Dive Dry with Dr. Bill

#280: The "Sheepish" Crab

Most of my "Dive Dry" columns talk about the natural history and ecology of a single species. However some of the most incredible aspects of the natural world, whether underwater or on land, are the ecological relationships between these species. Of course it is often easier for me to get good images of the critters sitting there doing nothing, instead of the more exciting munching and mating behavior I prefer capturing. While those two activities are critical for the survival of an individual... and of the species... they just don't happen 24/7.

I've been working on my current DVD project about the echinoderms so much of my attention has been focused on this group. It includes the sea cucumbers which often seem very lethargic and inactive, making them less than ideal for exciting behavioral footage. Despite this, I do occasionally see great footage of my favorite behaviors. Recently Matt Gieselman sent me a video he had taken of the warty sea cucumber casting its sperm (or eggs) into the water off Two Harbors. In 38 years of diving our waters, I've never seen this happen! However, you will have to buy my upcoming Echinoderms of Southern California to see the actual footage which Matt gave me permission to use.

Sea cucumber munching is nowhere near as exciting. They simply use their tentacles to capture soft bottom sediments in which there is organic matter for them to digest. They pass the sand or mud through their digestive tract and excrete the processed (cleaned?) sand as tubular excreta. Not very exciting. I recently had the unfortunate opportunity to dissect one and look closely at the digestive system which it had expelled. I hadn't dissected any critter in decades, preferring only to take pictures and leave bubbles when I'm underwater.

Many of the field guides state that these sea cucumbers do not have any known predators. Of course such statements are often made by scientists who spend more time in a laboratory or a library. Divers often spend more time underwater than some of my fellow marine biologists. Therefore they see things that some of the scientists may not observe. I'm kind of a hybrid... half research scientist and half dive bum, or is it 10% and 90%?

On a recent dive near the East End Quarry, I was ascending from my maximum depth of 130 ft. The visibility was not great, so I stayed very close to the bottom. After all great whites usually attack from below, and I've never heard of one burrowing into the sand or silt and popping out to surprise an unwary diver! Just in case, I always carry my camera on a dive as insurance... I know I'll never see a great white as long as I have it with me!

I came across something I'd seen a few times before... a mutilated sea cucumber. It had been sliced and diced very badly by some "food processor." I was quite sure it wasn't a fish predator, which might just swallow it, but a predator that had to tear it apart to feed. I quickly had my answer when I looked about 10 feet away to see a sheep crab nearby. And the crab looked very "sheepish" indeed... just like a five year old child does when they know they've done a "no no."

The sheep crabs began returning to our shallow water in February so they could do "the wild thing" in water that is a bit warmer. Although I haven't filmed them mating yet this season, other divers have seen them. On occasion over the past 8 years I've observed and filmed sheep crabs munching away on sea cucumbers. Heck, they're easy prey since they are kind of "sluggish." I'm pretty sure this crab had just feasted on the poor cuke!

Now this poses an interesting question (at least in my "fertile" biologist's mind). I've mentioned in the past that sea cucumbers are a favorite food item in many Asian markets. They are known as namako in Japan, beche de mer in Vietnam and trepang in Malaysia (and the Philippines, I believe). These cultures see the cucumber as not just a meal, but an aphrodisiac... at least in the eyes of many of their men. So I pose the following hypothesis. Was this "sheepish" crab feasting on the cucumber in anticipation of a tryst with some lovely 10-legged lady crab? Inquiring scientific minds want to know! Don't you?

© 2008 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 and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. 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," "Common Fish and Invertebrates of the Sea of Cortez" or the new "Sharks and Rays of Southern California" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

A young warty sea cucumber, the digestive tract of a cucumber showing the sandy contents;
the mutilated sea cucumber and the "sheepish" crab culprit in this dastardly event.

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia