Dive Dry with Dr. Bill

032: Navanax

A few months ago I invited my friend Sheena, who was visiting the island, up to my house to watch some exciting footage I had taken in the Dive Park that day. I thought she'd really be interested in seeing what I thought was a beautiful type of opisthobranch snail. I had forgotten that we marine biologists have slightly distorted views on what constitutes beauty (at least in the undersea world). As she and Iris watched the videotape, both responded with "yuck.... gross... disgusting." Perhaps they saw something I didn't.

Now to most biologists, the opisthobranchs are extremely beautiful. Some scientists have even called them the most beautiful group of animals on the planet, surpassing even the birds and butterflies. I've already written about one type in the Dive Park, the nudibranchs, in a previous column. Today's article is about one of the most vicious predators in the sea, their relative the cephalaspidean Navanax. Yes, I've finally had to resort to some scientific words since this group and species have no common names. You'll just have to accept the fact that every profession has its own special terminology and lingo... from marine biologists to plumbers and electricians to psychiatrists.

The opisthobranchs, especially the nudibranchs, are often brightly colored and very attractive (at least to marine biologists). The beautiful coloration and patterns, unique to each species, are an example of warning coloration telling predators the animal tastes bad or stings. "In the business," we refer to this as an aposematic warning which is a term I've only used once in my life... now, to impress the female readers of this column!

Unlike the shelled snails we all know well (including our common garden snail), opisthobranchs either have greatly reduced shells or none at all. Although the nudibranchs have no shell, Navanax actually has a small internal one. It is difficult to discern since the beautifully colored soft mantle of the body covers it. There are two color phases of this species: both are dark bodied with yellow stripes but one has bold yellow and blue spots while the other has many, much smaller spots on its body. The one in these pictures is the latter.

This species is found from Monterey Bay south to Laguna Manuela, Mexico and throughout the Gulf of California. It generally prefers mudflats, eelgrass beds and sandy bays over rocky reef areas. It feeds on other snails, especially some of its close relatives and is even cannibalistic on members of its own species. To do so, it uses special sensory structures on either side of the mouth to detect sugars in the slime trails left by its prey species. It is very fast and when it catches up to its prey, it swallows it whole. If it eats another opisthobranch with a shell, the shell passes through the digestive tract and is defecated unbroken.

Now that we've discussed the second most important function of any species, I can't end this article without mention of the first! Reproduction in Navanax, as in its relatives the nudibranchs, is quite interesting. Each individual has male and female sex organs. While an individual cannot fertilize itself, it can mate with any other Navanax it encounters along the way. Of course this is a great reproductive strategy since it doubles the probability of successful mating. Since there are only two real color variants of this species, they probably don't spend a lot of time evaluating individual mates. Hmmm... perhaps Henry Ford's philosophy was right... you can have whatever mate you want as long as (s)he has black hair (and that's all right with me).

© 2003 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.

Image caption: Navanax: one of the most vicious predators in the undersea world on the prowl

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia