Dive Dry with Dr. Bill

#252: The Munchability of Mullineri

Shakespeare had Juliet proclaim "a rose by any other name would smell as sweet." Eluard said "a fish by any other name would still know the sea." Of course the meaning of these phrases is simply that the name we place on a thing, living or inanimate, usually is simply a label we apply for our own purposes of communication. The plant we call a rose knows nothing of this name, nor do our fine finny friends realize we call them fish. They prefer their real names like Tony, Mary or Bubba. Thankfully the Discovery Tours boat glassbottom boat and semi-submersible tour personnel realize this when they talk about the marine life in Lover's Cove!

The names humans apply to the rest of Earth's residents are simply words we use to label them, and facilitate our communication... not theirs. After all, what self-respecting red sea urchin would come up with a label like Strongylocentrotus franciscanus? I rest my case. However, we humans seem to have an overwhelming need to come up with labels. Scientists refer to "undiscovered" species which have yet to be given a name, even though members of that species (as well as their predators... and prey) have been well aware of them for millennia.

I bring this up because the subject of this week's column is the nudibranch Peltodoris mullineri. The last time I mentioned this species in my column, it was referred to in the field guides as an undescribed species. I assume this was because it is not a common form of shell-less snail. In fact, at that time I had only seen it once that I remember... in Catalina's Casino Point Dive Park. I have seen it a few more times since then, most recently at Middle Reef off Catalina's Isthmus. The most recent edition of the same field guide now lists it by the name it was given in 2000. Does its scientific name reflect anything intrinsic in this snail? The Greek word pelto refers to a shield and dorids are the type of nudibranch this species belongs to, so this has some relevance. Its species name comes from a nudibranch specialist, Dave Mulliner. I doubt our sea slug has ever heard of this fellow.

Despite all this, I now have a name I can use to communicate with other scientists about sightings of this species. As far as communicating with the rest of you, there is no common name for this critter. It just isn't common enough to have one. You'll just have to use the scientific name like us scientists. Keep in mind that there are a lot of technical terms used by plumbers, electricians, architects or financial advisers that I have no clue about. You know, words like "profit" and "income." So now that our nudie has a name, let's talk about it.

One of my favorite field guides to these critters, Eastern Pacific Nudibranchs by David Behrens and Alicia Hermosillo, says this species is now known from Anacapa in the northern Channel Islands to Isla de Malpelo, Columbia. Its depth range is not listed, but I tend to find them in shallow water above 30 feet. Body coloration ranges from a creamy yellow to orange, although I've only seen the former color variant. The dorsal surface is covered with irregular blotches that remind me of melanomas, but these critters spend no time in the direct sun unless some collector removes them from the water. I take only pictures. The gills and rhinophores, the sensory appendages on the head, are also yellowish.

Originally this nudibranch was reported to feed on sponges. The first time I observed one, it was on the Casino breakwater rocks near several different types of sponges. However, more recent descriptions suggest that scientists really don't know what they feed on. However, I'm sure the nudibranchs do... they're just not telling! As for what eats them, I doubt any predator specializes on munching mullineri's. They just don't seem to be common enough to provide a steady diet for any marine critter. Who would want to spend hours and hours (or days and days) searching for one of these when there are abundant resources in the form of more common species?

It has long been said that nudibranchs have several defense mechanisms against predators. One of the suggested defenses is that they taste terrible. My marine biologist icon, "Doc" Ricketts of Cannery Row fame, actually tested this theory by tasting one... and concurred! However, I do have my own theory about this. Divers often question why Catalina waters have so few nudibranchs. They are much more common in mainland waters than here. My observations while diving between the recreational depth limit (130 feet) and my maximum depth of 200 feet have indicated that nudibranchs are actually very abundant in deeper water off Catalina.

Now deeper, colder water may be more nutrient rich, thus providing food for the critters like hydroids, bryozoa and sponges that many nudibranchs munch on. But there is another possible explanation. There are relatively few predatory fish like sheephead down at these depths, compared to the shallower waters where they are common in our kelp forests and sandy bottoms. Due to serious overfishing, pollution or other human causes, there are also relatively few sheephead in mainland waters compared to Catalina.

I believe sheephead and other predatory fish are not beneath munching on a few nudibranchs. Perhaps they see the bitter taste reported by "Doc" Ricketts as just adding a little spice to their diet! After all, our taste buds and theirs undoubtedly differ. I have seen garibaldi and sheephead grab nudibranchs, and rarely have they been spit out as unpalatable. In fact, a curious sheephead closed in on the nudibranch featured in this column... until I chased him away. I hate it when a sheephead or garibaldi munches my subjects as I'm filming them. Hmmm... this gives me an idea. Perhaps I should write a big budget grant to scientifically research and test this hypothesis about the "munchability" of nudibranchs.

© 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 Peltodoris mullineri showing body coloration, ring of external or "naked" gills;
the snail's foot on the underside, a sheephead looking to add some spice in his life.

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