Niphargus

I've commented before on the unexpectedly high diversity of animal species that can be found living in groundwater. Because dispersal through this habitat is, unsurprisingly, often difficult and bodies of groundwater are often isolated from each other, many groundwater-adapted species can have almost ludicrously small ranges. Hence, for instance, the diversity of the amphipod genus Niphargus.

Niphargus hadzii, copyright B. Sket.

The genus Niphargus is found across Europe, mostly south of what was the lower edge of the northern ice sheet during the Pleistocene, though it is replaced by a closely related genus in the Iberian Peninsula. A few species are also found in south-west Asia. They are usually eyeless and colourless (the genus name means 'snowy white'). Though clearly adapted for subterranean environments, they may also be found in associated surface habitats such as springs or the upper sections of streams (Fišer et al. 2015). Species of Niphargus vary considerably in size: the smallest interstitial species may be only two or three millimetres in length whereas some cave-dwellers reach forty millimetres (Karaman & Ruffo 1986). They may feed on organic particles filtered from the water, or they may predate on smaller animals.

Over 300 species of Niphargus are currently recognised, making it one of the largest genera of freshwater amphipods. Even so, this number is likely to be a significant underestimate of the genus' true diversity. A number of studies on Niphargus have identified evidence of previously cryptic species. A study by Fišer et al. (2015) of two species from the Istrian Peninsula in the north-west Balkans found genetic evidence for the existence of two strongly divergent populations within each, with the two populations of 'N. krameri' being morphologically as well as genetically distinct. Flot et al. (2010) found evidence of four distinct lineages within 'N. ictus' of Italy's Frasassi caves, representing at least three independent colonisations of the cave system from external sources. At least two of these lineages are unique among amphipods in living in a symbiotic association with sulphide-oxidising bacteria of the genus Thiothrix, allowing them to survive in Frasassi's sulphide-rich waters.

Niphargus aquilex from the River Till, copyright Lee Knight.

With such a large genus, attempts have naturally been made to divide it into more manageable units. About a dozen species groups have been recognised on the basis of morphology, albeit some poorly defined. However, a molecular phylogenetic study of the genus by Fišer et al. (2008) identified none of these groups as monophyletic. Instead, they found a higher correlation of phylogeny with geography than morphology. The picture suggested is one of poor dispersers, re-evolving similar forms on multiple occasions as they diverge to exploit their secluded habitats as best they can.

REFERENCES

Fišer, C., B. Sket & P. Trontelj. 2008. A phylogenetic perspective on 160 years of troubled taxonomy of Niphargus (Crustacea: Amphipoda). Zoologica Scripta 37 (6): 665–680.

Fišer, Ž., F. Altermatt, V. Zakšek, T. Knapič & C. Fišer. 2015. Morphologically cryptic amphipod species are "ecological clones" at regional but not at local scale: a case study of four Niphargus species. PLoS One 10 (7): e0134384.

Flot, J.-F., G. Wörheide & S. Dattagupta. 2010. Unsuspected diversity of Niphargus amphipods in the chemoautotrophic cave ecosystem of Frasassi, central Italy. BMC Evolutionary Biology 10: 171.

Karaman, G. S., & S. Ruffo. 1986. Amphipoda: Niphargus-group (Niphargidae sensu Bousfield, 1982). In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 514–534. E. J. Brill/Dr W. Backhuys: Leiden.

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.

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.

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 Grapsidae: From Sea to Shore

Sally Lightfoot, Grapsus grapsus, photographed by Victor Burolla. The vernacular name refers to their walking on the points of their legs.

In a post from back in 2008, I wrote about the group of crabs known as the Grapsoidea. As described in that post, the classification of the Grapsoidea has been shuffled in recent years, and the subjects of today's post, the Grapsidae, would have previously been classed as the Grapsinae within a larger Grapsidae. The more restricted Grapsidae has been supported by numerous recent analyses, both morphological (Karasawa & Kato 2001) and molecular (Schubart et al. 2000). Morphologically, grapsids are united by having an expanded anterolateral corner to the merus* of the third maxilliped, oblique ridges on the lateral surfaces of the meri of the pereiopods, and (in many species) oblique ridges on the dorsum of the carapace (Karasawa & Kato 2001). Studies of the larvae of grapsids have also identified distinctive characters by which grapsid larvae can be distinguished from those of other grapsoids (Cuesta & Schubart 1999).

*The merus is the first elongate segment of crustacean appendages, corresponding to the femur of other arthropods. The maxillipeds are feeding appendages; the pereiopods are the walking legs.

The Columbus crab Planes major, photographed by Denis Riek. This species comes in a wide range of colours, from brown to blue to almost white; the page linked to shows a number of examples.

Most grapsids are intertidal shore-dwellers, but there are some exceptions. Species of the genus Planes, known as Columbus crabs, are small oceanic forms. They live on objects floating in the open water: seaweed, driftwood and other debris, or even other animals such as by-the-wind sailors or turtles (Spivak & Bas 1999). Columbus crabs differ from other grapsids in having flattened pereiopod meri for swimming, and two of the three species lack oblique ridges on the carapace. The aforementioned phylogenetic analyses also agree in placing Planes as the sister group to other grapsids analysed.

Geograpsus grayi, from here.

Also distinctive are species of the genus Geograpsus, which are one of a number of crab groups to have developed a terrestrial lifestyle, found on islands of the Indo-Pacific and Atlantic. In the Indo-Pacific G. crinipes, it has been shown that dense bunches of setae between the second and third walking legs are long enough to contact the ground when the animal sits back on its haunches (McLay & Ryan 1990). Water on the surface of the ground is drawn up through the setae by capillary action and conducted into the gill chamber, keeping the gills damp and functioning. Terrestrial Geograpsus retain marine larvae as do many other terrestrial crabs; the larval development has been studied for the eastern Pacific G. lividus which goes through nine larval stages (eight zoeae and the megalopa) over the period of two months (Cuesta et al. 2011). This happens to be the longest developmental pathway of any known crab: the previous confirmed maximum was eight larval stages.

REFERENCES

Cuesta, J. A., G. Guerao, C. D. Schubart & K. Anger. 2011. Morphology and growth of the larval stages of Geograpsus lividus (Crustacea, Brachyura), with the descriptions of new larval characters for the Grapsidae and an undescribed setation pattern in extended developments. Acta Zoologica 92 (3): 225-240.

Cuesta, J. A., & C. D. Schubart. 1999. First zoeal stages of Geograpsus lividus and Goniopsis pulchra from Panama confirm consistent larval characters for the subfamily Grapsinae (Crustacea: Brachyura: Grapsidae). Ophelia 51 (3): 163-176.

Karasawa, H., & H. Kato. 2001. The systematic status of the genus Miosesarma Karasawa, 1989 with a phylogenetic analysis within the family Grapsidae and a review of fossil records (Crustacea: Decapoda: Brachyura). Paleontological Research 5 (4): 259-275.

McLay, C. L., & P. A. Ryan. 1990. The terrestrial crabs Sesarma (Sesarmops) impressum and Geograpsus crinipes (Brachyura, Grapsidae, Sesarminae) recorded from the Fiji Is. Journal of the Royal Society of New Zealand 20 (1): 107-118.

Schubart, C. D., J. A. Cuesta, R. Diesel & D. L. Felder. 2000. Molecular phylogeny, taxonomy, and evolution of nonmarine lineages within the American grapsoid crabs (Crustacea: Brachyura). Molecular Phylogenetics and Evolution 15 (2): 179-190.

Spivak, E. D., & C. C. Bas. 1999. First finding of the pelagic crab Planes marinus (Decapoda: Grapsidae) in the southwestern Atlantic. Journal of Crustacean Biology 19 (1): 72-76.

Life Among a Shrimp's Gills

Female of Schizobopyrina bombyliaster from Williams & Boyko (2004), with red box added on ventral view to indicate position of small male.

For today's random subject, I drew the marine isopod genus Schizobopyrina. Schizobopyrina is a genus in the family Bopyridae, and females of this genus were distinguished by Markham (1985) from those of the related genus Bopyrina by the presence of palp on the maxilliped (part of the mouthparts), by its more elongate oostegites (the lamellae forming the brood pouch in which eggs and larvae are incubated), and by the fusion of the pleomeres (posterior segments) on one side of the body. About ten or so species have been assigned to this genus from warmer waters around the world.

Mature bopyrids are parasites of shrimps and other crustaceans (Schizobopyrina has been found on hosts of the families Palaemonidae, Gnathophyllidae and Hippolytidae). Schizobopyrina and related genera are found in the branchial (gill) cavities of their host. Shrimp gills are developed from side-branches of the base of the legs, and are covered by an overhanging shelf of the carapace (if anyone is familiar with the process of preparing a crayfish or lobster, the gills are the 'dead man's fingers' that you have to remove before serving the crayfish). In a shrimp that is host to Schizobopyrina, the branchial cavity will become greatly protruding, as can be seen in this photo of a bumblebee shrimp Gnathophyllum americanum parasitised by Schizobopyrina bombyliaster (from Williams & Boyko 2004; scale bar equals 1.0 mm):

Bopyrids are released from the parent host as larvae that initially attach themselves to copepods. When they are approaching maturity, they leave the copepod and find an appropriate adult host. The first larva to attach itself to an appropriate shrimp will develop into a female, while any subsequent larva to attach itself will develop into a male (Cash & Bauer 1993). As can be seen in the figure at the top of this post, the female is considerably larger than the male. She is also noticeably asymmetrical in her body form, though a single species may include individuals bent to either the left or the right (Markham 1985). The female bopyrid attaches herself to her host before it reaches maturity: this puts her at risk of losing her place as the host moults, but studies of another branchial parasite bopyrid, Probopyrus pandalicola, indicate that as the host cuticle tears away during the process of moulting, the female is able to reattach herself to the new cuticle underneath and keep her place (Cash & Bauer 1993). The smaller male looks very different to the female, and is much more symmetrical. He attaches himself to the female, but whether or how he feeds is unknown. In Probopyrus pandalicola, the female moults, then produces eggs, after each moult of her host; the male has been observed crawling at this point into the brood pouch of the female, where he presumably fertilises her eggs.

Just as a further aside, the recent description of the species featured in the figures used in this post, Schizobopyrina bombyliaster Williams & Boyko 2004, was of further interest because the type specimen of this parasitic isopod was itself host to a hyperparasitic isopod, the cabiropid Cabirops bombyliophila. Which gives me an idea for a matryoshka design...

REFERENCES

Cash, C. E., & R. T. Bauer. 1993. Adaptations of the branchial parasite Probopyrus pandalicola (Isopoda: Bopyridae) for survival and reproduction related to ecdysis of the host, Palaemonetes pugio (Caridea: Palaemonidae). Journal of Crustacean Biology 13 (1): 111-124.

Markham, J. C. 1985. A review of the bopyrid isopods infesting caridean shrimps in the northwestern Atlantic Ocean, with special reference to those collected during the Hourglass cruises in the Gulf of Mexico. Memoirs of the Hourglass Cruises 7 (3): 1-156.

Williams, J. D., & C. B. Boyko. 2004. A new species of Schizobopyrina Markham, 1985 (Crustacea: Isopoda: Bopyridae: Bopyrinae) parasitic on a Gnathophyllum shrimp from Polynesia, with description of an associated hyperparasitic isopoda (Crustacea: Isopoda: Cabiropidae). Proceedings of the California Academy of Sciences 55 (24): 439-450.

Burrowing Beaky Amphipods

Oediceroides emarginatus, photographed by Gauthier Chapelle.

I've been out in the field for a couple of weeks, hence the momentary absence of regular posts. But I have returned, and shall kick off with a brief introduction to the Oedicerotidae.

The oedicerotids are another cluster within the systematic morass that is the gammaridean amphipods (other gammaridean families featured here and here). Members of the Oedicerotidae are marine benthic burrowing forms, appropriately solidly built (for an amphipod, at least), and most readily distinguished from most other gammarideans by their particularly long fifth pereiopods (the last pair of legs on the main body) (Barnard 1969). They also usually have a long peduncle on the third uropods (the 'tail' appendages), though one distinctive genus Metoediceros lacks the third uropod entirely (Barnard 1974). In many oedicerotids, the eyes have also moved upwards to become coalesced along the dorsal midline and the head often possesses a prominent rostrum. However, these features are absent from a number of Southern Hemisphere and deep-sea taxa (the latter of which generally lack eyes altogether).

Dorsal view of the head of Monoculodes borealis, showing the coalescent eyes, from Andrey Vedenin.

Oedicerotids of the genus Synchelidium have been shown to be predators of harpacticoid copepods (Yu & Suh 2006). The abundance of this food appears to determine their reproductive behaviour, as females produce larger broods in the spring when harpacticoids are more abundant than in the fall.

REFERENCES

Barnard, J. L. 1969. The families and genera of marine gammaridean Amphipoda. United States National Museum Bulletin 271: 1-535.

Barnard, J. L. 1974. Evolutionary patterns in gammaridean Amphipoda. Crustaceana 27 (2): 137-146.

Yu, O. H., & H.-L. Suh. 2006. Life history and reproduction of the amphipod Synchelidium trioostegitum (Crustacea, Oedicerotidae) on a sandy shore in Korea. Marine Biology 150: 141-148.

Flower-tails

The patterned anthurid Mesanthura astelia, from Museum Victoria.

Anthuroidea (or Anthuridea, depending on where you look) are small marine isopods that get up to a couple of centimetres in length. Anthuroids are distinguished from other isopods by their particularly narrow, elongate body form, as well as (in most species) their tail-fans with the component uropods arranged in a manner reminiscent of a 'five-petalled flower' (hence the name of the group, 'flower-tails'). The one exception is the recently described Leipanthura casuarina in which the uropod branches are cylindrical rather than flattened; Leipanthura is a very small species (less than 3 mm long) and probably represents a neotenous form retaining a juvenile tail morphology into adulthood (Poore 2009).

The question of whether this group should be called Anthuridea or Anthuroidea relates to different proposals on their phylogenetic position. The name 'Anthuridea' is older (replacing an even earlier name, Aberrantia, that does not appear to have any recent usage) but was changed to Anthuroidea by Brandt & Poore (2003) when they reclassified anthuroids from a separate 'suborder' of isopods to a 'superfamily' within the suborder Cymothoida. In a more recent analysis by Wilson (2009), the monophyly of Cymothoida was supported by morphological data alone, but not by molecular data or combined analysis (in particular, several parasitic 'cymothoid' families such as Gnathiidae and Bopyridae formed a clade in the latter analyses that was sister to all other isopods). As the composition of the anthuroids has never altered under either name, the question of orthography is largely academic.

Individual of Paranthura elegans, photographed by Peter J. Bryant. Paranthura belongs to a separate family (Paranthuridae) from Mesanthura (Anthuridae); the families are distinguished by the mouthparts of Paranthuridae being modified into piercing stylets, as opposed to the chewing mouthparts of Anthuridae.

Anthuroids live hidden among sponges, corals, seaweeds, etc. or burrowed into sand, where they are active hunters of smaller invertebrates (as evidenced by their raptorial forelimbs). They have most commonly been recorded from shallow waters, but are also known from the deep sea (Kensley 1982). It is unclear whether their supposed rarity in deep-sea collections reflects poorer investigation, or whether the morphological conservatism of anthuroids compared to other isopod groups has restricted their ecological diversity. At least some species are protogynous sequential hermaphrodites (Brusca et al. 2001), that is, they begin life as females and transform later into males.

REFERENCES

Brandt, A., & G. C. B. Poore. 2003. Higher classification of the flabelliferan and related Isopoda based on a reappraisal of relationships. Invertebrate Systematics 17 (6): 893-923.

Kensley, B. 1982. Deep-water Atlantic Anthuridea (Crustacea: Isopoda). Smithsonian Contributions to Zoology 346: 1-60.

Poore, G. C. B. 2009. Leipanthura casuarina, new genus and species of anthurid isopod from Australian coral reefs without a “five-petalled” tail (Isopoda, Cymothoida, Anthuroidea). ZooKeys 18: 171–180.

Wilson, G. D. F. 2009. The phylogenetic position of the Isopoda in the Peracarida (Crustacea: Malacostraca). Arthropod Systematics and Phylogeny 67 (2): 159-198.

Crabs That Cannot Scratch Their Heads (Taxon of the Week: Parthenopidae)

An elbow crab amongst seaweed, showing both its long reach and well-developed camouflage. Photo from Wild Shores of Singapore.

Lift up one arm, and bend your elbow. Reach with your fingers to a point on your back, between your shoulder-blades. Scratch. Not only will that work wonders for any annoying tingle that you might have been feeling, but you have just demonstrated your superior flexibility to an elbow crab.

Crabs of the family Parthenopidae are found in tropical and subtropical coral reefs and shelly sea bottoms. Most species have bodies that are roughly triangular in shape, and often highly ornamented with lumps, bumps and spines (this ornamentation makes them very difficult to see among coral and rocks; it also encourages the growth of algae and other camouflaging organisms on the crab). They also usually have very large and long chelipeds (pincers), which make it easy to see how they got the name of 'elbow crabs'. The merus (the 'upper arm' part of the cheliped) is proportionally much longer than in many other crab families, giving parthenopids a real gorilla-ish look (I found one website that labelled a parthenopid of the genus Daldorfia as the "King Kong crab"). Despite their extraordinary size and length, however, the range of mobility of an elbow crab's chelipeds is limited, hence the point about back-scratching above. An elbow crab cannot reach the middle part of the top of its carapace.


Furtipodia petrosa, a rather adorable-looking parthenopid from Guam that resembles a sponge-covered rock. Furtipodia is also one of a number of parthenopids in which the walking legs are hidden by the carapace, improving the disguise. Photo from here.

This lack of cheliped mobility is one of the features distinguishing members of the Parthenopidae from the spider crabs of the Majidae, which have a broadly similar superficial appearance (Ng & McLay, 2003). Other distinct features of the family include the fusion of the third to fifth segments of the male abdomen* (Tan & Ng, 2007); also, while female majids have a high-domed abdomen that forms an entirely enclosed brood chamber for her eggs, the parthenopid female's abdomen does not entirely seal the eggs away from the outside world. The similar adult appearance of Parthenopidae and Majidae, with their triangular bodies and pointed snouts, lead most early authors to regard them as closely related, but the similarities are now thought to be convergent. The larvae of parthenopids are more similar to those of other families than majids (Yang, 1971), while phylogenetic studies do not support their association (Brösing, 2008).


Normal parthenopids are remarkable enough, but Lambrachaeus ramifer looks like something out of a Japanese video game (making it appropriate that I found this photo on a Japanese website). This individual is a female carrying eggs - they're the orange mass on her underside.

The subfamilial classification of Parthenopidae was reviewed by Tan & Ng (2007) who recognised only two subfamilies of elbow crabs, the Parthenopinae and Daldorfiinae (earlier authors recognised more - some have been moved to other families, others have been synonymised). The two subfamilies are distinguished by only a single character, the relative length of the antennal segments, and a more formal analysis is still required to test their distinction. A separate subfamily had previously been recognised for the very distinctive Indo-Pacific species Lambrachaeus ramifer which has the front of the carapace extended forward into a long neck (Ng & McLay, 2003), but Tan & Ng (2007) placed this species in Parthenopinae, noting that it had been separated on the basis of its own peculiar autapomorphies rather than by lack of the features of other subfamilies.

*If you don't know where to find the abdomen of a crab, then look at the underside of one the next time you're able to. The much reduced abdomen is turned forwards and held on the underside of the cephalothorax. In males, it is a small, narrow segmented strip. In females, it is much larger and broader, and is used to hold her eggs.

REFERENCES

Brösing, A. 2008. A reconstruction of an evolutionary scenario for the Brachyura (Crustacea) in the context of the Cretaceous-Tertiary boundary. Crustaceana 81 (3): 271-287.

Ng, P. K. L., & C. L. McLay. 2003. On the systematic position of Lambrachaeus Alcock, 1895 (Brachyura, Parthenopidae). Crustaceana 76 (8): 897-915.

Tan, S. H., & P. K. L. Ng. 2007. Descriptions of new genera from the subfamily Parthenopinae (Crustacea: Decapoda: Brachyura: Parthenopidae). Raffles Bulletin of Zoology Supplement 16: 95-119.

Yang, W. T. 1971. The larval and postlarval development of Parthenope serrata reared in the laboratory and the systematic position of the Parthenopinae (Crustacea, Brachyura). Biological Bulletin 140: 166-189.

Snail Mimics and Marine Symbionts (Taxon of the Week: Pleustidae)

The pleustid amphipod Incisocalliope aestuarius. Photo by Marco Faasse.

The Pleustidae are a family of marine amphipods, distributed around the world. However, despite their seemingly cosmopolitan distribution and abundance, it has been surprisingly difficult to find information about this family online. Pleustids are one of the many families in the largest of the amphipod suborders, the Gammaridea (gammarideans include the sandhoppers and other sandhopper-looking crustaceans that are what most people think of when they think of an amphipod). Gammaridean interrelationships are a great tangled mess, and in many places fall into a state that we scientists technically refer to as "buggered beyond belief". Only slowly are researchers beginning to draw some sense out of things, and they'll probably be at it for a long time yet. For those who wish to track them down (for the most part, I haven't seen them), the main revisions of Pleustidae were published by Bousfield & Hendrycks (1994 and following) in Amphipacifica*. Stock (1986) (who very tentatively assigns three oddball stygobiotic species from Japan to the Pleustidae) gives the defining characters of the family as "the rudimentary condition of the accessory flagellum of the first antenna, the biramous third uropod (rami lanceolate), the elongate telson, the weak and more or less similar gnathopods 1 and 2, and... the bilobed condition of the labrum". Bousfield & Hendrycks (1994, 1995) divided the family into a number of subfamilies.

*The short-lived journal Amphipacifica ended up with three volumes published between 1994 and 2001 (I haven't found any indications that a planned fourth volume ever made an appearance). While I haven't found a complete contents listing, it appears that most (if not all) articles had the chief editor, Edward Bousfield, as author or co-author, and the majority were on amphipod taxonomy. Perhaps unjustly, the journal is not remembered for its contributions to crustacean systematics as much as it is for Bousfield's perhaps unwise foray into vertebrate taxonomy. Yes, this was the journal that saw the publication of the infamous Bousfield & LeBlond (1995), and the name Cadborosaurus wellsi - a paper so controversial that two of the journal's editors promptly handed in their resignations in protest. For more details, see Darren Naish's 2006 review.

Not having the resources on hand to give you a a decent overview of the family (sorry), I'm just going to give you a couple of the highlights that I have been able to locate - (as it says in the title) the snail mimics and the symbiotic taxa.


Pleustes panopla, a close relative of one of the snail-mimicing pleustids. Photo via here.

Snail mimicry has been recorded for two pleustids, Pleustes platypa and unidentified species of Stenopleustes (Field, 1974). Pleustes platypa lives in kelp beds and mimics the marine gastropod Mitrella carinata. Mimetic Stenopleustes (it is not known how many species are involved) live in beds of Zostera (seagrass) and mimic various species of snails of the genus Lacuna. Different Stenopleustes individuals may have different colour patterns, each matching a different Lacuna species. The amphipod clings tightly to the seagrass, moving slowly to match the speed of a snail. Occassional rocking back and forth mimics the rocking movement of the snails. And while amphipods are perfectly adept swimmers, snail-mimicing Stenopleustes would only swim under extreme provocation, preferring to crawl to the other side of the seagrass blade instead when possible. Obviously, starting to swim would quickly give away that the animal was not a snail!

Other intriguing pleustids are the species that live as symbionts (generally commensals) of larger marine invertebrates. Commensipleustes commensalis is a symbiont of crabs, with enlarged spines on the underside of the forelimbs against which the dactylus (claw) can be folded back, allowing the amphipod to hang onto the host's setae. Members of the genus Dactylopleustes, on the other hand, live on sea urchins, and their legs have notched claws that can be placed around the host's spines. Another species, Mesopleustes abyssorum, clings to the legs of sea spiders. But most remarkable of all is the lifestyle that has been inferred for the species Myzotarsa anaxiphilius (Cadien & Martin, 1999). Like Commensipleustes, Myzotarsa is a symbiont of crabs (in this case, king crabs of the genus Paralithodes), but while most crab-symbiotic amphipods live around the densely setose mouthparts, Myzotarsa lives underneath the crab's recurved abdomen. Without setae to cling on to, the claws on the walking legs of Myzotarsa bear special suckers to allow the animal to latch on. What makes Myzotarsa really remarkable is that not just any crab will make a suitable host - instead, the amphipod shows a strong preference for crabs that are parasitised by rhizocephalans (out of 179 specimens of Myzotarsa referred to by Cadien & Martin, 1999, 167 came from parasitised hosts while only six came from a non-parasitised host [the remainder came from hosts whose parasite status was not recorded). It seems that the diet of the little Myzotarsa is the eggs being incubated underneath the abdomen. While healthy crabs will only be carrying eggs if they're female and if it's the right season, the chemically-castrated, feminised (if male) and mind-controlled infected crabs will be carrying externa filled with yummy rhizocephalan eggs all year round...

REFERENCES

Bousfield, E. L., & E. A. Hendrycks. 1994. A revision of the family Pleustidae (Crustacea: Amphipoda: Leucothoidea). Part I. Systematics and biogeography of component subfamilies. Amphipacifica 1: 17-57.

Bousfield, E. L., & E. A. Hendrycks. 1995. The amphipod family Pleustidae on the Pacific coast of North America: Part II. Subfamilies Parapleustinae, Dactylopleustinae, and Pleusirinae. Systematics and distributional ecology. Amphipacifica 2: 65-133.

Bousfield, E. L., & P. H. LeBlond. 1995. An account of Cadborosaurus willsi, new genus, new species, a large aquatic reptile from the Pacific coast of North America. Amphipacifica 1 (Supplement 1): 3-25.

Cadien, D. B., & J. W. Martin. 1999. Myzotarsa anaxiphilius, new genus, new species, an atylopsine amphipod (Gammaridea: Pleustidae) commensal with lithodid crabs in California. Journal of Crustacean Biology 19 (3): 593-611.

Field, L. H. 1974. A description and experimental analysis of Batesian mimicry between a marine gastropod and an amphipod. Pacific Science 28 (4): 439-447.

Stock, J. H. 1986. Amphipoda: Pleustidae. In Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) (L. Botosaneanu, ed.) pp. 560-561. E. J. Brill / Dr. W. Backhuys: Leiden.

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.

Kneel before the Shrimp Queen

A pair of commensal snapping shrimps Synalpheus on their host crinoid. Photo from here.

As a child, snapping shrimp were one of my favourite things to find under rocks at the beach. The characteristic bang made by their enlarged pincer snapping shut never fails to fascinate. This 'snap' can often be heard for some distance, and the explosive force generated by it can be strong enough to stun small animals that get too close. Snapping shrimp form the family Alpheidae, and Synalpheus, with well over a hundred described species and counting, is one of the larger genera in that family.

Synapheus has a pantropically-centred distribution. Though it seems to be more abundant in the east Pacific and Atlantic Oceans than in the Indian, I'd be a little suspicious of the role collection bias has played in this. The various species of Synalpheus are retiring animals by nature, and sequester themselves in cryptic habitats, all the better to defend themselves against would-be intruders with that impressive claw. The best-known species of Synalpheus live within the body cavity of other animals such as sponges or corals, and a few species live on the underside of crinoids (VandenSpiegel et al., 1998). It is debatable to what extent the relationship between Synalpheus and their host should be regarded as commensal (with the shrimp feeding on food particles brought in by the host) or parasitic (with the shrimp feeding directly on the host tissue), as evidence exists for Synalpheus individuals doing both. Dardeau (1984) suggested that Synalpheus species could be divided into three broad levels of host association, from group I (generally free-living or opportunistically commensal species with very low or no host specificity) to group III (invariably commensal species with high host specificity). Many commensal-living individuals will do so as male-female pairs, aggressively excluding any conspecific competitors that attempt to settle in their home. Other species may be more tolerant, with numerous individuals in a single host.


Colonial Synalpheus on a sponge. Photo from Biology-Blog - this would appear to be a laboratory colony, with dabs of identifying paint on the individuals.

The most remarkable association of all, though, is found in certain species of what is called the Gambarelloides species group (after the species Synalpheus gambarelloides). The Gambarelloides group is a morphologically distinct association of species (most notably, they have a dense brush of setae on the smaller pincer) that was separated by Ríos & Duffy (2007) from the remainder of Synalpheus as their new genus Zuzalpheus. This separation was debated by Anker & De Grave (2008), but the complaint does not seem to concern the integrity of 'Zuzalpheus' itself, but that of the remainder of Synalpheus if the Gambarelloides group species are not included. Some of the group III sponge-dwelling species in this group (using Dardeau's grouping) form large colonies with hundreds of individuals in a single sponge. It was only recognised as recently as 1996 (Duffy et al., 2000, 2002) that these Synalpheus colonies actually qualify as eusocial, in the manner of bees and ants, representing the only known occurrence of eusociality outside insects other than mole rats. Reproduction within the colony is conducted by a single queen, though it remains unknown how the queen of a colony is established, and how she prevents other members of the colony becoming reproductive. The sexual ratio of the remainder of the colony remains unknown, as males are indistinguishable from non-egg-bearing females (and gender may be environmentally-determined rather than genetic), but the colony does include a number of larger individuals (called "males" by Duffy et al., 2002) that seem to be primarily responsible for the colony's defense, moving about the sponge more than the smaller juveniles seemingly on the lookout for intruders. The queen plays little part in defending the colony, and in one eusocial species, Synalpheus filidigitus, she lacks the large snapping pincer of the other individuals (Duffy et al., 2002). It is not yet established how fertilisation of the queen occurs, but allozyme analysis suggests that there may be only a single reproductive male in the colony (Duffy et al., 2000).

Phylogenetic analysis of the Gambarelloides group by Duffy et al. (2000) found that eusociality has evolved at least three times within the group. They suggested that it may have evolved as a response to severe competition for habitat. Where eusocial shrimps are found, almost all suitable hosts are home to a colony, so unoccupied homes are few and far between (offhand, how new colonies do become established is yet another unknown factor - eusocial Synalpheus lack a planktonic larval stage, so hatching offspring remain in the parent colony). A colonial group may be more effective at defending their host against would-be usurpers than a solitary individual or pair would be. With the large "soldiers" defending her, the queen is able to spend more time feeding and reproducing, safely hidden within the sponge.

REFERENCES

Anker, A., & S. De Grave. 2008. Zuzalpheus Ríos and Duffy, 2007: a junior synonym of Synalpheus Bate, 1888 (Decapoda: Alpheidae). Journal of Crustacean Biology 28 (4): 735-740.

Dardeau, M. R. 1984. Synalpheus shrimps (Crustacea: Decapoda: Alpheidae). I. The Gambarelloides group, with a description of a new species. Memoirs of the Hourglass Cruises 7 (2): 1-125.

Duffy, J. E., C. L. Morrison & K. S. Macdonald. 2002. Colony defense and behavioral differentiation in the eusocial shrimp Synalpheus regalis. Behav. Ecol. Sociobiol. 51: 488-495.

Duffy, J. E., C. L. Morrison & R. Ríos. 2000. Multiple origins of eusociality among sponge-dwelling shrimps (Synalpheus). Evolution 54 (2): 503-516.

Ríos, R., & J. E. Duffy. 2007. A review of the sponge-dwelling snapping shrimp from Carrie Bow Cay, Belize, with description of Zuzalpheus, new genus, and six new species (Crustacea: Decapoda: Alpheidae). Zootaxa 1602: 1-89.

VandenSpiegel, D., I. Eeckhaut & M. Jangoux. 1998. Host selection by Synalpheus stimpsoni (De Man), an ectosymbiotic shrimp of comatulid crinoids, inferred by a field survey and laboratory experiments. Journal of Experimental Marine Biology and Ecology 225 (2): 185-196.

Getting Crabs

The purple shore crab Leptograpsus variegatus of the southern subtropical Indo-Pacific ocean. Photo by Benjamint444.

