Although nobody likes to encounter a crab when relaxing on the beach or entering the water for a swim, crabs are definitely worth a closer look. They are all supposed to have 10 legs (four on each side for walking plus a pair of claws, also called pincers, nippers, or chelipeds). But take a closer look: do all crabs really have the full complement of appendages? No, they don’t; often legs are missing and in some cases, you might find only a tiny leg where you’d expect to find a big one. What’s going on? Is that another thing we can blame on global warming?
Not at all. The ability to shed legs (= known as autotomy) is common to all species of crabs, lobsters, many spiders and insects as well, but the ease and rapidity with which the process occurs varies from species to species. The so-called “daddy-long-legs” and cellar spiders like Pholcus phalangoides drop limbs very easily and since they have eight long walking legs, they can still escape with seven, six or even fewer. In the crab and other crustaceans the fracture plane is not necessarily the weakest part of the crustacean limb and Dr Alister McVean in the United Kingdom has examined how the controlled activation and rotation of one set of leg muscles versus another can lead to strains in the leg, which culminate in the preformed breakage plane to separate.
Although no differential susceptibility among walking legs or between left and right sides has been found in the crab Carcinus maenas, chelipeds are lost significantly more than walking legs. On the one hand this is understandable because the crab fights with them and they are at the front of the crab’s body, but on the other hand, to lose a claw ought to be a greater handicap for the crab than to lose a leg. Not only does the crab need the claw to defend itself, it also needs it to feed and that makes it harder to explain why crabs easily lose their claws. That younger and, thus, smaller individuals drop appendages, including claws, more readily than sexually mature, older animals are easier to explain because they are attacked more frequently on account of their smaller sizes and they can replace the lost limb and grow a new one much faster than older individuals. The re-establishment of connections between nerves and the regenerating muscles in the new leg is a fascinating neurobiological problem that also progresses more easily in younger individuals.
While autonomy appears to have immediate and valuable benefits (after all giving a leg is better than giving one’s life) in the long term it can render the crab more vulnerable. During moults of the crab legs may regrow, but the intervals between moults become increasingly longer as the crab ages so that the voluntary loss of a limb is less of a sacrifice to a youngster that it is to an adult. A crab’s goal in life is not to get old in perfect condition (and that’s one difference to us humans), but to reach sexual maturity at all cost. And whether that goal is reached with 10, 5 or no legs at all is immaterial as long as the crab still has the time and the strength to spawn and produce babies (and that’s another difference to us humans). Ultimately, it’s only that, which counts in the life of a crab, lobster, spider or autotomizing insect: to be able to produce and release fertile eggs.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2018.
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