Biologists uncovered why so many animals evolve into crabs but only true crabs walk sideways

The process of carcinisation, evolution of crabs, sideways movement, crustaceans, convergent evolution and decapods and true crabs are all involved in the intriguing scientific riddle that wonders how such varied organisms evolve into crabs, yet have just one common ancestor when it comes to sideways locomotion. It is said in evolutionary biology that although carcinisation has evolved several times independently among various organisms, the sideways locomotion of the present-day crabs has evolved only once.Carcinisation is the term used for the independent evolution of crab-like morphology in various groups of crustaceans, involving a flat body form, an abbreviated abdomen hidden underneath the thorax, and a large shield covering the carapace. The repeated evolution of this trait has been seen in various groups of decapod crustaceans, like hermit crabs and squat lobsters.As stated by a famous researcher, Joanna Wolfe,"Carcinisation represents one of the most impressive examples of convergent evolution on record"This repetition implies the presence of strong selective advantages due to the crab-like form, including enhanced defence mechanisms, increased manoeuvrability in small spaces, and efficient burrowing skills.Even though carcinisation has occurred multiple times, sideways walking seems to have occurred only once among true crabs (Brachyura). Sideways walking is possible due to the presence of jointed legs that can move laterally rather than longitudinally.According to the Journal of Experimental Biology, “The lateral gait of brachyuran crabs is related to an extraordinary reorientation of limb articulation in contrast to other decapods.”This means that while there might be other creatures that resemble crabs in physical appearance, they might not necessarily share their mode of locomotion.This is probably due to evolutionary trade-offs. Walking sideways is effective in terms of stability and efficiency at high speeds on rough terrain, but it necessitates extensive changes to the organism’s anatomy. In most crustaceans, forward walking persists as an adaptation since it is relatively easy to achieve and serves them well in their habitats.The research titled ‘Circadian rhythms in zebrafish (Danio rerio) behaviour and the sources of their variability’:“functional constraints limit the repeated evolution of certain locomotor strategies, even when body forms converge”In other words, evolving into a crab-like shape is easier than developing the ability to walk sideways.Crab-like structures come with a variety of advantages, such as reduced vulnerability due to a smaller body size, protection provided by a hard shell, and efficient contact with the environment through limb positioning. This is the reason why carcinisation continues to recur.Additionally, crab-like sideways locomotion provides an additional element of efficiency, making true crabs particularly effective in coastal and intertidal environments, where speed and agility are crucial for survival and manoeuvrability.While the phrase “everything evolves into crabs” may seem to reflect the truth, it misses the point. Whereas carcinisation shows how nature likes to recycle proven design concepts, sideways movement is a novel evolutionary trait.