When I was but an ickle lad, and my family would camp over Christmas at the beach by the estuary beneath the house of my great-grandparents, I would spend many hours turning over rocks and catching the crabs that I found underneath them. The most common variety I would find was the tiny grey-brown mud crab (Helice crassa), which could be handled easily, but if I managed to turn over one of the really big rocks then I would be able to find the larger purple shore crabs (Leptograpsus variegatus), which required a more careful approach lest they inflict great pain. One thing I didn't know at the time about either animal, however, was that they were both members of the superfamily Grapsoidea.

Grapsoidea is a grouping of crabs including at least seven families. The classification of Grapsoidea is currently undergoing something of a revision, and has shifted about a little in recent years. While most grapsoids were once included in the single family Grapsidae, the recognition of the latter as paraphyletic to the Gecarcinidae has lead to the elevation of the various prior subfamilies of Grapsidae to separate families. The family Glyptograpsidae was only established in 2002 (Schubart et al., 2002), while the genus Xenograpsus was moved into its own family within the past year (Ng et al., 2007). Other families in the group are Sesarmidae, Varunidae and Plagusiidae. The majority of grapsoids are found on the shoreline, but some (such as the Chinese mitten crab Eriocheir sinensis) move into fresh water. At least one genus, Planes (Grapsidae), is pelagic, while Xenograpsus has been found to depths of 270 m (McLay, 2007). Xenograpsus is found in association with hydrothermal vents, and populations of X. testudinatus living on sulphur vents near Taiwan make their living by feeding on the rain of dead zooplankton killed by toxic discharges from the vents (Ng et al., 2007).


Gecarcoidea natalis, Christmas Island red crab migration. Photo from here.

Some members of the Gecarcinidae live their adult lives terrestrially as adults on tropical islands. Nevertheless, all grapsoids (as far as I can tell) retain the ancestral state of marine planktonic larvae, so all terrestrial gecarcinids must return to the coast to spawn. The Christmas Island red crab, Gecarcoidea natalis has become renowned for the vast numbers that can be seen in its mass migrations, as the entire island's population of crabs (more than 40 million when estimated in 1995 - Adamczewska & Morris, 2001) moves down to the coast over the course of a week or so. Tragically, recent years have seen a population explosion on Christmas Island of the introduced yellow crazy ant* (Anoplolepis gracilipes), which was estimated to have killed off some 15 million-plus crabs by 2003 (O'Dowd et al., 2003), and has essentially eliminated crab populations wherever it has established colonies. Foraging crabs are attacked in large numbers by crazy ants defending their nests, and poisoned with large amounts of formic acid. Crazy ants will also occupy crab burrows, removing their former inhabitants with extreme prejudice. Not only are resident crabs killed, but crabs migrating from elsewhere have been destroyed as they crossed crazy ant-infested locations on their way to the coast. Where red crabs have been eliminated, the forest vegetation structure has begun to change significantly, as seedlings that would have once been grazed by crabs are able to establish a dense undergrowth.

*So called because of the seemingly random way in which they wander about when foraging.


Xenograpsus testudinatus at the base of a sulphur vent. Photo from here.

The Grapsoidea are closely related to another shore-crab family, the Ocypodoidea, and apparently species included in these two superfamilies were once united (back in the 1800s) under the taxon name Catometopa (Schubart et al., 2006), a name that I think deserves resurrection (just try saying it a couple of times - "Catometopa!"). While it seems to be universally accepted that these two superfamilies form a clade, the molecular phylogenetic analysis of Schubart et al. (2006) indicated that each of the "superfamilies" was polyphyletic within this clade, and recommended that they not be recognised as distinct. So far, I haven't been able to find what are the characters that are supposed to separate the two groups. Davie & Ng (2007) stated that morphological data maintained the monophyly of Grapsoidea, but omitted to cite any details in support of this statement.

REFERENCES

Adamczewska, A. M., & S. Morris. 2001. Ecology and behavior of Gecarcoidea natalis, the Christmas Island red crab, during the annual breeding migration. Biological Bulletin 200: 305-320.

Davie, P. J. F., & N. K. Ng. 2007. Two new subfamilies of Varunidae (Crustacea: Brachyura), with description of two new genera. Raffles Bulletin of Zoology Supplement 16: 257-272.

McLay, C. 2007. New crabs from hydrothermal vents of the Kermadec Ridge submarine volcanoes, New Zealand: Gandalfus gen. nov. (Bythograeidae) and Xenograpsus (Varunidae) (Decapoda: Brachyura). Zootaxa 1524: 1-22.

Ng, N. K., P. J. F. Davie, C. D. Schubart & P. K. L. Ng. 2007. Xenograpsidae, a new family of grapsoid crabs (Crustacea: Brachyura) associated with shallow water hydrothermal vents. Raffles Bulletin of Zoology Supplement 16: 233-256.

O'Dowd, D. J., P. T. Green & P. S. Lake. 2003. Invasional 'meltdown' on an oceanic island. Ecology Letters 6 (9): 812-817.

Schubart, C. D., S. Cannicci, M. Vannini & S. Fratini. 2006. Molecular phylogeny of grapsoid crabs (Decapoda, Brachyura) and allies based on two mitochondrial genes and a proposal for refraining from current superfamily classification. Journal of Zoological Systematics and Evolutionary Research 44 (3): 193-199.

