The spiny skinned invertebrates, known scientifically as echinoderms, include the sea stars, brittle stars, sea urchins and sea cucumbers. All have hard parts made of calcium carbonate, present either as the rigid exoskeleton of the starfish and sea urchin, or embedded within their skin as in the sea cucumbers. The sea stars are often avid predators seeking out other critters like bivalves and snails to munch on, and brave the bites of predators like sheephead to search for their munchables. Brittle stars feed on muck and hide away under rocks for protection. Sea urchins usually have very sharp spines which offer them a formidable defense... until a starving sheephead, or dastardly diver, turns them over allowing access to the softer, less protected underside.
Like other members of its group the subject of this week's column, a sea cucumber in the genus Holothuria, is soft bodied and lacks the obvious defenses of other echinoderms. Yet, for the most part, sea cucumbers reside out in the open where even the most dull witted predator could find and munch on them. The calcium carbonate spicules in their skin offer no real protection, although they may make them seem a bit less delicious to predators unless they prefer their fare crunchy. Only occasionally do I see sea cucumbers with obvious evidence of having been attacked by something wishing to make it a "happy" meal. I can't speak for all species found in our waters, but will elaborate on the interesting defenses of this species.
This species of Holothuria is more commonly found to the south in Mexican waters. The only portion of Catalina's waters where I see it frequently is in the Goat Harbor-Twin Rocks region, although Lorraine Sadler has seen it in a few other locations. One of its defenses offers a partial explanation as to why it may not be readily observed. It is great at looking like something totally inedible (no, not my cooking)! In fact, when I give briefings on the King Neptune to tell divers what critters they might encounter, I refer to this sea cucumber as looking "like a piece of dung with algae growing on it." Now there aren't many critters that would find that menu item appealing. I might add that I've seen a related species in Mexican waters that is commonly referred to even by scientists as the "donkey dung sea cucumber."
The appearance of this sea cucumber renders it difficult to identify as something living. When I first encountered one, I thought it was some strange alga I hadn't observed before. It was only when I touched the "bump" on the rock that I realized it was alive. When I turned it over, I could see the tube feet that gave it away as a member of the echinoderm phylum. Its appearance, and the presence of what appears to be living algae on its dorsal surface, certainly seems to be a good example of camouflage. It's hard to eat what you can't see... unless, perhaps, you can smell it cooking!
Now let's assume there's a submarine predator with the wisdom of this sharp-minded marine biologist. Yes, I know that is hard to imagine, but bear with me on this one. Like the biologist, the predator realizes that this seaweed covered piece of poop is a potential palate pleaser. It tries to grab the cucumber with its teeth, claw or whatever body part it uses to capture its prey. Suddenly the predator is startled when a pile of pale pasta noodles comes streaming out of the animal's anus! Whoa, hold on Nellie...what have I gotten myself into? But that's not the half of it. The "pasta noodles" are even stickier than my undercooked vermicelli, and adhere to the would-be muncher's mouth, head and eyes causing it to react with some predatory panic and paddle off. Or, if it is cool headed, it may start to munch on them like an Italian on the Via Venuto, even though they don't come with meatballs or even sauce. If so, it is distracted from the sea cucumber as a tasty treat.
Like many of its fellow sea cucumbers, this species has a very interesting defense mechanism called evisceration. In some species portions of the intestinal tract, respiratory trees or even the gonads are expelled through either the anus or mouth when the animal is disturbed. In this species, the eviscerate is actually produced by specialized organs known as Cuvierian tubes. I assume this is to avoid the digestive dead end of a self colostomy. The sea cucumber can continue to fully digest its food without having to regrow or regenerate a portion of its internal structures. The specialized organs fabricate new noodles to use the next time such a defense is necessary. Now that is using one's noodle to survive in the mutual eating society.
In my research for this column, I learned that many sea cucumbers have a toxin referred to as holothurin in their bodies. This chemical is technically a saponin, a group of naturally occurring surfactants most commonly associated with desert plants. Digitalis is one commonly known saponin which, in small doses, can help patients with heart disease. In larger doses it can be a deadly poison, and was used on spear and arrow tips by native tribes. Perhaps the holothurin is one reason why sea cucumbers are not generally attacked by most potential predators. Apparently holothurin is also found in the eviscerated structures of this sea cucumber species.
Thinking of sea cucumbers eviscerating takes me back to Jean-Michel Cousteau's Project Ocean Search back in the mid 1970's. At the end of each session of these educational programs, the staff would put on a "no talent show." I might add this was an aptly named event, especially after the punch Jean-Michel made that night took effect. One year Henry Genthe, Carol Cable and I came up with an amusing skit. I would play a sea cucumber by hiding inside my sleeping bag. Henry and Carol would-be predators who came upon me and started poking the sleeping bag. Suddenly I would expel several lily white white bed sheets through the opening of the bag. Yep, yours truly once played an eviscerating sea cucumber... to rave reviews, I might add!
© 2007 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles," "Gentle Giants: Giant Sea Bass" or "Common Fish and Invertebrates of the Sea of Cortez" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!
The undisturbed sea cucumber and one beginning to eviscerate; the mess
of "noodles" and my gloved hand with them sticking to it.
This document maintained by
Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia