Dive Dry with Dr. Bill

#417: The "Christmas Crab"

My night dives were cut short this fall and it has been over two months since I last submerged to explore and film on my own. The only exception was a dive back in late October when I filmed a snorkeler exploring our kelp beds for the cable TV show "Out and About with Roger Martin." Now I have nothing against filming a pretty lady down under, but if that's all I wrote about my readers would probably become quite bored. I know how much they prefer to read of crabs and slugs and worms from the briny deep (well, at least half my readers).

With no current observations to share, I'm digging back into my nocturnal dives of early fall for this week's story. Since it is almost Christmas time, I thought I would chose a critter that is bright red in color and often contrasts with the green seaweeds on the rocks it frequents. That critter is the southern kelp crab (Taliepus [formerly Epialtus] nuttallii). This crab is truly red... you don't have to put it in a pot of boiling water to get that color like you do with our spiny lobsters.

The southern kelp crab reaches a width of about four inches. The hard carapace is somewhat convex, bulging outward a bit. It can be distinguished from the more northerly kelp crab, Pugettia producta, by the prominent rostrum (nose like structure) with a well defined triangular notch. The southern species is known from Santa Barbara down to Magdalena Bay on the Baja California Peninsula. The other kelp crab inhabits the cold waters of Yakutat, Alaska, down into Punta Asuncion, Baja.

I rarely see these crabs while diving during the day, but I've had a number of encounters with them on my night dives. Usually when the sun is out, if I see them at all they are wedged deep in the rocks where predators like the sheephead can't reach them. Then, when our fine finned fishy friends like Oscar go to sleep at night, these crabs come out, apparently to do one of two things... munching or mating!

As my readers know, red is the first color that gets filtered out in the water column. Therefore these crabs, as well as other critters including fish that have red bodies, appear almost black... until I turn on my video lights! And what do I see when I turn on the lights? Well, nothing the Beatles would sing about. These bright red crabs usually start moving quickly along the bottom to escape the beam. Most often I've interrupted their munching behavior. Their diet is herbivorous and they favor the giant kelp (Macrocystis) and feather boa kelp (Egregia). Hmmm... I wonder if they would have liked the kelp muffins I used to bake in the 1970s? They use their claws to rip and tear mouthfuls of these seaweeds, and to defend against potential predators... and unsuspecting videographers.

They seem to spend almost equal time doing the other "M" word based on my observations. Males are about twice the size of females, which is counter to the rule that larger females produce more young. This suggests to me that they either have very small eggs (which are carried under their tail-like telsons) or just do not have to produce as many. I have seen these kelp crabs mating both within small "caves" in the rocky reef where they might be protected from some predators during this vulnerable period, as well as high up on the giant kelp stipes which may also allow for added safety high in the canopy of the kelp "forest."

One night I observed a larger male holding onto his dainty lady by her claw. He was not very gentle with her, dragging her along as he tried to escape from my video light beam. Now I think she had a good case for spousal abuse. However, as I watched, a large moray came darting out of that cave and undoubtedly would have enjoyed this two course meal had they remained stationary. Perhaps this male actually was a red knight, saving his lady in distress.

Years ago I remember watching a large moray go through the maze in the rocky reef. It entered a small cavity. The moray kelp hammering into the cavity, trying to reach further into it, and I could not see what it was after. I looked around and found a tiny slit I could just shine my video light through, but could not film what was going on. There in the cavity was an octopus trying to fend off the morays gnashing teeth. A few of its tentacles (actually referred to as arms) were holding a southern kelp crab in front of it like a shield. Now that would have been a gourmet meal had the moray succeeded, but it gave up and suddenly burst out of the reef scaring my poor dive buddy, Gary Garibaldi!

© 2010 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 DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Images of the southern kelp crab at night; male dragging his fair damsel out of cave, and
comparison of carapaces of the southern and "northern" kelp crabs.

This document maintained by Dr. Bill Bushing.
Material and images © 2010 Star Thrower Educational Multimedia