Dive Dry with Dr. Bill

115: Leafy Hornmouth

We often think of snails as slow moving vegetarians. While most of them are indeed slow, some prefer a bit more "meaty" diet than your garden plants or seaweed. The leafy hornmouth, a local species of murex, is one such snail. It would rather have a big Mac than a healthy McSalad. Obviously the name "leafy" does not come from its diet! The hornmouth is found along the Pacific coast from Alaska to San Diego in rocky areas and kelp beds, usually in areas exposed to surf. They are less common south of Pt. Conception. It may be seen from the intertidal down to 150 feet.

The hornmouth has a unique shape. Like other snails (gastropods), it has a single shell with an opening within which the snail itself lives. This opening can be sealed use their operculum like a trap door to seal the shell and prevent access to the soft body parts by predators. The hornmouth improves on its defense with three distinctive "leafy" or wing-like extensions on its shell. One is on the top of the shell and the other two on each side. Biologists suggest that these extensions are to ensure the snail lands on its "foot" if dislodged from the rocks. That helps protect it from its predators. The wings also serve to strengthen the shell making it more difficult for predators like sheephead to crush. Some believe the wings are sharp enough to cut the mouth of a fish that might try to eat these snails. It is my belief that the three "wings" also help stabilize the snail on the rocky surfaces by preventing it from tipping in the face of waves and surge.

Shells range from two to nearly four inches with the maximum size occurring at about four years depending on food supply. Of course the very young hornmouths are smaller! After all, humans don't start life at 6 feet tall and hornmouths don't suddenly appear at 2 inches long. The juvenile shells have a different appearance than the adults with the shells exhibiting a gradual transition as the individual ages.

Once they reach maximum size, the shells may start to decrease in length due to the actions of fouling animals and boring sponges which burrow into it. As in all snails, the shells are built of calcium carbonate which is deposited by the animal as it grows. Most larger individuals have a "tooth" or horn at the outer edge of the shell's opening.

In the northern part of its range these snails feed largely on barnacles and small clams. Here in southern California they are believed to feed primarily on tube snails, a gastropod that cements its worm-like shell to a rock and therefore can't move. They use their tooth to bore into the shells of their prey.

When mating time arrives, these snails form breeding groups. All the females in a group will attach their eggs in one large yellow cluster. Each adult female may produce 40 capsules of eggs a year, each containing about 50 eggs. During her lifetime the average female may produce more than 15,000 eggs. Unlike many other species, the larvae produced do not enter the plankton but develop within the egg capsule and emerge as small juvenile snails. This development takes about four months.

Hornmouth snails have a different pattern of movement than many other gastropods. Like their relatives, they use a strong muscular "foot" to crawl along the rock surface. However the foot does not move uniformly. The two sides of the foot move out of synch with one another. Because of this, hornmouths do not have the same ability to grip the rocks tightly and are more susceptible to being dislodged.

© 2004 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.

Images of hornmouth snails showing three wings and operculum.

This document maintained by Dr. Bill Bushing.
Material © 2004 Star Thrower Educational Multimedia