss 2002). Though phytotelmata allow the crabs to inhabit regions of the rainforest that might otherwise be off limits, they are not the most forgiving of habitats. The combination of their small size together with an accumulation of organic matter means that the water in them tends to be quite acidic, a definite problem for a crab that relies on its calcitic exoskeleton for protection. The crabs feed on snails found in litter of the rainforest floor, and emerge from their home hollows to hunt at night or on cloudy, wet days. After eating a snail, they carry its shell back with them to their phytotelma and drop it in. The lime from the snail shell helps to neutralise the acidity of the water in the phytotelma, as well as supplying much-needed calcium that the crab will itself absorb when the time comes for it to moult to a new exoskeleton.

REFERENCES

Bayliss, J. 2002. The East Usambara tree-hole crab (Brachyura: Potamoidea: Potamonautidae)—a striking example of crustacean adaptation in closed canopy forest, Tanzania. African Journal of Ecology 40: 26–34.

Brandis, D., & S. Sharma. 2005. Taxonomic revision of the freshwater crab fauna of Nepal with description of a new species (Crustacea, Decapoda, Brachyura, Potamoidea and Gecarcinucoidea. Senckenbergiana Biologica 85 (1): 1–30.

Cumberlidge, N., & W. Wranik. 2002. A new genus and new species of freshwater crab (Potamoidea, Potamidae) from Socotra Island, Yemen. Journal of Natural History 36: 51–64.

Klaus, S., & M. Gross. 2009. Synopsis of the fossil freshwater crabs of Europe (Brachyura: Potamoidea: Potamidae). N. Jb. Geol. Paläont. Abh.

Yeo, D. C. J., P. K. L. Ng, N. Cumberlidge, C. Magalhães, S. R. Daniels & M. R. Campos. 2008. Global diversity of crabs (Crustacea: Decapoda: Brachyura) in freshwater. Hydrobiologia 595: 275–286.

All that is Silver is not Fish

Common silverfish Lepisma saccharina, copyright Christian Fischer.

The insects are deservedly recognised as one of the most successful groups of organisms on the planet. Thanks in no small part to their unlocking the ability of flight, insects can be seen today in almost every part of the planet above sea level. But not all insects, of course, are flighted; many remain firmly on the ground. A large proportion of these are the descendents of flighted ancestors that returned to a terrestrial existence but there are also some whose ancestors never took to the skies. For most people, the most familiar of these original land-huggers are likely to be the silverfish of the family Lepismatidae.

Silverfish are long-bodied insects with a covering of reflective scales—hence the 'silver' part of their name. The 'fish' part probably refers to the manner of their movement; speaking from my own experience collecting them, these buggers move fast, slipping along the ground like a silver minnow. There are over 250 known species of Lepismatidae (Mendes 2002); probably many more remain to be described. They comprise over half the known species of the insect order Zygentoma (sometimes referred to as the Thysanura though most current entomologists tend to avoid that name due to its previous history referring to a now-obsolete grouping of the Zygentoma with the superficially similar Archaeognatha); the other families in the order are commonly subterranean and less commonly encountered by the average person. The highest diversity of silverfish occurs in tropical and subtropical parts of the world, particularly in arid or semi-arid regions. Adaptations of the rectal epithelium allow silverfish to absorb moisture straight from the atmosphere (or, to put it another way, they drink through their butt), making them ideally suited to tolerating the dryness of deserts. They are also suited to tolerating the relatively dry habitats offered by the interiors of human houses and several species have become our associates (in cooler parts of the world, these synathropic species are often the only lepismatids around). These include the common silverfish Lepisma saccharina and the giant silverfish Ctenolepisma longicaudata. The firebrat Thermobia domestica is a colourfully patterned human associate that likes it particularly warm; it is usually restricted to places like the backs of stoves or alongside hot-water cylinders where it can find the heat it craves. Being detritivores (that is, they feed on dust), human-associated silverfish are usually quite innocuous though they may cause problems if their numbers get too high or if they get into stored foodstuffs.

Firebrat Thermobia domestica, copyright David R. Madison.

In areas where they are native, silverfish may be quite diverse. Watson & Irish (1998) conducted a study of an area of the Namib Desert that was home to eight different species of silverfish. They found a tendency for the species to differ in their preferred microhabitat within the area: some were restricted to the upper parts of the sand dunes dominating the region, others were restricted to the rocky hollows separating the dunes. Those found in rocky lower zones resembled the familiar human-associated species (indeed, they included members of the same genus as the giant silverfish, Ctenolepisma) in being elongate and slender. In contrast, those species found higher in the dunes themselves were shorter and more flattened with well-developed spines covering the legs. These features allowed the dune silverfish to effectively 'swim' through the sand, using the spines on the legs to dig about and their flattened form to slip between grains.

REFERENCES
Mendes, L. F. 2002. Taxonomy of Zygentoma and Microcoryphia: historical overview, present status and goals for the new millennium. Pedobiologia 46: 225–233.

Watson, R. T., & J. Irish. 1998. An introduction to the Lepismatidae (Thysanura: Insecta) of the Namib Desert sand dunes. Madoqua 15 (4): 285–293.

