SOME SEA TURTLES ARE GLASS EATERS

by Whit Gibbons

December 26, 2004

A new book just hit the market—Sea Turtles: A Complete Guide to Their Biology, Behavior, and Conservation by James R. Spotila, published by Johns Hopkins University Press (2004). If you did not get a copy during the holidays, get one now. The price is $24.95, for 240 pages and more than 110 color photos that include many awesome sea turtle pictures.

Most of the seven kinds of sea turtles in the world today are in big trouble. The book reveals what is known about their ecology and behavior and focuses on how the information can be applied to develop prudent conservation programs. The biggest problem in dealing with some endangered species is our ignorance of their ecology. Often we know so little about an organism's relationship with its environment that we are uncertain about why the species declined in a region or throughout its range. Special measures taken to save them may then be too late. This has happened many times throughout the world, as species become rare or extinct. Sometimes we lose a species without ever finding out for certain what its environmental role or contribution may have been.

Human impacts inflicted on most species of sea turtles are not sustainable. With sea turtles, most of us are aware of problems on beaches, where efforts by females to lay eggs are thwarted by resort developments and by people who collect the eggs for food. Thus, environmentalists have taken many measures to protect sea turtles during the nesting period. However, sea turtles spend almost all of their time in the ocean itself, a part of their ecology about which we know relatively little. One of the biologists mentioned in Sea Turtles, Anne Meylan, is a sea turtle expert I have long admired. Her research has given insight into what at least one species of sea turtle does at the bottom of the sea, and how human activities could influence its survival.

The hawksbill turtle is an endangered species that has suffered more than most sea turtles as a result of human commerce. Adult hawksbills are at risk because of their shells, which are quite beautiful. Hawksbills are the source of the highly prized "tortoiseshell." Thus, the species has been collected for commercial purposes, greatly depleting the populations in certain regions. Although it is illegal to bring tortoiseshell or any part of a hawksbill into the United States, commercial markets are still open in other parts of the world. With the double impact of destruction of eggs and adults, hawksbills don't need any other problems. But Anne Meylan's fascinating studies suggest that a specialized diet may be one more form of jeopardy for the species.

Hawksbill turtles predominantly eat sponges that grow on coral reefs. Sponges eaten by these turtles contain silica, in essence glass. The amount of silica consumed by hawksbills is higher than that in the diet of any other vertebrate. In addition, some sponges are toxic to other animals that attempt to eat them. However, an intestine full of glass spicules is no problem for a hawksbill. They thrive on this natural food source inedible to most other animals. Also, they serve the important role of opening up coral areas by removing or breaking sponges and thus exposing food for fish and other creatures on the ocean floor. A major threat to the hawksbill is the worldwide pollution or physical destruction of coral reefs where sponges live. The ocean ecosystem is enormous, but the parts are held in a very delicate and fragile balance. An effect on one part, such as disruption of a coral reef, can significantly impact organisms that depend on that part for survival.

The plight of the hawksbill should be taken as one more signal that we humans are having an adverse planetwide influence on many plants, animals, and ecosystems that may never be able to recover. The triple menace of egg destruction, tortoiseshell collection, and pollution of coral reefs is clearly a threat to the hawksbill. Jim Spotila's book makes it clear we need to be attentive to perils faced by the other sea turtles as well.



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