Dive Dry with Dr. Bill

#198: Xantus' Swimming Crabs

Many of you are probably familiar with the most noticeable crab on our shoreline, the striped shore crab. It can be seen on the rocks above the water line using its claws to graze on algae. I call it the "sissy crab" because it avoids going very far into the water where it might get munched by a hungry sheephead. Today I'm going to talk about a more daring species, one that dwells at deeper depths where it has to fend for itself... Xantus' swimming crab.

The swimming crabs have modified rear legs with broad paddles at the end which are used to swim. There are several different species found in our waters. This one can be identified by the longer, sharp spines along the sides of its body and a prominent spine on each claw. After all, if it is going to venture into the realm of crab connoisseurs, it needs some good defense! In addition to the spines, it often escapes by quickly running or swimming away. These movements are done sideways rather than forward or backward. If that doesn't work, they can quickly bury themselves in the sand.

The Xantus' crab is found from Santa Barbara south to Topolobampo, Mexico. It prefers sandy bottoms or eelgrass beds at depths down to nearly 600 ft. Their bodies are somewhat small, no wider than three inches, and are gray in color occasionally with white markings and red or purple on the claws.

The swimming crab feeds on sand crabs which can be very abundant on sandy bottoms. Their hunting method is quite interesting and was described years ago by an early California biologist couple, the MacGinities. The crabs walk over beds of sand crabs and will pause to push one of their claws into the sand. If they connect with a sand crab, they grasp it and run around in a circle around the claw to pull the crab up to the surface. As an interesting historical aside, both MacGinities served as directors of Cal Tech's Kerckhoff Marine Laboratory from the early 1930's to the late 1950's. If memory serves me well, G. E. MacGinitie visited the pre-War II Catalina Island School for Boys at Toyon Bay to give a talk on marine life. Of course I wasn't on staff back then... or even born!

Dive buddy Andrea and I came across a mating pair on the sandy bottom of La Jolla Shores. Although my references indicate both sexes are about the same size, the female in this pair was much smaller than the male. I wondered whether this lecherous male had literally robbed the cradle! They remained coupled as I filmed, holding their long claws out in a threatening manner to ward off any further advances on my part. Egg-bearing females are seen from May through September, so I must have encountered a pair getting an early start on the mating season. My dive buddy Missy, who dives that location often, says she frequently sees these crabs mating.

I had to do some research to find out who Xantus was. Of course I was familiar with the name from the Xantus' hummingbird, Xantus' murrelet, and several fish species. John Xantus de Vesey was born in Hungary and affected an aristocratic title. His activities as a rebellious Army officer there forced him to flee to America for his life. Here he pursued several different occupations: bookseller, druggist, teacher, hospital steward, a State Department consular official in Mexico, and assistant surgeon. In the latter role he worked under future U.S. Surgeon General W. A. Hammond whose interest in natural history was contagious. The rest is history. He ended his varied career serving as the Director of the Zoological Garden in Budapest. I wonder what kind of degree one needs to be so diversified! Why he didn't even go to Harvard where Renaissance men (and now women) are born.

© 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!

A pair of mating Xantus' swimming crabs on the sandy bottom.

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