Just What is a Frog Crab?

Here's a strange aquatic animal that you've probably never heard of: the frog crab. No, it's not some horrific amphibian-crustacean hybrid cooked up by mad scientists. Instead, it's just one very odd looking crab, characterized by an unusually flat and frog-like body. Now researchers are investigating the evolution of these strange and poorly understood creatures.

The frog crab supposedly earned its deceptive name from the way in which it holds itself. Ranina ranina (pictured above) happens to be one of the best-known species of these crustaceans, and sometimes assumes angry frog-like poses. However, many have argued that the stunningly flat bodies of these animals make them look more like giant ticks than anything else. Still, with a prickly exoskeleton, eye stalks, 10 legs, and iconic pincers, they are no doubt crabs.

So what sets frog crabs apart from most of their other crustacean cousins? For one, most crabs boast bodies that are as wide as they are long and scuttle sideways to get around. Frog crabs, however, lie flat on the ocean floor, pushing their unusually disk-like bodies forward and back through loose sediment.

This keeps them well hidden from predators and prey, but also makes them difficult to find and observe for researchers.

However, Javier Luque, of the University of Alberta in Canada, knows enough about these creatures from fossil records dating to about 125 million years ago to assert that the strange crabs were once practically everywhere, found from Greenland to the Antarctic. Now, however, they are only common in tropical parts of the world.

So what happened? In a study recently published in the Journal of Systematic Palaeontology, the researcher suggests that ocean floor conditions changed, where the burrowing lifestyle of these animals eventually proved less efficient compared to the more active scavenging of upright crabs.

This, he writes, backs the theory that the "crab" in "frog crab" came first, where its unusual frog-like qualities reflect "a derived condition related to a specialized burrowing lifestyle." When this lifestyle was no longer preferable, the frog crab quietly retreated to more specialized habitats, while their scuttling ancestors exploded into the many crab species we see today.

In an interview with New Scientist, the expert added that low oxygen levels in water could also have had something to do with it, as burrowers often evolve for such conditions.

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