Dive Dry with Dr. Bill

#268: Just Where is That Nudi Beach?

Many of my mainland dive friends complain about the lack of nudis in Catalina waters. Now I'm not referring to the lovely young ladies who might be sunbathing in the buff at the more secluded beaches around the island. I'm talking about my favorite group of snails... the shell-less ones known as nudibranchs due to their exposed or "naked" gills. It has been my hypothesis that nudibranchs are rarer at recreational diving depths (above 130 feet) in our waters because we have higher densities of fish here. Despite what the scientists say about nudibranchs tasting so bad they won't be eaten, I think a hungry sheephead will try a few. Heck, you ought to see what I eat in my kitchen.

One of the reasons I continue my deep dives around Catalina is that is where I see nudis in our waters. Sometimes the bottom may be carpeted with them at certain sites in the 130-160 ft range or deeper. I'm going to focus today on the two closely related species that I most commonly see at these depths. The red-tipped dorid (Acanthodoris rhodoceras) and the Hudson's dorid (Acanthodoris hudsoni) are two fairly similar looking nudis I encounter. Both barely reach an inch in length and have white to greyish bodies, but they differ somewhat in the details. The red-tipped dorid has red-and-black tipped rhinophores, or sensory structures on the head, and gills; while those of the Hudson's dorid are tipped with yellow. The "bumps" or papillae on the dorsal (upper) surface are tipped in black on the former and with yellow on the latter.

The Hudson's dorid is known from as far north as Alaska to its southernmost distribution here on Catalina and off San Diego. The red-tipped dorid has a slightly more southern distribution from Oregon to Bahia Magdalena, Baja California. Both feed on tiny colonies of animals known as bryozoa.

I often see these two species in mating position (see image). You may remember that nudibranchs are almost all simultaneous hermaphrodites, each having both male and female sex organs in their mature state. Despite this, they cannot fertilize their own eggs and must rely on finding another to mate with. Of course any individual of their own species is a possible mate! Makes Saturday nights much easier. Their sex organs are located on their right side, so they "cozy up" head-to-tail with their right sides pressed close together. Even though they double their pleasure, double their fun while mating, it is highly unlikely that a videographer like myself could make an X-rated movie starring them. Remember... they are snails, and mate at a snail's pace.

So much for mating, now let's look at munching. Nudibranchs have a scraping, tongue-like structure known as a radula that they use to rasp their prey to death. I'm sure it is horrific for the poor little bryozoa, but I can barely observe it myself. I guess I really need to dive with a prescription mask... and maybe a microscope. For every critter that munches, there is the "equal but opposite reaction" of being munched. It is in this arena nudibranchs shine as they focus on "defending their life." These snails have certainly evolved some nifty little mechanisms to prevent becoming a snack for someone else.

Some nudis incorporate toxins or stinging cells from the critters they feed on into their own bodies. They then defend the nudi from being munched. Some nudibranchs I see at depth feed on sponges, which are notorious for creating toxins to prevent critters from munching on them. The nudibranch incorporates these toxins into its body, and fish generally avoid them. Many nudibranchs I see in shallower waters feed on hydroids, which possess the same kinds of stinging cells (nematocysts) as their jellyfish relatives. Fish have been observed swallowing these nudis, and immediately spitting them out as the stinging cells trigger inside their mouth... a little too much "spice" for their tastes. These nudibranchs often have bright coloration and markings, known as warning coloration, to help the fish associate them with the unpalatable taste.

Since both of the featured nudibranchs in this column feed on bryozoa, which lack stinging cells or toxins (to the best of my knowledge) I'm not sure how they defend themselves from predators. They also lack the usual warning coloration of many of their nudi relatives. Of course I see far fewer carnivorous fish at deeper depths, so maybe a good defense isn't as necessary. Unlike my marine biologist icon, Dr. Edward F. "Doc" Ricketts of Cannery Row fame, I've never tried to "taste" one. Besides, my taste buds have been desensitized by decades of eating my own cooking!

© 2008 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 "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!

One of my "high definition" images of the Hudson's dorid mating (upper left) and three of the red-tipped
dorid (right) and Hudon's (lower left) by underwater photographer extraordinaire Ken Kopp. .

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia