Dive Dry with Dr. Bill

#201: Nothing Could be Fina'
than Tylodina fungina

There are some scientific names that I just enjoy saying in my warped Latin. The first name that I felt this way about was Strongylocentrotus droebachiensis, the scientific name for the green sea urchin that I used to see on New England's rocky shores in my Harvard days. At the time it was the longest scientific name in existence, but just rolled off my tongue. When I moved out to Catalina, I acquired a few more including Melibe leonina, the lion nudibranch. The subject of today's column is a third such name... Tylodina fungina. Why, "nothing could be fina than" to say Tylodina fungina in the morning. It is said the species name, fungina, comes from its fungus- or mushroom-like shape. Even engaging my most creative imagination, I don't really see this resemblance.

I was not aware of a common name for this snail species. While researching on the Internet (thank you classmate Al Gore), I found several: the mushroom sidegill, the yellow umbrella snail and the yellow snot snail. None of them sound near as pleasing as its scientific name, so I'll stick with that... which means you are stuck with it too. The first common name comes from its evolutionary grouping with the sidegilled sea slugs, which as their name implies have gills along the side of their body.

Perhaps the lack of a consistent common name is because this snail just isn't that common in our waters. It is considered rare in southern California although its range extends from Cayucos and the northern Channel Islands through Mexican waters down to the Galapagos. In addition to being rare, it is also relatively small. Most of the specimens I observe in our waters are between 1/2" and 3/4" long. Their bodies are a bright yellow color and that alone should make them easy to spot. However, they feed on the yellow sulfur sponge and are well hidden when munching away on top of their prey. I have tried several times to film them feeding on this sponge, but my aging camera has lost the ability to close focus on such small subjects. Come to think of it, so have my own aging eyes. Gulp, time for a prescription mask?

A week ago I was completing a deep dive at Sea Fan Grotto. I had descended to 180 feet looking for a yellow nudibranch, but didn't see any at that depth (don't try this at home or anywhere else for that matter). I turned around (on a dime) and began ascending to the relative safety of the shallows. I was disappointed that I wasn't seeing any of "the usual suspects" I've enjoyed filming at these "sub-recreational" diving depths. However, by going deep, I consumed more air and therefore had less remaining bottom time to find and film other critters. Since my experience allows me to manage the gas in my SCUBA tank carefully, I ended up doing a one hour dive with lots of time in the shallows to "off gas" the excess nitrogen acquired at depth... and find some "shallow" critters.

Tylodina looks like a limpet snail since it has a small brownish cap or "shell" on top of its soft body. Don't let looks fool you... this snail is more closely related to the sea hares which include the largest known marine snail, the black sea hare. At a size rarely exceeding 1", this tiny snail could hardly hold a candle to its gargantuan relative which can exceed three feet in length and 30 pounds in weight. Divers often think it is a nudibranch, but it is not. It is in the opisthobranchs, the same larger group as the nudibranchs, but it is just related to them.

Now, I've never observed this species mating, but I have often found it munching. I guess its priorities are the opposite of mine, since I already munch too frequently. Whenever I see a cluster of yellow sulfur sponges on a rock, I look carefully to see if there are any Tylodinas feeding on it. My patience is rewarded often enough that I do keep looking (positive reinforcement sure helps). By feeding, the snail excavates depressions into the sponge mass that fit its foot quite well.

Tylodina's yellow color is an example of protective coloration, where an animal closely matches the color of its habitat. However, their yellow color probably comes from a nearly exclusive diet of yellow sponges. Once a Tylodina has eaten all of the yellow sponge, it may stand out like a sore thumb. It was this color that caught my attention as I was leisurely doing my safety stop in the shallow waters of this dive site. I passed over the boulder bottom and immediately saw one of the biggest Tylodinas I'd ever seen... easily 1 1/2 inches. This behemoth must have eaten a lot of sponges... good thing Sponge Bob Square Pants is probably a synthetic rather than living sponge (sorry to disappoint you kids... and kids at heart).

Through my research I discovered that a diver I know found one twice as big (3" long) as this giant. He located it at Shaw's Cove in Laguna Beach and said there was lots of its only food present there. A researcher in Australia indicated that divers occasionally see "giants" of a species of Tylodina found there as well. Never knew sponges were so good for your health. I think I'll stick to my daily pan dulce and an occasional steak... I'm already enough of a "giant" myself!

© 2006 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" or "Gentle Giants: Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Two views of the large (1 1/2") Tylodina fungina snail I discovered; its only food item, the yellow sulfur sponge;
and a Tylodina (inside oval) feeding on a sulfur sponge.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia