Most people are familiar with a lizard’s ability to drop its tail off at will. Also known as autotomy, reflex amputation is an adaptive behavior developed by a wide range of organisms as a means of escaping from predators. It is especially common among various groups of marine invertebrates.
The main distinction between reflex amputation and your average run-of-the-mill amputation is that the former is intentional. The animal purposely detaches the appendage, be it a tail, leg or claw, as a way of escaping from a larger predator, losing just a limb rather than its life. Of course, autotomy wouldn’t be much use if the animal didn’t have regenerative abilities as well.
The lizard in the above example is exceptional among vertebrates in being able to re-grow an entire limb. But this ability is a cinch for most of the lower backbone-less organisms. I’ll get into regeneration a bit more in a later diary, but for now it is simply the ability of an animal to replace a lost body part. We’ll start with an animal that is perhaps most well-known for this: The American lobster.
Lobsters, like all decapod crustaceans, have ten legs. The first pair of legs are actually huge claws. Each claw is used for a specific purpose. The heavy, blunt-toothed one is called the crusher, and is used to crush things. The lighter, sharp-toothed one is called the shredder and is used to tear soft foods into pieces small enough to swallow. It is not uncommon for fishermen to find a lobster with only one of these two claws. When sold for food, these one-armed animals are known as "culls", and often a culled lobster is the result of autotomy.
An adult lobster has few enemies other than a lobsterman’s trap. Lobsters are, however, highly territorial as well as cannibalistic. When two lobsters meet the smaller one will almost always flee. If it doesn’t a fight will break out and the winner is usually the one that manages to leave the ring with the most limbs intact. Again, the claw that is lost in these fights is not actually pulled off by the victor, but is self-amputated by the loser as a way of escaping from the battle. The winner eats the spoils. This one just lost but doesn’t seem to want to part with his missing part.
Nearly all crustaceans, including crabs and shrimp, are able to amputate reflexively. Regeneration in this group of animals doesn’t happen gradually, as we’ll see in echinoderms down below. Crustaceans have to shed their exoskeleton as they grow and the regenerative process takes place beneath this shell layer. The lobster will be without the claw for weeks or months, but the claw is regenerating out of sight. After the animal molts the new appendage will appear, although it will be about half the size of the original. It will take one or two more molts for the regeneration to be completed, after which you won’t be able to tell that it ever lost the limb in the first place.
Echinoderms, especially the large group of them known as sea stars, are especially good at regenerating. Any sea star can lose an arm to a predator and grow it back, and nearly all can autotomize as well, although most do so only when environmental conditions are so bad that survival is a lost cause anyway. For example, a sea star placed in fresh water, just before it dies, will detach all five of its arms. The arms will crawl away in different directions leaving only the main central disc behind. Then all the parts die, although if you quickly transfer it to saltwater there’s a chance the disc can survive and regenerate five new arms.
However, the masters of reflex amputation is a related group of echinoderms known as brittle stars (see separate essay on them here.) As you can see on this one I happen to own, it lost one of the arms and it has nearly grown back. The regeneration process takes only about three months with this species.
Think back to the lizard from the beginning of this essay and you can probably guess what happens to the tail after it falls off. As part of the defense the tail will wiggle around quite a bit, distracting the predator and giving the reptile a little more time to make its escape. Watch this and you’ll see what I mean:
The brittle stars can do the same thing. Once detached the arm will move around for quite a while. The one above lost its arm about two months before I took this photo. The severed arm crawled around the aquarium for nearly two weeks before it stopped moving.
Competing in the mollusk category is the octopus, although these animals have such short life spans that they usually die before the limb fully regenerates anyway. But octopus males give the concept of autotomy a twist. On the third right arm of many species is sac-like extension of the limb known as a hectocotylus, which is filled with sperm. During mating this arm is placed into the gill cavity of the female and the distal part of the arm is dislocated. The octopus mutilates itself for the sake of its offspring.
Fun Fact: When a lizard’s tail grows back the new one is supported by cartilage. Bone doesn’t regenerate.
Other diaries in this series can be found here.
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