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Dive Dry with Dr. Bill

#604: Such Sensitive and Mysterious Creatures

Two summers I wrote about the light sensitive sea cucumbers I'd "discovered" in the dive park. You know, like Columbus "discovered" the New World... it was already there and the natives knew it (as well as a number of ancient cultures from Asia, Europe and possibly even Egypt). This sea cucumber has probably been there a long, long time but I just hadn't encountered them previously... even on my night dives in earlier years. Well, now I look for them on most night dives but hadn't seen many this past summer until recently. I thought I'd give you a bit more information about these mysterious echinoderm relatives of the starfish and sea urchins.

I contacted Dr. Gordon Hendler who is Curator of Echinoderms at the Natural History Museum in Los Angeles and sent him a few images I had taken. He wisely said that it was impossible to identify this "cuke" without a specimen since a definitive answer required looking at its "dermal ossicles." I studied echinoderms back in the 1960s at Harvard under Dr. H. Barraclough Fell, so I knew exactly what Dr. Hendler meant. However, I'd better explain it for you, my readers, because it will give further insight into why sea cucumbers are relatives of those other spiny critters.

The dermal ossicles are calcareous structures embedded in the skin (dermis) of sea cucumbers. They form the calcium carbonate exoskeleton that is typical of echinoderms. Starfish (OK, sea stars for the PC crowd) have them embedded in their skin while sea urchins generally have fused calcium carbonate plates that form the "test" or outer skeleton to which the spines are attached. Sea cucumbers also have many other characteristics of that phylum, but this is the one I'm focusing on today since the ossicles are critical in identifying a cuke to the genus and species level.

Since I found these light sensitive cucumbers in the dive park, a marine protected area, I cannot legally take one, preserve it and send it to specialists. It also goes against my preferences as a biologist... one who studies life... so I am torn on this issue. I want to solve the mystery of this critter's identity, but I don't want to have to kill one to do so. Perhaps with the proper permit I can take a sample of its skin containing the ossicles so they can be observed under a microscope without killing a cuke. I kill enough "cukes" for my sandwiches.

On my previous dives I had photographed only the oral (mouth) end of the cucumber as it projected out of the stony rubble to feed. I had no idea how large one of these might be. Based on what I saw, they appeared rather thin (about 1" in diameter) so I did not expect them to be very long. Given that, I had initially assumed they might be juveniles of the more common cukes of the genus Parastichopus, the warty and the California sea cucumber, which are frequent sights in the waters here.

On a dive last summer I filmed one quickly withdrawing into its hole, but noticed it did not fully pull into the protection of the rocks. About two inches of it remained exposed. I cautiously began removing the 5-8" rocks from around it and ended up exposing about a foot of the cucumber. Then I continued removing the rocks and gently drew the rest of it out of the hole to avoid injuring it. WOW... was I surprised! It was a good two feet or more in length and the buried portion was much thicker than the exposed portion. The bulbous rear end (Baby Got Back... "I like big butts" NOT!) seemed adapted to serving as an anchor preventing predators from extracting the entire critter for dinner. Since I don't believe sea cucumbers are good for my sex life (as some other cultures do), I had no desire to eat this one. Besides, I'd need to find a mermaid as well. No such luck.

Once out in the open, I was able to get video and still images of the entire critter which I hoped would help in identification. I spent about 55 minutes of my 85 minute dive extracting, filming and observing this single cucumber. I could finally look at the tentacles around the mouth in detail (see images). They are brush-like and used to capture organic matter on the rocks and in the surrounding sediment when the oral end is exposed (before my lights strike it). I'm guessing they do most of their feeding at night since the organic matter inside their holes might be exhausted and not replenished very quickly.

Dr. Hendler had previously suggested the cuke might be a member of the scientific genus Holothuria. We have several other species from that genus in Catalina waters although some tend to be fairly rare. Years ago I found one species out at Twin Rocks that has still not been identified by the experts (again because I won't take a living sample for them to study). While gently handling this specimen, I was surprised to see it release what are known as Cuvierian tubules. These tube-shaped structures come from the respiratory area that opens in the anus. I'm sure a number of you know that when some sea cucumbers are disturbed or attacked by a predator, they may release part of their digestive tract... giving the predator something to munch on as they slowly crawl away. But then they have to regenerate the digestive tract. The Cuvierian tubules serve the same purpose of distracting the predator, yet they are not critical to digestion or respiration so they can slowly regenerate while the sea cucumbers continues to function at a 100% level (or 110% level as one local company used to tout).

Last October I did five night dives in a row without spotting a single one of these strange echinoderms. On my previous night dives I had always seen at least two of them and as many as seven. Their absence posed a new mystery to me. Are they now sensing my video lights from a distance at which my eyes can't detect them, and withdrawing before I get within sight? Another hypothesis was that they might have been spawning previously which is why I was seeing them frequently. Our more common species raise their "head" regions up off the bottom to spawn, so perhaps these were doing something similar. Hopefully I'll find answers to these questions and solve the mystery of the light sensitive sea cucumber! Stay tuned.

© 2014 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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The light sensitive sea cucumber dug out of its hole and stretching its oral end towards the camera;
feeding tentacles and the discharged Cuvierian tubules.

This document maintained by Dr. Bill Bushing.
Material and images © 2014 Star Thrower Educational Multimedia