Dive Dry with Dr. Bill

003: Nudibranchs

Nudibranchs are creatures with a special fascination for many marine biologists. The name means "naked gills" and you can see the many exposed gills looking like a shag carpet in this week's photo. Unlike humans who draw air into their lungs, many marine species have their breathing structures outside where oxygen in the water can easily pass over them.

Although often incredibly beautiful, they really are vicious predators... at least to the tiny hydroids, bryozoa, sponges and other invertebrates they feast on. Imagine being faced underwater with a predator that was one hundred times your size! With a rasp-like tongue they scrape the flesh off these small defenseless animals.

Nudibranchs actually are shell-less snails with no obvious defenses. They often are brightly colored and easy to spot, causing many to ask why fish don't eat them. Many species actually incorporate the stinging cells or nematocysts from the hydroids (jellyfish relatives) they eat into their gills where they are still capable of stinging. These then become their defense against most potential predators. Other species are said to taste bitter, and this serves as their defense just like Monarch butterflies protect themselves from bird predators. My icon, marine biologist Ed "Doc" Ricketts, once tasted a nudibranch and verified this fact for himself!

As I mentioned in a previous article, eating and reproducing are the two key functions of any species. Nudibranchs have a rather strange and interesting sex life. Each individual is both male and female. When two nudibranchs mate, the male part of one fertilizes the female part of the other and vice-versa. Of course there are many benefits to this strategy, known as hermaphrodism. If you encounter another nudibranch in the sea, you know you're compatible mates! I once saw several thousand lion nudibranchs mating simultaneous in the kelp off Catalina's East End. Kind of reminded me of the love-ins from the 60's (which I can't really remember because I was there... strictly as an observer of course!).

© 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: Beautiful Spanish shawl nudibranch in search of prey

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