Dive Dry with Dr. Bill

#194: The Slender or Graceful Crab

I mentioned in a previous column that the sandy bottom off La Jolla Shores had been a very productive place to dive recently. Today's column will feature another "new" species I observed and filmed there, the slender or graceful crab. These crabs are sometimes mistaken for the larger and more tasty Dungeness crab since they are similar in appearance. However the distribution of the Dungeness crab stops at Santa Barbara and does not extend as far south as San Diego. The graceful crab we found was crawling along the crest of the South Wall, our second dive site.

Crabs are decapods, meaning that they have ten appendages. The legs of this species are slender (my preference), hence the common name of slender or graceful crab. They have purple colored legs and white tipped claws. The perimeter of the carapace or shell is lined in white. Males may be up to 4.5" across while females are about an inch smaller. They are found from Prince William Sound in Alaska to Bahia Playa Maria in Mexico on sand or mud bottoms from the low intertidal down to nearly 500 feet. They prefer muddier habitats than the Dungeness crab, and cannot tolerate the lower salinities found in estuaries. However, they do frequent eelgrass beds.

Graceful crabs feed on animal remains, small clams and other bivalves, as well as barnacles. Commercial oyster farmers consider them a pest if present in their beds. I'm referring to the oyster beds, not the farmer's... this isn't that type of crab! The young are a major food source for the starry flounder. Adults are also eaten by wolf eels and occasionally by humans.

Slender crabs mate in late summer to early winter, apparently depending on the water temperature where they are found. Females group together in relatively small areas where mating is most intense. The females are a bit forward, often initiating mating with the male. I guess they are a bit more liberated than the human females I encounter, or less intelligent... sigh. Size does not seem to play a role in this. However, in competition between males prior to mating, the larger one usually wins. Males which successfully mate with a female may place sperm plugs in her to prevent other males from successfully copulating and fertilizing her eggs.

Afterwards the males remain near the females, apparently to protect them. One scientist believes this may also prevent other males from mating with her. Both ensure his genes are the ones passed on in her offspring. I think I've seen similar behavior at the Chi Chi Club in the past. The eggs hatch into planktonic larvae which drift as they develop. The advanced larval forms and very young juveniles are often seen on large jellyfish such as the purple striped sea jelly. They may also be seen on floating debris. It is believed the larvae may "hitchhike" on these things to reach nearshore waters where they settle out.

Both the Dungeness and graceful crabs are members of the scientific genus Cancer, which means "hard shell." The hard shells of these and other crustaceans mean that their soft bodies are capable of growing only to a certain point. When this is reached, the hard shell is cast off through a process known as molting. The new underlying exoskeleton is soft until it becomes calcified. Of course the crabs are more vulnerable in this state. Lost legs are replaced during molts, although they are initially much smaller than the original and increase in size with subsequent molts. Wouldn't it be great if we had the ability to regenerate lost arms and legs? Think of the wounded soldiers returning from the Middle East. This could be a very rewarding scientific research project.

© 2006 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" or "Gentle Giants: Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Graceful Andrea's gloved hand prepared to let the graceful crab loose
to walk the peak of La Jolla Shore's south wall.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia