Convergent evolution is a process whereby over time, similar features evolve in different and unrelated species. It’s been observed in many species of animals and plants − for example, there are remarkable structural similarities in the eyes of humans and octopuses − but one form of convergent evolution has attracted popular attention in recent years.
‘True’ crabs are decapod crustaceans belonging to the infraorder Brachyura. They include common and commercially important species, such as Carcinus maenas, the shore crab, Cancer pagurus, the edible crab, and Chionoecetes opilio, the snow crab. They have a thick exoskeleton, wide carapace, four pairs of walking legs, a single pair of pincers, and a short, often hidden, tail. These traits, however, are not limited to the Brachyura – they have also evolved in several other distinct groups in a process called ‘carcinisation’. The internet might have you believe that the crab body plan is the ultimate end-goal of evolution for all animals, but this is not quite true. Thus far, it has been limited to decapod crustaceans, and particularly the infraorder Anomura.
All members of the Brachyura generally share the typical crab body plan described above. Meanwhile, such similarity is rare among members of the Anomura. For example, squat lobsters that have a long pair of pincers and a long, segmented body, hermit crabs exhibiting unique soft, spiral-shaped bodies designed to fit into a mollusc shell, and king crabs that have wide, spiny carapaces, long legs, and a large pair of pincers are all Anomura. The wide variability demonstrated by the different groups within this order makes it all the more interesting that several of them have developed to resemble true crabs. What is it about the shape of a crab that makes it so evolutionarily advantageous to crustaceans?The internet might have you believe that the crab body plan is the ultimate end-goal of evolution for all animals, but this is not quite true.
As with many concepts in science, there is no clear answer to this. However, based on our understanding of convergent evolution and adaptation, we might be able to theorise about the advantages of resembling a crab. Convergent evolution, when similar characteristics evolve in unrelated animals, occurs in species that share similar ecological niches – that is, they might live within similar habitats or use similar food sources. For example, bees and flower beetles are not related, but both have a proboscis that allows them to consume nectar. Returning to the crustacean realm, let’s take porcelain crabs and the hairy stone crab as examples. Both are members of the Anomura and are prominent examples of carcinisation, but the latter evolved from hermit crabs, while the former are believed to have evolved from squat lobsters. Like the shore crab, porcelain crabs and the hairy stone crab live on rocky seashores and share a flat, squat body and tough exoskeleton. Their body shape allows them to hide easily from predators in crevices between rocks, while their exoskeleton protects them from desiccation due to the harsh conditions of the seashore, such as extreme temperature variations.
With the development of such similarities between these groups, how can a true crab be distinguished from a member of the Anomura? The answer is quite simple – a true crab walks on four pairs of legs. The Anomura, on the other hand, only walk on three pairs of legs. Their fourth pair is shrunken and hidden beneath their carapace, so the next time you spot a crab on the shore, take a look at its legs. If only six are visible, then it might not be a true crab after all!
Rachel Herbert-Goddard is a freelance writer based in the UK.