Echinoderm Defenses
Posted April 1, 1998
Echinoderms are eaten by crustaceans, gastropods, and fishes. How do echinoderms protect themselves from predation? When you look at a sea urchin, it may seem obvious
that its spines are features that keep sea urchins safe from some predators. Starfishes and brittle stars have calcium deposits under their skin and thick body walls that provide defense
against potential predators. Pincerlike appendages called pedicellarias also aid in the defense of starfishes and sea urchins. Sea cucumbers have no calcium deposits, but they are protected
by thick body wall tissues.
Although these physical defenses work well enough to deter some predators, echinoderms also have chemical defenses. Some echinoderm species have large pedicellarias
that release poisons into unwary predators. The body walls of most echinoderms also contain saponins, toxic glucosides that produce a soapy lather. Sea cucumbers have more chemical defenses
than other echinoderms, including unique saponins that are extremely toxic. Perhaps this is why sea cucumbers have few natural predators.
Scientists have been studying the chemical defenses of echinoderms to identify what types of defenses protect them against certain predators. In a recent study, body
wall sections of a variety of echinoderms were fed to fishes and crabs, common starfish predators. Both types of predators ate the sections with no hesitation. This study showed that
the amount of calcium and the thickness of the body wall were not in themselves enough to deter predators of echinoderms. But when scientists made food pellets of chemicals extracted
from the body wall tissues of echinoderms and fed these to fishes and crabs, the fishes rejected them. Crabs, however, ate the pellets. Scientists hypothesize that the chemical defenses
of echinoderms are effective against predatory fishes, whereas predatory crabs may be able to tolerate, or deactivate, these chemicals.
Meanwhile, other scientists are studying the chemicals of echinoderms for other properties. Chemicals from one species of starfish have been found to inhibit bacterial
growth and inhibit the settlement of barnacle larvae on a substrate. Researchers continue to investigate the bioactivity of echinoderm chemicals, screening them for their antifungal,
antibacterial, antiviral, and anti-inflammatory properties.
References
Bryan, Patrick J., James B. McClintock, Thomas S. Hopkins. "Structural and chemical defenses of echinoderms from the northern Gulf of Mexico," Journal of Experimental
Marine Biology and Ecology, 210 (2), March 1, 1997, pp. 173-186.
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