Sea Slugs Protect Themselves From Predators By Self-Cutting

To escape the deadly grasp of a crab's claw, the long-named Melibe leonine hooded sea slug can readily sever its paddle-shaped back appendages and swim away unharmed. Researchers from the University of Victoria, in Canada, believe this process of self-amputation, or autotomy, is a slug's way of deterring predators from attacking other, more vulnerable, parts of its body. 

Melibe leonina is a marine snail that lacks a protective shell. These critters live on eelgrass and use a large tentacled oral hood like a Venus flytrap to feed on small planktonic shrimp. While such a snail has back appendages - scientifically called cerata - that simply "pop off" its body easily, each is much more vulnerable in the area of its oral hood, according to a news release.

"When they are handled roughly or pinched, the tissue at the base of the cerata seems to melt away and they just pop off," Louise Page, study researcher and University of Victoria developmental biologist, explained in the release. "I've often wondered if the cerata may be acting as a decoy, to attract the first attack from a fish or a crab, to deflect it away from the hood, because the oral hood is so vulnerable."

This self-amputating behavior is similar to how lizards and amphibians shed body parts when caught by a predator. When a back appendage is removed, the snail is able to simply re-grow the missing plate later.

"Autotomy is a voluntary loss of an appendage, and animals must also have a way to seal off the wound so they don't bleed to death," Page added.

In the latest study, Page investigated the area at the base of individual ceras, known as the autotomy plane. Here she discovered several unique characteristics, including special granule-filled cells that are directly connected to nerves and found nowhere else in the animal.

"It looks suspiciously like these cells may be involved in releasing something that is breaking down connective tissue," Page said, adding this would make it much easier for the tissues at the base of the ceras to pull away. 

Page and her team of researchers then studied the signals involved in initiating autotomy and found that when self-cutting is triggered, the longitudinal muscles contract strongly and put tension on the autotomy plane. This releases granulated material from the cells, breaking down connective tissues. Two muscular sphincters along the autotomy plane then contract to cut the ceras and close the wound, preventing the hooded sea slug from dying.  

While further research is required, researchers believe their findings could lead to medical breakthroughs in helping wounds to heal. Page and her students recently presented their study at the 2016 annual meeting of the Society for Comparative and Integrative Biology in Portland, Oregon.

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