Dive Dry with Dr. Bill

#241: Southern Kelp Crab

A few weeks ago I was diving Long Point off the King Neptune. I decided to drop down to the base of the wall at the end of the point, and had a good dive at about 125 feet. As I slowly ascended the wall to decompress, I rounded the tip of the point and came across a bright red object. Of course red is not commonly seen underwater since it is one of the first wavelengths to get absorbed in the water column. However, this object was at a depth of only 40 feet and it was definitely red. It got even redder when I turned my video light on to start filming.

So what was it I found? It was a southern kelp crab... or more accurately, the discarded exoskeleton of one. This one did not appear to have been munched since everything was intact. Most likely it was the molt of a kelp crab that had simply grown too big for its britches, and split the shell towards the back so it could crawl out and grow a new, larger exoskeleton. Other crustaceans like lobster do this as well. I could relate to this since I've split more than a few pair of pants and had to buy ones with a "skoch" more room in them! Good thing wetsuits are quite stretchable... especially the hyperstretch kind.

Southern kelp crabs are found along the rocky bottoms under giant kelp forests. They are often seen climbing on the giant and other kelps in our region. It is assumed that they are less common today than in previous centuries when kelp was more abundant... and perhaps even less so in the future due to the likely effect of global warming on kelp in southern California. However, they may extend down to depths of about 300 feet, far exceeding the depth to which giant kelp can grow (or I will dive). Their geographic range goes from Santa Barbara to Magdalena Bay on the Pacific coast of Baja.

These crabs have a pronounced rostrum or pointed structure above their mouth and between their eyes. The rostrum has a v-shaped notch in it which helps distinguish this crab from the similar shield-backed kelp crab also found in our waters. The body may be up to four inches across in males and about half that in females. Adults are often reddish in color like the one I found. Being decapods, they have ten legs with the forward pair having pincing claws at the end for feeding and defense. The southern kelp crab munches mainly on large algae including the giant and feather boa kelps.

These crabs are limited to temperate waters in the northern hemisphere. However, there is a closely related crab found in southern temperate waters from northern Chile and Peru. Geographic distributions of closely related species like this often suggest that they may have been a single species at one time. It is possible that during one of Earth's ice ages, when cooler waters extended closer to the equator in both hemispheres, the distribution of their ancestor ranged across the warmer equatorial waters. They may then have became isolated when waters warmed and the tropics expanded to form a wide barrier to the dispersal of these cooler water species.

I was shocked to find very little information on this species when I researched this column. I even tried searching using the former scientific name and found nothing of substance to pass on to my readers. I guess that means I'll have to come up with a little tale from my own diving experience to pad this column to a reasonable length. No, I'm not paid by the word. If I were, these columns would be billions of words long, and I'd be a wealthy man instead of a dive bum!

Hmmm, let's see. Ah, yes. One day I was filming in the dive park when I saw a moray in a crevice hammering away at something hidden from my eyes. I found a small hole in the rocks and was able to shine my video light through it. I got quite a surprise when I saw the moray's head trying desperately to reach an octopus which was holding a live southern kelp crab in front of it like a Roman soldier's shield! My guess is that the octopus had captured the crab to munch on, but was surprised by the moray which sensed a two course meal. So close, yet so far away. The moray never got either of them... nor was I able to film this amusing incident. You'll just have to take my word on the crab and octopus that got away... well, at least the octopus (with the crab's soft parts inside it).

© 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 the fairly intact molted exoskeleton of the southern kelp crab... almost looks alive!

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