Dive Dry with Dr. Bill

#312: A Commoner... The Kelp Snail

Avalon, along with the rest of the country, is experiencing the effects of a lack of discipline in our financial markets, and in many over-extended consumers. Now I avoided economics class at Harvard, preferring ecology over that subject, so I'm no expert. However, I think it is safe to say you can't get a good feel for our "economy" by studying the buying habits of the upper two percent. Neither Bill Gates nor Warren Buffet could be considered typical of the average consumer (although Buffet is probably much closer to my thinking). Of course as a professional dive bum I'm at the other end of the spectrum, but it is the "typical consumer" that gives you the best picture of the state of the economy. In many ways this is also true when studying the ecological systems of a region

In past columns I've written about a strange behavior some biologists, especially taxonomists (look that up in your Funk & Wagnalls), exhibit with respect to the species found in different ecosystems. I'm referring to their focus on the rare and unusual species, those which may be exceedingly difficult to find and therefore seem to titillate their "collector" mentality when they are able to chalk them off their "life list" of species. I don't keep a "life list," but I'm also interested in the rare and exotic as well, primarily of the human species. But I'll leave that topic to my "tell all" autobiography. In a few more decades I should have enough juicy stories to fill a dozen pages... with pictures as well.

Generally animals that are rare have relatively little impact within an ecosystem. Unless they fulfill some unique and vital role within the biotic community, they just aren't present in numbers sufficient to affect many of the other community members or ecological cycles. To study the dynamics of an ecosystem, one often needs to look closely at the less glamorous members in it. You know, the common species that, often by virtue of sheer numbers, have the greatest impact on the rest of the biotic community.

The huge schools of plankton feeders like blacksmith, or jack mackerel (which are really jacks rather than mackerel) have tremendous impact on local communities. They munch away on the plankton and thin out their numbers (don't worry, we'll make more). In the case of blacksmith, they transport nutrients from open water to the reef as well. After feeding on tiny drifters all day, they retreat to rocky holes and crevices at night. Here they defecate and enrich the reef with nutrients that promote the growth of algae and detritus feeders.

One such common critter found in our local kelp forests is the kelp snail, also called Norris' top snail. Their shells are often spotted high up in the kelp fronds by observant divers. It is said that these snails will start at the base of a kelp plant, by the holdfast, and climb up the stipes to munch on the juicy young upper blades near the canopy. Just like the Flying Wallenda Brothers, they do so without a safety net. When dislodged, they fall through the water column back to the bottom where they start the process all over (assuming no hungry sheephead crunched them on the way down). Escargot anyone?

These snails have reddish-orange to reddish-brown spiral shells up to 2 1/2 inches in diameter. The snail's foot, often seen adhering to the kelp, is bright red. On the underside of the shell, the umbilicus (the region around which the shell spirals) is green in color. The soft body of this gastropod is protected by a horny operculum or "trap door" that covers up the hole when it retreats inside the shell. They are found from the intertidal to depths of about 100 feet from Pt. Conception to Isla Asuncion in Baja.

Although strongly associated with the giant kelp upon which it feeds, these snails are also seen on the southern sea palm kelp and other species. In fact it will feed on most brown algae. Some scientists have asserted that they prefer the feather boa kelp over the deeper water Laminaria kelps, and the broad blades of the bottom-dwelling Laminaria to those of giant kelp, whose most tender blades are in the growth regions near the surface making them more difficult to reach. The abundance of its principle food sources, and its fairly adequate defense mechanisms, may well be reasons for its commonality.

Researchers have detected an unusual behavior here in Catalina waters. These snails, especially those located high up in the giant kelp, move down towards the bottom during the day. It is said that by about 3:00 pm the majority of the individuals will be found on the lower portions of the giant kelp. Then in late afternoon, especially after dusk, the snails begin moving back towards the top of the kelp. Before midnight they are largely concentrated in the upper 2/3rds of the kelp forest.

Now I'm going to go out on a limb... er, kelp frond... and suggest a hypothesis that might explain this. During daylight these snails may be more visible to predators like the California sheephead when they are high in the kelp. Moving down probably brings them into areas where the sheephead don't find them as easily. The sheephead is a strictly diurnal fish, like most divers. At night it seeks the shelter of the rocky reef and its protective crevices and holes where it sleeps until just before daylight. While their predators sleep, it is safe for the snails to crawl back into the upper kelp plant without fear of being crushed by the "jaws of death."

Certainly divers are no different from some of the scientists I mentioned at the beginning of this column. They also seek the rare and unusual. After all, we're only human. They spend their bottom time looking for the spectacular including this snail's relatives, the colorful nudibranchs. But if you really want to comprehend the dynamics and functioning of your local ecosystem, spend some time with the "commoners." Next time you dive, hover close to a Norris' top snail and watch it closely to better understand this rather influential member of our incredible kelp forests. If you do that on a night dive, the lobsters will be glad you did!

© 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 DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

Two kelp snails piggy-backing, kelp snail on kelp showing orange foot; snail emerging from
shell showing operculum, scar on sea palm kelp left by feeding kelp snail

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