When Mayflies Last for Millions of Years

Mayfly Paraleptophlebia prisca preserved in amber, from Penney & Jepsen (2014).

Of all the media available for the preservation of fossils, none approaches perfection anywhere near as close as amber. There is little structural that amber does not preserve: external apperance, soft tissues, even cellular structure may potentially be examined. Amber offers us a window into the past unlike any other. The first amber deposits to go on the record (going back as far as the ancient Greeks), and the largest deposit yet discovered, was the Baltic amber of northern Europe, formed in the Eocene or Oligocene epoch* from the sap of a relative of the modern pines.

*Calculating the age of amber deposits is not easy. The amber itself cannot be dated directly (it is too old to be carbon-dated, of course, and it usually contains no mineral sediments that can be dated by other means) so aging it depends on indirect methods such as comparison of enclosed fossils with other aged samples, or dating of the deposits in which the amber is buried. Fossil comparisons suffer from the fact that the types of organisms that tend to be found preserved in amber are usually different from those preserved by other means (so we may know that two amber deposits are of similar age as each other, but we may still not know what that age actually is). Dating of surrounding deposits may be more straightforward, but is complicated by the fact that amber's relative buoyancy makes it prone to reworking (when a geological specimen becomes eroded out of its original formation and reburied in a younger one, thus making it appear younger than it is). Of course, if an amber deposit was produced over a long period of time, it may be impossible to tell if a particular piece comes from early or late in that period. Current consensus seems to indicate an Eocene age for the Baltic amber, but older references may refer to it as Oligocene or even Miocene.

Most of the insects we find preserved in Baltic amber are similar to those we find today (though differences in climate between now and then mean that the amber contains a number of groups that we would not expect to find so far north today). The specimen shown at the top of this post is assigned to a genus of mayfly (of the diverse family Leptophlebiidae) that remains widespread in Europe and North America today, of which it represents the earliest record. Like other members of the genus, it is a relatively small mayfly with a wingspan of less than 15 millimetres (I came across this fly-fishing website referring to the frustration of anglers trying to handle such small flies). Several specimens of this species are known from the Baltic amber, representing the subimaginal and imaginal stages of both sexes (mayflies are unique among living insects in that their wings become functional before they are fully mature; when a mayfly first emerges from the water it is as a near-mature subimago, subsequently moulting to a fully mature imago). Presumably, like modern mayflies, Paraleptophlebia prisca emerged as synchronised swarms, many individuals of which may have found themselves landing on an unwisely chosen tree-trunk and trapped within weeping sap.

This species was first described in 1856 by F. J. Pictet-Baraban along with other species of 'Neuroptera' from Baltic amber (at the time, the order Neuroptera included a wide range of insects with relatively unspecialised wings such as mayflies, dragonflies, bark-lice and lacewings). Pictet assigned it to the genus Potamanthus, another Recent mayfly genus albeit one in a different family Potamanthidae. He did express some uncertainty about this placement, an uncertainty that was later borne out by Demoulin (1968) who re-identified it as Paraleptophlebia on the basis of wing venation and male genital characters. Subsequent authors have agreed with Demoulin's assessment, regarding the mayflies of today as little different from those you might have found flying about over 50 million years ago.

REFERENCES

Demoulin, G. 1968. Deuxième contribution à la connaissance des Éphéméroptères de l’ambre oligocène de la Baltique. Deutsche Entomologische Zeitschrift, N. F. 15 (1–3): 233–276.

Penney, D., & J. E. Jepson. 2014. Fossil Insects: An introduction to palaeoentomology. Siri Scientific Press: Manchester.

Pictet-Baraban, F. J., & H. Hagen. 1856. Die im Bernstein befindlichen Neuropteren der Vorwelt. In: Berendt, G. C. Die im Bernstein befindlichen organischen Reste der Vorwelt vol. 2 pp. 41–126. Nicolaischen Buchhandlung: Berlin.

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.

A Crab Out of Water

Crabs are, of course, one of the most instantly recognisable groups of crustaceans. We all know what they look like, and we all know where can find them: under rocks at the beach, among seaweed,... climbing trees?

The Sri Lankan climbing crab Ceylonthelphusa scansor, copyright Harsha Meemaduma.

Though most of us probably think of crabs as animals of the seaside, there are several crab lineages that are found further inland, either in bodies of fresh water or among damp forests. One such group is the Parathelphusinae, an assemblage of freshwater crabs found in south-east Asia and the Indian subcontinent. A single genus, Somanniathelphusa, is found in southern China as far north as Taiwan and the adjacent mainland. Another, Austrothelphusa, is found in Australia. The group is diverse and new species continue to be described at a fair rate of knots. Most are found in swamps or on the banks of water bodies, in which they dig burrows up to a metre in depth (Davie 2002). They often emerge from the water to forage terrestrially, and at least one species, the Sri Lankan Ceylonthelphusa scansor, has been found in association with phytotelmata (water-filled hollows) in trees (Ng 2005). Parathelphusines are distinguished from the other subfamily of the Asian freshwater crab family Gecarcinucidae, the Gecarcinucinae, by the presence of a strong lateral groove on the male's second gonopods (Klaus et al. 2006). Until recently, most sources have treated these two groups as distinct families, but phylogenetic studies have suggested the Gecarcinucidae in the restricted sense to be non-monophyletic. The situation is further complicated by the diagnostic gonopod groove becoming reduced in some genera, so their gonopods look superficially more like gecarcinucines'.

