Dive Dry with Dr. Bill

#258: Up the Bean Stalk... er, Kelp Stipe!

A prehistoric looking local member of the spider crab family, the sheep crab, enters our nearshore waters during the winter months to escape the cold of their usual deeper water habitats. Like it's any different temperature-wise down there during summer or winter! These crabs, which get up to several feet from leg tip to leg tip, are quite as treat for divers to encounter and observe. At times they may just rest on the bottom like a rock (not so entertaining), but often they exhibit interesting behavior making them worth spending a good part of your dive with.

One reason they enter the shallows is to search out a mate and... well... mate. This is the reason to escape the cold... eggs and young develop faster in warm water. Regular readers of my column know that, as a marine biologist, I feel "munching" and "mating" are the two most interesting behaviors exhibited by any species. Although breathing is also important, it is nowhere near as interesting and the latter "M" word may take it away at times. As for defense, I lump that in with the munching "m" word since it is an attempt to avoid that particular activity by a potential predator.

Sheep crabs are found from Marin County to Punta San Bartholome, Baja California. Males reach 6-10" across the carapace, and tend to be larger than females which are under 5" across. Depending on season, they may be found from the lower intertidal to 500 feet (where Dr. Bill dares not go... yet). They frequent rocky reefs or may be out in the open on sandy or silty bottoms.

Males also have larger claws than the females. In the event their body or claw size doesn't give away their gender, divers who read this column know that if you turn them over (carefully, please) a positive ID may be made. Males have a deep v-shaped telson (tail like structure) while females have a broad oval-shaped one which is used to hold the eggs under the body while they develop. One well respected scientific field guide referred to these crabs as looking "ludicrous." Good thing they don't sound like the rapper Ludacris or I'd probably stay out of the water when they're around.

While researching this crab, I found that fossil specimens first appeared off the Washington coast, then in central California and finally, during the last Ice Age, in southern California. This strongly suggests it is a cold water species which moved further south as temperatures decreased during the Pleistocene. Living at depth in our region certainly reinforces this assumption. So I wonder what will happen as global warming proceeds? Will these critters migrate north to get away from the warmth? If so, their brains and physiology are radically different from yours truly!

The sheep crab is both scavenger and carnivore, and will take food opportunistically... that is when it falls on its "plate." I have seen them feed for days on dead bat rays and sharks, and have also observed them feeding on sea cucumbers, starfish, gorgonians (sea fans) and other critters. In turn, there are predators that feed on them. That includes humans, and there is a small sheep crab fishery based in Santa Barbara and Ventura which fishes the northern Channel Islands. It had greater impact on the crab populations before nearshore gillnet bans went into effect in California between 1990 and 1994.

In one scientific study at Scripps, 26% of the sheep crabs captured for the study had missing legs. Looks like predators like crab legs as much as I do. They may take them just after the crabs have molted their hard exoskeletons and are still very soft. It will take several molts before the lost leg is regenerated back to full size. However, some of the leg losses may be due to competition, especially between the aggressive males. I call these crabs the Cassius Clay Crab (Muhammad Ali for you "youngsters") because if you approach them in a threatening manner, they start boxing at you... before fleeing as a secondary defense mechanism.

Over the past few months I've observed a means of defense against predators that I hadn't really noticed previously. I've found a number of female sheep crabs, often with "berries" (eggs) under their telsons, which have climbed up various species of kelp. While I'm merely proposing an unproven hypothesis here, I believe these females have chosen a different mechanism to avoid predation. They have climbed up the proverbial bean stalk... er, kelp stipe... just like Jack climbed down one to escape The Giant. Then again, perhaps the females were just trying to avoid the aggressive Cassius Clay males who had no intention to "munch," just to "mate." Either hypothesis makes sense to me.

© 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!

Images of sheep crab females in southern sea palm kelp (Eisenia) and giant bladder kelp (Macrocystis) .

This document maintained by Dr. Bill Bushing.
Material and images © 2007 Star Thrower Educational Multimedia