You might think of crabs mainly as food, but this group is also highly instructive in terms of convergence…

Along with the lobsters, the crabs represent probably the most familiar examples of crustaceans. There are almost 7000 species of true crabs (infraorder Brachyura), most of which inhabit the marine realm, but about 850 species live in freshwater or are terrestrial or semi-terrestrial. And they provide a rich source of insights into convergence…

Crab morphology

Crabs are characterised by a reduced abdomen that is tucked under the calcified carapace. This is evidently a first-class design, because it has evolved at least five times independently amongst the decapod crustaceans, once in the Brachyura and four times in the Anomura (the group that includes the hermit crabs). As well as providing a striking example of convergent evolution (and we should note in passing that the extinct and somewhat enigmatic arthropods known as the Cycloidea also show a number of convergences with the crabs), this repeated emergence of a rather distinctive morphology raises some particularly intriguing evolutionary questions. The crab-like body form appears to be an irreversible choice – crabs will always be with us. And as this particular morphology has evolved independently in a remarkable range of environments, it isn’t just a good fix in one context. Interestingly, some authors have used the word “innate” when speaking of crab evolution (Morrison et al. 2002, Proceedings of the Royal Society of London B, vol. 269, p. 345), implying that is there is a particular predisposition to this solution.

Asymmetric feeding

A rather more unusual convergence involves some crabs that have specialised on a diet of snails. Snails come in two different forms – the shell typically spirals in a clockwise direction (dextral snails), but there are variants where the shell is coiled the other way round (sinistral snails). And some predators have evolved structures that are specifically designed to deal with the more common dextral snails – they have essentially become right-handed. This asymmetry has evolved convergently in at least two groups of crabs, a Cretaceous crab known as Megaxantho and the extant box crabs (Calappa). Here, one of the claws is enlarged and operates with a scissor-like action that facilitates peeling open the snail. Some pareatine and dipsadine snakes as well as the larvae of the water beetle Hydrophilus acuminatus have independently come up with asymmetrical mandibles that enhance feeding effectiveness on dextral snails.

Terrestrialisation and lungs

Although primarily marine, crabs have repeatedly moved onto land, mainly in tropical and subtropical regions (and, curiously, the traps employed by human hunters to capture the land crabs of the Caribbean and the Philippines both employ a very similar bamboo technology…). Members of the genera Uca (the fiddler crabs), Sesarma and Armases, for example, have all invaded brackish egg-laying habitats independently. Such terrestrialisation, which is generally highly convergent among animals, obviously required a number of physiological, morphological and behavioural adaptations, for instance the evolution of air-breathing structures. All air-breathing crabs have retained their gills (which seem to be important mainly for ion regulation, but remain involved in gas exchange in at least some species), but have additionally developed simple vascularised lungs that show certain parallels to the lungs of pulmonate molluscs.

Trinidad mountain crabs (Pseudothelphusa garmani) represent a remarkable exception. Their paired invaginated lungs are of high structural complexity and ventilated by a flow-through mechanism, rendering them extremely efficient and unique among invertebrates. Thus, they are more analogous to the lungs of higher vertebrates, particularly birds, than to the lungs of other land crabs and have been described as “a quantum leap in the evolution of air-breathing in invertebrates” (Innes et al. 1987, Comparative Biochemistry and Physiology, vol. 87A, p. 7).

And it is amongst the terrestrial crabs that we find other intriguing cases of convergence…

Breeding in “ponds” and parental care

The grapsid land crab Metopaulias depressus lives in a rainforest-covered karst area in western Jamaica, an environment of extreme hostility. There are no permanent water resources in this region, apart from the small rain-filled “ponds” that nestle within epiphytic bromeliads. And this is where these crabs spend their entire life. Even more remarkably, the females provide elaborate care for their young, which significantly enhances offspring survival and growth. They prepare a leaf axil as a nursery, regulating the pH of the water by accumulating snail shells and controlling oxygen and carbon dioxide levels, probably by removing organic debris. The developing brood is then not only guarded from predators, such as damselfly nymphs and large spiders, but also fed by the female.

However, it seems that Metopaulias is not the only crab showing this kind of behaviour. In the East Usambara region of Tanzania, a potamoid freshwater crab was found to inhabit water-filled tree holes. And on the opposite side of the continent, a similar ecological niche is occupied by the Liberian tree-hole crab Globonautes macropus. Both species clear their tree holes from organic debris, and East Usambara tree-hole crabs were observed to introduce snail shells into their holes. There are further intriguing parallels with some bromeliad-breeding frogs, such as the strawberry poison frog (Dendrobatus pomilio), which shows a complex brood-feeding behaviour.

Arboreal lifestyle

Apart from providing sources of freshwater in hostile environments, a life on trees might offer other advantages to a crab, such as escape from low tide predators, shelter or food. In the families Grapsidae and Sesarmidae, several species climb mangrove trees but adapt this arboreal lifestyle to different degrees. Some have become completely arboreal, thriving in the canopies of trees and consuming fresh leaves. Among these are three sesarmids from different parts of the world, the American species Aratus pisonii, the African crab Armases elegans and Parasesarma leptosoma from the Indo-Pacific region, as well as a grapsid, Metopograpsus latifrons. These tree climbers share at least two morphological characters – specially adapted walking legs and a flattened triangular carapace with an invaginated sternum. Molecular analyses have suggested that ecological and morphological adaptations to an arboreal life have evolved independently at least three times in the Sesarmidae and once in the Grapsidae.

In addition, there are important physiological changes that enable these crabs to feed on leaves, thereby recalling the problem faced by many groups of herbivores that have to grapple with cellulose and a low nitrogen diet. To date, however, there is no evidence for fermentation activities in the gut that have evolved multiple times in other animals.

Olfaction

The omnivorous coconut or robber crabs (Birgus latro), which belong to the hermit crabs (Paguroidea), are the world’s largest terrestrial arthropods. As they have a marine larval stage, these crustaceans present an opportunity to study the changes to sensory systems occurring at the transition from water to land. The olfactory sense is of particular interest, because these mainly nocturnal omnivores have an excellent sense of smell that allows them to detect potential food sources such as fruits or rotting meat from a distance. Intriguingly, the olfactory system of robber crabs is clearly different from that of marine crustaceans but remarkably analogous to that of insects in terms of physiology, morphology and behaviour. Thus, as Marcus Stensmyr and colleagues note, “the insect nose of the robber crab is a striking example of convergent evolution and nicely illustrates how similar selection pressures result in similar adaptation” (Stensmyr et al. 2005, Current Biology, vol. 15, p. 116).