Paddyfield crab Parathelphusa convexa in west Java, copyright Wibowo Djatmiko.

The Gecarcinucidae differ from the grapsoid terrestrial crabs referred to in earlier posts in that they do not need to return to the sea to release their eggs to hatch into larvae. Instead, gecarcinucids produce relatively large eggs that hatch directly into miniature crabs, that are brooded for a short period by the females before being released to face the world. Because of the lack of a planktonic stage, some parathelphusines have quite restricted ranges, and many are threatened by human developments.

REFERENCES

Davie, P. J. F. 2002. Zoological Catalogue of Australia vol. 19.3B. Crustacea: Malacostraca: Eucarida (part 2): Decapoda—Anomura, Brachyura. CSIRO Publishing: Collingwood (Australia).

Klaus, S., C. D. Schubart & D. Brandis. 2006. Phylogeny, biogeography and a new taxonomy for the Gecarcinucoidea Rathbun, 1904 (Decapoda: Brachyura). Organisms, Diversity and Evolution 6: 199–217.

Ng, P. K. L. 1995. Ceylonthelphusa scansor, a new species of tree-climbing crab from Sinharaja Forest in Sri Lanka (Crustacea: Decapoda: Brachyura: Parathelphusidae). J. South Asian nat. Hist. 1 (2): 175–184.

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.

Proasellus: Life Under Water

Proasellus slavus, photographed by Hans Jürgen Hahn K. Grabow (see comments below re credits).

The animal in the picture above is not quite the animal that I was planning on telling you about today, but I couldn't find an image of my particular target species. As long-time readers of this page will know, once a week I pick some random taxon to look at, and for this week I picked out the freshwater isopod Proasellus vignai. Most of you will know isopods as the woodlice that you may find in your garden, but the woodlice are really only one small part of the broad range of mostly aquatic isopod diversity. Proasellus belongs to a group of isopods known as the Asellota; as you can see in the picture above, asellotes differ from woodlice in (amongst other things) having the dorsal shields of each segment less tightly pressed together.

Proasellus is a genus of freshwater asellotes found around the Mediterranean: Europe, western Asia, northern Africa. Proasellus vignai is one of a number of species of Proasellus that are found in subterranean habitats, like P. slavus shown above. Both P. slavus and P. vignai, like most other subterranean animals, have lost the pigment and eyes of their surface-dwelling relatives. However, not all subterranean habitats are equal, and not all subterranean animals live in 'caves' as you might usually imagine them. Some Proasellus species are indeed found living in caves, but P. vignai and P. slavus are inhabitants of the hyporheic zone, the ground around rivers and streams where the water from the river soaks into the surrounding groundwater. Cave-dwelling Proasellus species tend to be broader and have more elongate limbs, so that they can maximise their chances of finding food in the nutritionally sparse cave waters. Hyporheic species, on the other hand, are narrower and more elongate, making them better suited for squeezing through the gaps between sediment particles.

Proasellus vignai seems to be a little known species (hence the lack of an available illustration). It is only known from the hyporheic zone of the Melfa river, in the Appenine mountains of the Lazio region of Italy (Bodon & Argano 1982). The Melfa is not a long river, only about 40 km long, so P. vignai may be a very localised species. It is a close relative of P. slavus, which lives in the water catchment of the Danube River. Other related species include P. ligusticus in the Ligurian Alps, P. sketi in Greece and P. boui in Languedoc in southern France. The scattered nature of the species of the P. slavus group, all of them hyporheic, suggests a certain degree of relictualism. Like other habitats that represent the edge of things, the hyporheic environment can be an uncertain one, vulnerable to outside influences. Should something change the nature of the Melfa river, Proasellus vignai might be taken with it.

REFERENCE

Bodon, M., & R. Argano. 1982. Un asellide delle acque sotterranee della Liguria orientale: Proasellus ligusticus n.sp. (Crustacea, Isopoda, Asellota). Fragm. Entomol. 16 (2): 117-123.

Majids: Crabs with Stylish Hats

Aggregation of large spider crabs Leptomithrax gaimardii, photographed by Peter Fuller.

The subjects of today's post, the Majidae, commonly go by the names of spider crabs or decorator crabs. The first of those names might sound like some people's ultimate nightmare, but I doubt that anyone could complain about the latter. Majids are characterised by having a carapace longer than wide, often with a covering of bristly hooked setae and relatively long legs (hence the name 'spider crab'). They get their alternate name of 'decorator crab' from the habit of many species of using the aforementioned hooked setae to attach algae and other bits of organic matter to themselves. The primary purpose of this adornment is to provide camouflage, and a decorated spider crab can be inordinately difficult to see when not moving. A secondary use of the crab's organic covering, however, is that they will also feed on material from it in times of need*.

*It is perhaps fortunate for Gaga that the question was never raised of her doing the same.

Triangle crab Eurynolambrus australis, from here.

The circumscription of the Majidae is more than a little fluid: at times, it has been used to include all the spider crabs of the superfamily Majoidea, but the more common practice these days is to divide the majoids between a number of families. Unfortunately, authors have disagreed about what those families should be. Ng et al. (2008) united the subfamilies Majinae and Mithracinae within the Majidae on the basis of shared features such as a well-developed protective orbit around the eyestalk. However, a direct relationship between majines and mithracines is not currently supported by molecular (Hultgren & Stachowicz 2008) or larval (Marques & Pohle 1998) data, though both these latter data sources are themselves limited by the relatively small number of studied taxa. Two smaller subfamilies included by Ng et al. (2008) in the Majidae, the Planoterginae and the isolated species Eurynolambrus australis, have not yet been analysed molecularly. Eurynolambrus australis is a particularly unusual little majid, so much so that it looks more like a parthenopid than a majid. Eurynolambrus also lacks hooked setae and so does not decorate itself; instead, it relies for disguise on its resemblance in colour to the coralline algae amongst which it lives (and on which it primarily feeds, though it is omnivorous overall—Woods & McLay 1996). Ng et al. placed it in the Majidae nevertheless owing to the resemblance of its larval stages to those of Majinae.

Channel clinging crab Mithrax spinosissimus, photographed by Nick Hobgood.

The two main subfamilies, the Majinae and Mithracinae, can be distinguished by the development of the orbit around the eyestalk. In the Mithracinae, the orbit is broadly expanded both above and below (with the lower margin formed from an expansion of the basal antennal segment), almost entirely enclosing the eyestalk and giving the front of the carapace a distinctly broad appearance in dorsal view. In the Majinae, the basal antennal segment is not expanded to form an underside to the orbit, so the eyestalks are contained from above only (Davie 2002). The Majinae are most diverse in the Indo-West Pacific, with only a handful of genera found outside this region. Some majines are quite large: the Australian Leptomithrax gaimardii reaches a leg-span of about 70 cm. The Mithracinae are more pantropical inhabitants of shallow water reefs.

REFERENCES

Davie, P. J. F. 2002. Zoological Catalogue of Australia vol. 19.3B. Crustacea: Malacostraca: Eucarida (part 2): Decapoda—Anomura, Brachyura. CSIRO Publishing: Collingwood (Australia).

Hultgren, K. M., & J. J. Stachowicz. 2008. Molecular phylogeny of the brachyuran crab superfamily Majoidea indicates close congruence with trees based on larval morphology. Molecular Phylogenetics and Evolution 48: 986-996.

Marques, F., & G. Pohle. 1998. The use of structural reduction in phylogenetic reconstruction of decapods and a phylogenetic hypothesis for 15 genera of Majidae: testing previous larval hypotheses and assumptions. Invertebrate Reproduction and Development 33 (2-3): 241-262.

Ng, P. K. L., D. Guinot & P. J. F. Davie. 2008. Systema brachyurorum: part I. An annotated checklist of extant brachyuran crabs of the world. Raffles Bulletin of Zoology 17: 1-286.

Woods, C. M. C., & C. L. McLay. 1996. Diet and cryptic colouration of the crab Eurynolambrus australis (Brachyura: Majidae) at Kaikoura, New Zealand. Crustacean Research 25: 34-43.

The Stone Mantis

Lithomantis carbonarius, as illustrated by Woodward (1876).

In 1876, Henry Woodward published the description of a large fossil insect found in a Scottish clay-ironstone nodule. This insect, when alive, would have had a wingspan of well over ten centimetres: Woodward measured the longest preserved wing at two and a quarter inches long, and a sizeable piece of the end was still missing. Believing it to be an ancient relative of the modern mantids, he named it Lithomantis carbonarius, the 'stone mantis from a coal measure'. Woodward's interpretation of his new fossil was to prove incorrect: it was not a mantis, but a member of those spectacular wonders of the Palaeozoic, the palaeodictyopteroids. More specifically, Lithomantis has been placed in a group of palaeodictyopteroids distinguished by Sinitshenkova (2002) as the Eugereonoidea.

Reconstructed wings of Lycocercus goldenbergi from Kukalová (1969), showing the overlap between the fore and hind wings; note also the bold colour patterning (often preserved in insect wings).

The palaeodictyopteroids are a group long overdue a truly comprehensive revision, and many aspects of their higher classification remain debatable. Of the current default classification, that of Sinitshenkova (2002), Prokop & Nel (2004) somewhat snarkily commented that, "Sinitshenkova’s classification cannot be considered based on the cladistic method, even if it uses the cladistic terminology". Nevertheless, Sinitshenkova defined the Eugereonoidea by a number of wing characters: wings that were about 2.5 times as long as broad, a subcostal vein reaching the costal vein near the wing apex, medial and cubital veins with little-branched anterior forks but much-branched posterior forks, and a tendency for the archedictyon (the net-like array of veinlets running amongst the major wing veins in palaeodictyopteroids) to become simplified or replaced by direct cross-veins. Members of the Eugereonoidea are known from the Upper Carboniferous and Lower Permian (Sinitshenkova 2002; Prokop & Nel 2007) and, like other palaeodictyopteroids, would have inhabited tropical latitudes in life.

Wings of the eugereonid Peromaptera filholi, from Kukalová (1969).

Like other palaeodictyopteroids (and, indeed, many other Palaeozoic insect groups in general), Eugereonoidea are mostly known from fossils of the wings, but those that are more completely known are large-bodied insects with relatively long sucking beaks. One species, Eugereon boeckingi, had a beak over three centimetres long; that of Lithomantis was a bit more restrained at only just over one centimetre. They used these impressive weapons to attack the stems of the ferns and seed ferns of the time in search of sap. Other palaeodictyopteroids, including the eugereonoid Lycocercus goldenbergi (Kukalová 1969), had much shorter beaks, and would have probably fed from spores or seeds. Eugereonoids had fairly broad-based wings with the fore and hind pairs of wings originally little differing from each other. The pronotum bore well-developed paranotal lobes that have lead to descriptions of these insects as 'six-winged'; though the pronotal lobes could not actively flap in the manner of true wings, Wootton & Kukalová-Peck (2000) suggested that they were somewhat movable, and could have been used to stabilised pitch. In the family Lycocercidae, the two pairs of wings overlapped to a degree unknown in any living insects; in Notorhachis wolfforum, the forewings overlapped the hind wings almost entirely. As argued by Wootton & Kukalová-Peck (2000), these species would have flown quickly but with relatively little manouevrability, like insectoid turkeys. In contrast, members of the families Eugereonidae and Megaptilidae developed relatively long narrow forewings followed by shorter, broader hind wings. Like modern insects with comparable wing morphologies (such as bees and butterflies), members of these families probably beat the two pairs of wings in concert, and would have been more manoeuvrable compared to the lycocercids. However, with an estimated wingspan of over a foot, the eugereonoid Megaptilus blanchardi was by far the largest insect ever known to develop this mode of flying.

REFERENCES

Kukalová, J. 1969. Revisional study of the order Palaeodictyoptera in the Upper Carboniferous shales of Commentry, France part II. Psyche 76: 439-486.

Prokop, J., & A. Nel. 2004. A new genus and species of Homoiopteridae from the Upper Carboniferous of the Intra-Sudetic Basin, Czech Republic (Insecta: Palaeodictyoptera). European Journal of Entomology 101: 583-589.

Prokop, J., & A. Nel. 2007. New significant fossil insects from the Upper Carboniferous of Ningxia in northern China (Palaeodictyoptera, Archaeorthoptera). European Journal of Entomology 104: 267-275.

Sinitshenkova, N. D. 2002. Superorder Dictyoneuridea Handlirsch, 1906 (=Palaeodictyopteroidea). In History of Insects (A. P. Rasnitsyn & D. L. J. Quicke, eds) pp. 115-124. Kluwer Academic Publishers: Dordrecht.

Woodward, H. 1876. On a remarkable fossil orthopterous insect from the coal-measures of Scotland. Quarterly Journal of the Geological Society of London 32: 60-65.

Wootton, R. J., & J. Kukalová-Peck. 2000. Flight adaptations in Palaeozoic Palaeoptera (Insecta). Biol. Rev. 75: 129-167.

Snap! goes the Termite

The snapping termite Cavitermes tuberosus, from Wiki Termes.

For the subject of today's post, I drew the termite subfamily Termitinae. Termites are extraordinary animals: socially complex, ecologically vital, dietically remarkable. Personally, I'm rather found of these communal cockroaches.

Termites of the family Termitidae (commonly referred to as the 'higher termites') differ from other, 'lower' termites in the nature of their gut biota (without which they would not be able to digest their cellulose diets): instead of having flagellated protozoa in their gut, termitids carry symbiotic bacteria. This difference in symbionts is reflected by a difference in diet. Higher termites feed on more decayed wood or plant matter than lower termites; some higher termites feed directly on organic-rich soil that contains little or no plant material (Inward et al. 2007). Subfamilies within the Termitidae are also distinguished on the basis of their gut anatomy: members of the Termitinae have what is called a 'mixed segment' on the outer edge of their intestine (Lo & Eggleton 2011). In the mixed segment, instead of the division between the mesenteron (the middle section of the intestine) and the proctodaeum (the posterior section) being simple and straight across, the mesenteron wall extends backwards along one side of the gut only; it has been suggested that the mixed segment functions to pump alkaline fluids into the gut, maintaining appropriate pH and fluid levels for the symbiotic bacteria in the hindgut (Bignell et al. 1983).

Workers of Amitermes dentatus repairing a damaged nest, from here.

