Nature created crabs that aren’t actually crabs
It’s all just a beautiful example of convergent evolution known as carcinization…
By Mark Pan
You have probably seen a crab before. They are hardy arthropods with a thick exoskeleton, imposing pinsers, and inhabit a variety of environments: from the subzero temperatures of the Arctic Ocean to the Ankarana limestone formations in northern Madagascar. They range from the massive Tanner Crabs prowling the frigid waters of Bristol Bay; to the tiny Boxer Crab wielding minuscule anemones like pom-poms. But aside from the dazzling variety, the standard ‘crabby’ anatomy has evolved independently several times, and carcinization is the reason.
Crabs are decapods from the order Decapoda, which includes lobsters, prawns, crayfish and related crustaceans. A ‘true crab’ (e.g. the Dungeness Crab) has the quintessential boxy appearance. There are several other members of Decapoda which are routinely confused with true crabs, but are taxonomically different: porcelain crabs, king crabs, and the Hairy Stone Crab. All of them possess crab-like attributes, including pincers, spindly appendages, and a compact central body; and several, most notably porcelain crabs, may seem indistinguishable a first glance. However, all these groups have evolved separately; and in an interesting case of convergent evolution they have all developed characteristics which are uncannily similar.
The benefits of a crab-like appearance
This example of convergent evolution is known as carcinization. First christened by the English zoologist Lancelot Alexander Borradaile in 1916, he described it as “one of the many attempts of Nature to evolve a crab.” Simply put, carcinization describes the evolutionary process in which non-crab forms develop crab-like attributes. While certainly true, there are several rationales as to why the crab configuration is such a successful fad.
Take a lobster, for instance. There are several crucial elements of the body plan of the lobster: there is the cephalothorax (the combined section of the head and the thorax); the pincer limbs; the abdomen; the tail fin; and the main walking legs and swimmerets. Whilst the cephalothorax area is protected with thick shell and the pincers, the abdomen is relatively vulnerable and prone to attack. Since the abdomen is composed of smaller individual segments that enable flexibility and movement, the exoskeleton is segmented and therefore cannot provide a continuous shell of protection. Therefore, it would be an advantage to tuck the abdomen underneath the cephalothorax: between the legs, and thus giving the animal a defensive advantage.
Porcelain crabs
Porcelain crabs are regularly mistaken for true crabs: defining characteristics that differ them from true crabs include 8 legs (6 walking legs), and a pair of antennae. They are, in fact, related to squat lobsters, which are excellent examples of a partially tucked abdomen. Unlike traditional lobsters which have a trailing pleon (abdomen) with a tail fan, squat lobsters have pleonal segments which decrease in length after each succession, and the pleon curls inwards underneath the thorax.
King crabs
King crabs are descendants of hermit crabs: such evolution is suggested to the be the most renowned example of carcinization. The rapid loss of a shell apparatus in exchange for a hardy exoskeleton is not unique to the king crabs, though it is the most dramatic. King crabs, like porcelain crabs, only have 6 walking legs, differing form the 8 walking legs true crabs possess. A study (Cunningham et al, 1992) suggests that the complete carcinization of king crabs has taken more than 13 million years.
Hairy Stone Crab
A contentious topic which remains between researchers is the taxonomic classification of the Hairy Stone Crab: whether it is a distant relative of king crabs or squat lobsters. Recent studies have noted that it may be related to the family Aeglidae (freshwater crustaceans that resemble squat lobsters). Therefore, this is proven to be true, it can be assumed that Hairy Stone Crab’s evolutionary path is similar to that of porcelain crabs.
Conclusion
Although aquatic arthropods are seemingly enthusiastic over evolving into crabs, the same cannot be said for their terrestrial counterparts. It seems that the craze for crabs is, at least for now, not too relevant for land-dwelling organisms. Although several terrestrial arthropods, like the arachnids, may seem morphologically similar at first glance, they have taken very different evolutionary paths. Thus, it seems carcinization is exclusive to these multi-legged, pincer-wielding decapods.
How much significance does carcinization have on your life? Probably not very much. However, armed with this knowledge, the next time you find yourself at an awkward gathering, you can dazzle your friends with the fascinating case of carcinization and nature’s fondness for crabs.