Why Crustaceans Keep Turning Into Crabs
Have you ever been to the beach, and lifted a rock, or looked into a tide pool? Among the minnows, sea stars, and algae, you likely saw a crab, scurrying away to a crevice in the rocks. Found across the world, these boxy, flat, and pinching crustaceans are iconic. They are also globally popular in seafood, making them economically and culturally significant for those who live around them.
But what if I told you that the crab you found in that tide pool, or the one you ate at a restaurant, was not really a crab? That, in fact, many crab-like impostors roam the sea, almost indistinguishable from the real thing. Take these two, for example:
To the untrained eye, these crustaceans appear very similar. Both definitely appear to be crabs, and both are commonly sold as crabs in food markets. But the Norway King crab is actually an entirely different crustacean. Despite their remarkable resemblance, the King crab shares a closer common ancestor with the hermit crab. True crabs, such as the European spider crab, belong to the classification Brachyura and can be easily identified by their four pairs of walking legs. In contrast, King crabs, and their hermit crab cousins, belong to the sister group Anomura, which only contains crustaceans with six visible walking legs.
Porcelain crabs, delicate and small rock-dwelling creatures found throughout the world’s non-polar waters, appear to be true crabs at first glance. They have all the markings of one, except for their six walking legs and their suspiciously long antennae. Genetic evidence suggests that they are actually a highly specialized lineage of squat lobster, a group usually characterized by slender, tailed crustaceans with an appearance in between a lobster and a shrimp.
Perhaps it is unfair to call true crabs the “real” crabs. After all, the crab body plan has independently evolved among crustaceans five times, including the already mentioned true crabs, porcelain crabs, and King crabs, as well as the coconut crab and the deep-sea hairy stone crab. True crabs were not the first to develop this way either. Hundreds of millions of years ago, in the Carboniferous period, the Cyclida roamed. These ancient crustaceans pre-date any present-day crab, yet share their iconic look, and are suspected to have lived a similar lifestyle.
So why do so many crustaceans eventually evolve into crabs? Zoologist Lancelot A. Borradaile answered this question in 1918 by coining the term “Carcinization,” which he defined as “the many attempts of Nature to evolve a crab.” He suspected that the same pressures of natural selection drove each instance of carcinization.
He also suggested that this process begins with the pleon — the segmented tail which many crustaceans use to swim. Seldom visible on crabs, the pleon visually distinguishes lobsters and shrimp. Crabs lack that visible tail because they fold their pleon underneath them. In a study published in the Biological Journal of the Linnean Society, Jonas Keiler et. al. suggest that this adaptation is the first step towards carcinization. Crab ancestors began with a longer, lobster-like pleon, which shortened and flattened until the animal could fold it to its body. Simultaneously, the crab’s body grew wider and flatter. This increased the crab’s mobility across the ocean floor and allowed them to more easily hide between rocks. The change also protected otherwise vulnerable organs within, like the intestine, from being easily consumed by a predator.
Organisms fill specific roles in the environment — known as a biological niche. Over time, as different lineages of crustaceans converged to become crab-like, they all also filled the niche of the bottom-feeding omnivore. Why? When an animal fills a niche, it must become fully optimized, otherwise, it goes extinct. So, when these crustaceans began to lose their pleons, they became less effective at their old niche and began to adapt towards a new one. For a crustacean becoming the bottom-feeding omnivore, efficiency means becoming a crab.
While evolution often produces fantastic diversity, some roles must be played a specific way. In these cases, unrelated organisms will develop into remarkably similar forms difficult to distinguish. So, next time you are flipping rocks at the beach or buying seafood, consider for yourself — are you looking at a real crab or a convincing fake?