The Termitinae have also been distinguished on the basis of the morphology of their soldiers, with most genera having soldiers with elongate mandibles that have relatively few large teeth. These are used to bite and slash at threats to the colony. However, phylogenetic analyses have contradicted this distinction (Inward et al. 2007). The Termitinae are paraphyletic with regard to the Nasutitermitinae, who have developed a very different method of defense: the mandibles are reduced, and instead the front of the head is drawn out into an elongate 'nose'. At the end of the 'nose' is a glandular opening from which the soldiers squirt a sticky glue at their opponents. Also nested within the Termitinae are the Syntermitinae whose soldiers combine both methods of defense: they retain sickle-shaped mandibles that are used to pierce the cuticle of attackers while the protruded glandular opening is used to apply toxic secretions. Chemical defenses are also not unknown among more standard termitines: soldiers of Globitermes sulphureus were dubbed 'walking bombs' by E. O. Wilson due to their explosive (and often self-destructive) discharge of toxic chemicals from hypertrophied labial gland reservoirs in the abdomen. It should also be noted that a small number of termitines do not produce soldiers at all: they may live in association with other soldier-producing termites, like the Australian Invasitermes, or they may feed on low-nutrient soils (presumably making the maintenance of a soldier caste too nutritionally expensive), like the Indomalayan genera Protohamitermes and Orientotermes.

The mushroom-like mound of Cubitermes, a major soil-feeding genus in Africa, photographed by Marco Schmidt.

Another mode of defense that is found only among the termitines (though phylogenetic analysis indicates that it has evolved multiple times) is the production of soldiers with elongate snapping mandibles. In these termites, soldiers store kinetic energy through muscular deformation of the mandibles, allowing them to be suddenly closed with great force (Prestwich 1984). So great is the force involved, in fact, that it seems to be not uncommon for the jaws to become completely crossed over as has happened to the individual at the top of this post. Snapping termites generally live in subterranean colonies, and even after the soldier has been 'spent' on the discharge of its mandibles, its body acts as a physical barrier in the confined tunnel. In some snapping termites, the mandibles are strongly asymmetrical, so the force of the closure is channelled through the left mandible only with doubled force. Asymmetrical snappers of the genus Neocapritermes, in fact, are able to knock out fairly large ants with a single blow. The video below shows a soldier of Planicapritermes attacking an ant: Or you can see Neocapritermes in action in this video. Keep a close eye on the screen around the 20-second mark...

REFERENCES

Bignell, D. E., H. Oskarsson, J. M. Anderson & P. Ineson. 1983. Structure, microbial associations and function of the so-called "mixed segment" of the gut in two soil-feeding termites, Procubitermes aburiensis and Cubitermes severus (Termitidae, Termitinae). Journal of Zoology 201: 445-480.

Inward, D. J. G., A. P. Vogler & P. Eggleton. 2007. A comprehensive phylogenetic analysis of termites (Isoptera) illuminates key aspects of their evolutionary biology. Molecular Phylogenetics and Evolution 44: 953-967.

Lo, N., & P. Eggleton. 2011. Termite phylogenetics and co-cladogenesis with symbionts. In: Bignell, D. E., et al. (eds) Biology of Termites: a modern synthesis pp. 27-50. Springer.

Prestwich, G. D. 1984. Defense mechanisms of termites. Annual Review of Entomology 29: 201-232.

Mayflies in their Spring

Armoured mayfly Baetisca obesa, photographed by Jason Neuswanger.

Mayflies have occasionally put in an appearance here at CoO, most notably in an earlier post where I explained how the one thing that everyone 'knows' about mayflies is simply not true. In this post, I thought that I'd look briefly at the fossil context of mayflies.

The basalmost relationships among insects have been subject to some discussion over the years, but the current majority view is probably that mayflies were the first of the living winged insect lineages to diverge from the rest. Evidence for this is their retention of some plesiomorphic features such as the presence of three caudal filaments at the end of the abdomen, and a sliding rather than fixed inner mandibular articulation in the nymphs (adult mayflies don't have functional mouthparts). Mayfly nymphs, offhand, are known as naiads. Naiads were originally supposed to be nymphs that inhabited freshwater springs, so at some point the term 'naiad' was transferred from this:

Hylas and the Nymphs, by John William Waterhouse, in which our hero is fatally tempted by a septet of skinnydipping broads.
to this:

Drunella cornuta, photographed by Jonas Insinga.
Which I'm sure came as something of a disappointment to Hylas (though, of course, had Hylas been more disappointed, he may have also been less dead).

As discussed in an earlier post on stoneflies, there is some uncertainty whether aquatic nymphs are ancestral or derived for winged insects. However, mayflies were spending the first part of their lives in water by at least the Permian (Kluge & Sinitshenkova 2002; Grimaldi & Engel 2005). Representatives of the mayfly crown group (i.e. the group stemming from the most recent common ancestor of all living mayflies) are not known until the Jurassic; earlier species all belong to the stem group. The Carboniferous Syntonopterodea may also be stem-mayflies, but in superficial appearance the large, broad-winged syntonopterodeans may have looked more like the contemporary palaeodictyopteroids.

Reconstruction of Protereisma permianum, one of the best known of the Permian stem-mayflies, via here.

