Sea squirts use their digestive tracts to avoid being digested
A sea squirt is a small, marine invertebrate animal that you could easily confuse for a squishy chunk of a coral reef. The tube-shaped creatures live attached to a fixed surface of some sort, then passively filter-feed on particulate in the water. This is not to say that their life is without drama— when danger is detected, the sea squirt Polycarpa mytiligera can take the concept of elective amputation to a whole new level and eject their digestive tract out at the potential predator, hopefully frightening it away. Or at least making it loose its appetite.
When the sea squirt feels like something is about to bite it off its perch, in about 16 seconds it will regurgitate it’s digestive tract out its mouth, possibly into the mouth of the predator. This can include everything from the stomach down to the rectum, leaving the squirt a small, slightly crumpled version of itself. Aside from the startling visual, the guts repel fish because they apparently taste bad, and will be spat out by fish who give them a nibble. Of course, this can only considered a defensive maneuver if the sea squirt’s survival rate goes up as a result, and remarkably, they can survive nearly turning themselves inside out.
Regenerated rectum, intestines, and stomach
After 12 days of a defensive ejection, sea squirts were found to have already grown digestive organs. Not only that, but they seemed to be immediately functional, as feces was already collecting in the new rectum, meaning they had been eating for some time already. There must be some delay, of course, because sea squirts had been known to be occasionally found in a gutless state for many years. The defensive concept was only solidified for researchers after they accidentally triggered ejections when pinching squirts to remove them for study.
Regeneration in marine life isn’t unique to sea squirts like Polycarpa mytiligera. Sea cucumbers can also eject organs on demand. Sea stars have a slightly tangential set of abilities, as they regularly invert their stomachs to eat, although they don’t really amputate them. For actual regeneration, sea stars just regrow limbs.
Source: Nervous Sea Squirts Squirt Out Their Stomachs and Grow New Ones by Elizabeth Preston, Inkfish
