Sea cucumbers are not, however, spiny to the touch. Slimy is a better descriptor but I’m getting away from talking about their butts. Okay, I wasn’t quite talking about their butts yet but their cloaca. The cloaca is a pouch that, as I was so eloquently stating at the beginning of this mini essay, has one opening to the outside and two tubes from the inside of the animal that end at the cloaca. The two internal tubes are the ends of the reproductive system and the digestive system - I knew we would find their butt eventually. Sometimes tubes from the urinary system also end here. So into this pouch dumps the animal’s waste and the animals reproductive products (eggs or sperm). I should probably be more sciencé (pronounced Sci Ents E) here and start referring to the butt as the anus. I think sea cucumbers must have decided this idea (waste and reproductive end products into a single bucket) was gross because their cloaca does not contain tubes from the reproductive system (so maybe its not officially a cloaca). Evolution seems to think this as well as the ‘trend’ appears to be to separate the the tubes into separate exits. A cloaca is found in the evolutionary holdout the platypus of course. Quick side note: sea cucumber reproductive systems have only one gonad so in them the reproductive “tubes” are just one tube.
Okay back to the butt: In sea cucumbers the pouch (cloaca but maybe not officially) may also include a respiratory tree (or an opening into a respiratory tree), which is fine feather like extensions (very small tubes) that enable oxygen exchange. Oxygen diffuses into the body fluid - sea cucumbers do not have blood exactly - and that oxygen is used, of course, to keep the electron transport system (and therefore the body) running. The muscles around the external cloaca opening contract and relax to allow water exchange refreshing the oxygen supply.
But wait, there’s more. Some sea cucumbers - like the Five-toothed Sea Cucumber Actinopyga agassizii - can bring water into the respiratory tree and by putting it under pressure expel a portion of the tissue. The expelled mesh of tissue shoots through the anus. The purpose is to distract, ensnare, or poison a predator. The tissue is general sticky and sometime poisonous (as it is in the Five-toothed Sea Cucumber). Echinoderms, including sea cucumbers, are well know for being able to regenerate body parts. This they must do after expelling parts to ward off predators.
But wait there’s more. The five-toothed Sea Cucumber A. agassizii also sometimes is found to have a fish that takes up residence in its cloaca. The fish, the Pearlfish Echiodon rendahli pushes itself tail-first into the anal opening and takes up residence in the base of the respiratory tree. Pearlfish venture forth to feed at night. Pearlfish don’t just pick on sea cucumbers but find any protected vestibule, in clams, sea squirts, or sea stars. Hiding out inside other animals is not just done by Pearlfish but some worms and even some crabs do similar things. I think there must be more animals that do similar things but, as with the Pearlfish in A. agassizii patient watching is typically required to figure it out.
These relationships seems to almost always be commensal - that is they benefit the animal doing the hiding but do not harm the host. One does wonder why the Pearlfish doesn’t trigger the sea cucumber to expel part of its respiratory tree as it might with predators.
One more thing I wanted to make sure I pointed out; the “teeth” of the Five-toothed Sea Cucumber are around the anus not the mouth.
I’ll leave you with this picture of the anal “teeth” of a Sea Cucumber.