The Permian and Jurassic Ephemeroptera themselves had some notable differences from crown-group mayflies. Modern mayflies have heteronomous wings, with the fore- and hind wings differing in size (in some mayflies, the hind wings have almost disappeared entirely). Permian mayflies, in contrast, had homonomous wings, with the two pairs more or less identical; the hind wings became shortened in Triassic stem-mayflies (Grimaldi & Engel 2005). At least some stem-mayflies also retained well-developed mouthparts as adults; this suggests that they may well have lived longer as adults than modern mayflies. While Grimaldi & Engel (2005) included Permian and Triassic species in the Ephemeroptera, Staniczek et al. (2011) restricted that name to the crown group and its nearest and dearest, placing most of Grimaldi & Engel's stem-group 'Ephemeroptera' into an extinct clade Permoplectoptera.

REFERENCES

Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press: New York.

Kluge, N. Yu., & N. D. Sinitshenkova. 2002. Order Ephemerida Latreille, 1810. The true mayflies (=Ephemeroptera Hyatt et Arms, 1891 (s. l.); =Euephemeroptera Kluge, 2000. In History of Insects (A. P. Rasnitsyn & D. L. J. Quicke, eds) pp. 89-97. Kluwer Academic Publishers: Dordrecht.

Staniczek, A. H., G. Bechly & R. J. Godunko. 2011. Coxoplectoptera, a new fossil order of Palaeoptera (Arthropoda: Insecta), with comments on the phylogeny of the stem group of mayflies (Ephemeroptera). Insect Systematics and Evolution 42: 101-138.

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.

A Devonian Pterygote?

I was going to write a post today on Strudiella devonica, the new fossil insect described from the Late Devonian of Belgium in today's Nature (Garrouste et al. 2012). Unfortunately, there's a limit to what I can really say. The stratigraphic significance of the specimen is undeniable: it sits well within a gap of about sixty million years that previously divided the earlier known insect fossils from the lower Devonian from the earliest known unequivocal winged insects in the mid-Carboniferous. Unfortunately, and I say this in the nicest possible way, the specimen itself is roadkill:

Photograph and interpretative drawing of Strudiella devonica, from Garrouste et al. (2012). Scale bar equals 1 mm.
Ah well, we simply have to work with what we're given, don't we? I think it's fairly reliable that this is, indeed, an insect: there seems a clear separation into a well-defined head, thorax, and legless abdomen, with no more than three legs visible on a single side of the thorax. The forward-protruding mandibles and antennae with broader basal segments are also insect-like rather than entognath-like (so it's not a stem dipluran or something like that). However, Garrouste et al. suggest that this specimen not only represents an insect, but also a crown-group pterygote. This I feel is a little more problematic.

Assignment of Strudiella to pterygotes relies on two characters: the relatively elongate legs, and the appearance of the mandibles. I suspect that it would not be that difficult for elongate legs to evolve convergently, and the supposed Carboniferous dipluran Testajapyx appears to have relatively long hind legs at least (Kukalova-Peck 1987). The mandible structure is a bit more difficult to hand-wave, though: Garrouste et al. interpret Strudiella as having an orthopteroid mandible, which is believed to be a synapomorphy of the Metapterygota, the particular clade within the pterygotes uniting the Neoptera and Odonata.

Hexapod phylogeny with representative mandible types, from Engel & Grimaldi (2004).
The earliest hexapods possessed a mandible with a single articulation (the condyle) to the head; such a mandible is still present in springtails, diplurans and bristletails. The clade uniting silverfish and pterygotes developed a second articulation (the acetabulum) on the inside of the mandible. In silverfish and mayflies, the acetabulum is anterior to the condyle, and the acetabular articulation is relatively loose. In the metapterygotes, the acetabulum has moved back to become more level with the condyle, and the mandible's articulation with the head is a lot more solid. The fossil remains of Strudiella do not appear to show the mandible articulation itself, but the general shape and orientation of the triangular mandible is more similar to the metapterygote arrangement than to the more basal morphology. Besides, such a morphology is has more clearly been demonstrated in the Lower Devonian Rhyniognatha, known only from a pair of preserved mandibles that are even older than Strudiella (Engel & Grimaldi 2004).

The ultimate question, then, is: is this one character enough to cement these taxa as crown pterygotes, with the implication that winged insects must have evolved considerably earlier than their fossil record currently indicates? Strudiella itself shows no sign of wings; Garrouste et al. suggest that it may be a nymph of a winged adult. I would counter that it also doesn't appear to possess any incipient wing buds, but of course it is debatable whether the preservation is good enough to be confident on this point.

If winged insects have been around since the Early Devonian, why do we find no direct evidence of them until the mid-Carboniferous? Wings are among the most commonly preserved insect remains—to the extent that if, as the adage goes, mammalian palaeontology is all about 'the tooth, the whole tooth, and nothing but the tooth', insect palaeontology often threatens to be 'all in vein'. For my part, I'm not inherently opposed to the idea of Devonian winged insects, but I don't think I'd really be willing to accept them until we're shown the actual wings.

REFERENCES

Engel, M. S., & D. A. Grimaldi. 2004. New light shed on the oldest insect. Nature 427: 627-630.

Garrouste, R., G. Clément, P. Nel, M. S. Engel, P. Grandcolas, C. D’Haese, L. Lagebro, J. Denayer, P. Gueriau, P. Lafaite, S. Olive, C. Prestianni & A. Nel. 2012. A complete insect from the Late Devonian period. Nature 488: 82-85.

Kukalova-Peck, J. 1987. New Carboniferous Diplura, Monura, and Thysanura, the hexapod ground plan, and the role of thoracic side lobes in the origin of wings (Insecta). Canadian Journal of Zoology 65: 2327-2345.

The Stoneflies: Old or New?

Little snowfly Capnia nana, from here.

Despite being a working entomologist, I have to confess that there are some insect groups with which I am not entirely familiar. The stoneflies, Plecoptera, are one of those groups. I work in arid northern Australia, but stoneflies are associated with cool waters. The highest diversity live in temperate regions of the world; those whose ranges extend into lower latitudes are found higher in the mountains, away from the heat.

Stoneflies live in their favoured waterways as nymphs, emerging when they develop to adulthood (at least one species, Capnia lacustra of Lake Tahoe, appears to also be aquatic as an adult). The adults are large, long-bodied insects that are often better runners than they are fliers. Nymphs are primarily detritivores, but many species are carnivorous to a greater or lesser extent. Adults of some species do not feed; others feed on such things as encrusting algae or lichen or rotten wood. Depending on species, adult stoneflies may have full-sized wings, reduced wings or no wings at all; in some species, both flying and flightless morphs may be present. Two European species, Perla bipunctata and Perlodes microcephala, are solely brachypterous in Britain but may be either brachypterous or macropterous elsewhere in their range (Hynes 1976). Winged females of many species lay eggs while in flight, either dropping them into water or gliding to the water surface and letting the eggs be washed off from the end of the abdomen. Other species attach their eggs to stones underwater or insert them into crevices or rotting wood.

Tasmanian stonefly, Eusthenia sp., photographed by Nuytsia@Tas. More colourful than most other stoneflies, Eusthenia species raise their forewings when threatened to reveal brightly patterned hindwings.

Most recent authors have supported a division of the stoneflies between two lineages, the Antarctoperlaria and Arctoperlaria, that are both morphologically and geographically distinct (Zwick 2000). The Antarctoperlaria are found in South America, Australia and New Zealand. The Arctoperlaria, in contrast, are primarily found in the Northern Hemisphere (except for members of two families, the Perlidae and Notonemouridae). Many species of the Arctoperlaria signal to potential mates by drumming the abdomen on a substrate, a behaviour unknown in the Antarctoperlaria.

Nymph of Acroneuria abnormis, photographed by Michel Gauvin.

Stoneflies have often been regarded as one of the most primitive groups of winged insects, and their position remains contentious. The two main theories are that they are the sister group to all other neopteran insects (insects that are capable of folding the wings back flat over the body), or that they belong to the group known as Polyneoptera that also includes grasshoppers and cockroaches. Which of these is correct has been regarded as potentially significant in understanding how flight evolved in insects as a whole. As discussed in an earlier post, it has been suggested that insect wings are homologous with articulated gills in aquatic nymphs. As well as Plecoptera, the two living non-neopteran insect orders Odonata (dragonflies) and Ephemeroptera (mayflies) are aquatic as nymphs, and if Plecoptera are basal to other neopterans then it suggests that this life history may be ancestral for winged insects as a whole. However, differences in nymphal morphology between the three groups may indicate that the aquatic lifestyle has been independently acquired in all three from terrestrial ancestors, which would also be more likely if stoneflies are derived polyneopterans. Molecular studies have supported a polyneopteran relationship for stoneflies, but not with rock-solid support (e.g. Terry & Whiting 2005); morphological studies are equivocal and do not clearly point either way (Zwick 2009). The fossil record is also unclear: while a number of early insect groups have been connected to stoneflies, whether they are true stem-Plecoptera or closer to other polyneopteran lineages is debatable (Béthoux et al. 2011). It is also worth pointing out that while similarities between stonefly and mayfly gills have been cited in relation to their supposed homology with wings, different families of stoneflies have different gill types, and we still do not know whether and what kind of gills were ancestral for Plecoptera. Also, in those stoneflies with plate-like gills, the gills are not articulated like wings and incapable of independent movement (Zwick 2009).

REFERENCES

Béthoux, O., Y. Cui, B. Kondratieff, B. Stark & D. Ren. 2011. At last, a Pennsylvanian stem-stonefly (Plecoptera) discovered. BMC Evolutionary Biology 11: 248.

Hynes, H. B. N. 1976. Biology of Plecoptera. Annual Review of Entomology 21: 135-153.

Terry, M.D., & M. F. Whiting. 2005. Mantophasmatodea and phylogeny of the lower neopterous insects. Cladistics 21: 240–257.

Zwick, P. 2000. Phylogenetic system and zoogeography of the Plecoptera. Annual Review of Entomology 45: 709-746.

Zwick, P. 2009. The Plecoptera–who are they? The problematic placement of stoneflies in the phylogenetic system of insects. Aquatic Insects 31 (suppl. 1): 181-194.