Shells and their Predators

by Lucy Kile

The Guam Shell Club had a most interesting guest speaker on April 3, 1979. Dr. Gary Vermeij, an evolutionary biologist from the University of Maryland, spoke on mollusks and their predators. He showed examples to illustrate the different ways predators kill shells.

The first category includes predators that swallow shells whole. This form of predator is relatively rare in tropical waters.

A category that is more commonly seen on Guam is the group of predators that ingest the soft parts without harming the shell. Cones, thaids, and cymatiums are examples of predators that do just that.

Most of us have found shells on the beach or in the ocean that were seemingly in perfect shape, except for a tiny hole. Moon snails, murex, and octopus are some predators that use radula and a secreted enzyme to drill a hole and reach the soft parts. This method is slow and rather inefficient. It is most prevalent in the grassy and sandy areas. There are some types of shells that one seldom, if ever, sees drilled. Cowries, drupas, or cones are probably immune because of the thickness of their shells. Strombus can make their escape by simply "jumping" away using their muscular foot.

The final kind of predator is distinctly tropical in distribution. It crushes shells, resulting in many of the colorful fragments found abundantly on Guam. Some examples of shell crushers are puffer fish with a platelike mouth, wrasse with teeth in their throat, crabs often with incredible power in their claws, and mantis shrimp which hammer shells. The shells have made many adaptations to survive the crushes. Thicker shells, lower whorls, and growing spines and knobs are some ways. The shape of the aperture may also make a difference. Note: Terebra (Augers) with their long spiral shells pull back inside so if the end of the shell is crushed, it often escapes destruction.

After hearing Dr. Vermeij's talk, I find myself more conscious for the structure of the shells and noticing things I'd previously overlooked. Now when I find a Terebra with the top of the shell broken, but still living, I wonder what kind of predator he has escaped from and I find myself searching dead shells for marks, indicating what may have killed them.

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Additional Reading

Vermeij. Geerat J. 1995. A Natural History of Shells. Princeton University Press, Princeton, New Jersey.
In particular, refer to Chapter 5 "Predators and their Methods" and Chapter 6 "Coping with Enemies: The Shells as Protection."