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.