Schubart, C. D., J. A. Cuesta & D. L. Felder. 2002. Glyptograpsidae, a new brachyuran family from Central America: larval and adult morphology, and a molecular phylogeny of the Grapsoidea. Journal of Crustacean Biology 22(1): 28-44.

Southern Crustacean Relicts

Eophreatoicus from Kakadu in the Northern Territory of Australia. Image from here.

This week's highlight taxon is the Phreatoicidea, a suborder of isopods restricted to freshwater habitats in ex-Gondwanan continents. This is not a particularly large group - only about a hundred species have been described, though it is estimated that at least that number again remain undescribed. A reasonably high proportion of the species are known from subterranean habitats*, including the first species to be described, Phreatoicus typicus. Knott (1986) listed eleven subterranean species, which at the time was about a quarter of the total known diversity (over half the known species have been described since then). The diversity of the suborder is also heavily centred in Australasia - 94 species have been described from there, in contrast to four species from South Africa and only two from India (Wilson, 2008), but again this is probably heavily biased by the fact that almost all taxonomic work on this group has been conducted in Australia. For instance, Knott (1986) refers to possible undescribed species from India - these species seemingly still have not appeared in print twelve years later. The supposed Gondwanan distribution of phreatoicids also makes their apparent absence from South America very interesting, but how confident can we be that they are truly absent from that continent?

*I should make it clear that "subterranean" does not necessarily mean "cave-dwelling". Caves actually only make up a small proportion of the subterranean habitat, and only one cave-dwelling phreatoicid species is known (Knott, 1986). The majority of subterreanean species are sediment-dwelling forms whose habitats can extend right down into groundwater aquifers. Phreatoicus typicus, for instance, was originally described from a well near Christchurch in New Zealand, into which it would have emerged from the surrounding bedrock.

Most people imagine isopods as dorsoventrally flattened animals, like their most familiar representatives the woodlice and slaters. Phreatoicids, however, represent an exception to this rule, being fairly high-vaulted, narrow animals. Stygobiotic forms tend to be more elongated. Phylogenetically, phreatoicids are one of the most basal groups of isopods, and have one of the earliest fossil records. The Palaeozoic phreatoicids (or, technically, stem-phreatoicids) Palaeophreatoicidae are known from marine sediments, but since the Triassic all known representatives have been freshwater. Phreatoicids are detritivores feeding primarily on decaying vegetation or on the micro-organisms associated with the former, but may occassionally be carnivorous (Wilson, 2008). Phreatoicids are a significant part of the pholeteros - the specific faunal assemblage of organisms associated with the burrows of larger animals such as freshwater crayfish.


Pilbarophreatoicus platyarthricus, a potentially subterranean form from the Pilbara in Western Australia. Pilbarophreatoicus was described from an intermittent stream (i.e. one that dries up outside the rainy season), but shows features usually associated with subterranean habitats such as blindness and elongated body form. Like many subterranean phreatoicids in arid regions, it probably emerges from the ground when standing water is available and retreats back into the groundwater during the dry season. Figure from Knott & Halse (1999). shale bar = 1 mm.

Taxonomically, the Phreatoicidea have been a difficult group. For many years the classificatory sytem used was that established by G. E. Nicholls in the early 1940s, which divided phreatoicids between two families, each divided into a number of subfamilies. Unfortunately, the features used to separate these taxa have been shown to be largely artificial, and a high degree of variation can occur between closely-related species or even within examples of the one species. Wilson & Keable (2002)revised the classification somewhat through phylogenetic analysis, recognising three families and abandoning Nicholls' subfamilies. The suborder as a whole seems to be characterised by fairly slow morphological evolution. Gouws et al. (2004) showed that at least one supposed species, the South African Mesamphisopus capensis, is divisible on genetic and morphometric grounds into a number of potential cryptic species.

Like many freshwater and subterranean organisms, many phreatoicids have very restricted distributions and are placed at significant risk of human activities. Genetic studies show that each separate aquifer may have its own isolated population (Wilson, 2008). Indeed, a number of species are believed to have already gone extinct, due to factors such as increasing groundwater salinity as a result of deforestation (Knott, 1986) or alteration and exhaustion of water supplies and aquifers (Wilson, 2008). Unfortunately, the lack of taxonomic resolution within the group, as well as the difficulty of surveying the habitats of subterranean species in particular, make it very difficult to assess the risk to individual species.

REFERENCES

Gouws, G., B. A. Stewart & S. R. Daniels. 2004. Cryptic species within the freshwater isopod Mesamphisopus capensis (Phreatoicidea: Amphisopodidae) in the Western Cape, South Africa: allozyme and 12S rRNA sequence data and morphometric evidence. Biological Journal of the Linnean Society 81: 235-253.

Knott, B. 1986. Isopoda: Phreatoicidea. In Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) (L. Botosaneanu, ed.) pp. 486-492. E. J. Brill / Dr. W. Backhuys: Leiden.

Knott, B., & S. A. Halse. 1999. Pilbarophreatoicus platyarthricus n.gen., n.sp. (Isopoda: Phreatoicidea: Amphisopodidae) from the Pilbara Region of Western Australia. Records of the Australian Museum 51: 33-42.

Wilson, G. D. F. 2008. Global diversity of Isopod crustaceans (Crustacea; Isopoda) in freshwater. Hydrobiologia 595: 231–240.

Wilson, G. D. F., & S. J. Keable. 2002. New Phreatoicidea (Crustacea: Isopoda) from Grampians National Park, with revisions of Synamphisopus and Phreatoicopsis. Memoirs of the Museum of Victoria 59 (2): 457-529.