Counting falling starfish in California.

The batstar starfish is usually so abundant along the coast of California that in the northern regions it's been dubbed the "junkfish." But starting in 1978, a number of marine scientists began to notice that the batstar and other starfish species in southern California waters were dying off in droves. An epidemic disease ravaging the starfish population caused lesions to form on the animals' backs and rays, exposing the inner organs and making the bodies literally fall apart within days.

"You could swim along the bottom and just see bodies lying all over," says marine ecologist John D. Dixon. "It was kind of eerie."

Dixon and Stephen C. Schroeter, both in the Department of Biological Sciences at the University of Southern California in Los Angeles, recently documented the starfish's demise. They estimate that the batstar population density plunged from 1 to 2 per square meter in 1980 to very close to zero in May 1984 in three study areas. This year, the starfish seem to be making a comeback, but scientists don't know why and are still uncertain as to what caused the epidemic. Preliminary experiments conducted several years ago by microbiologist Kenneth H. Nealson at Scripps Institution of Oceanography in La Jolla, Calif., suggest that the villain may be a bacterium.

Whatever the cause, Dixon and Schroeter have discovered that the disease appears to work its worst in warm waters. Starfish were wiped out in the warmer coastal waters at and to the south of Santa Barbara. Moreover, the malady hit hardest during the summer and early fall, when the sea is the warmest. Fatalities were greatest between 1981 and 1983, when the El Nino episode warmed the Pacific. In a series of experiments, Dixon and Schroeter also found that a sick starfish would heal when it was placed in cold water.

The researchers observed that healthy animals became ill more rapidly when caged with a sick starfish than when caged alone. They suspect that the disease spreads faster in such close conditions because the healthy animals eat the sickly ones. Batstars, while aggressive, are normally not thought to be cannibalistic. The scientists suggest that the diseased animals are mistaken for prey when they become lethargic and are unable to protect themselves from healthy animals. Dixon hopes to test this idea by seeing what develops between two healthy starfish when one is sedated.

According to Dixon, diseases have driven a number of echinoderm species practically to extinction in the last few years. While this may be a new phenomenon, he says it's more likely that such epidemics have consumed marine populations periodically for a long time, and that researchers, who weren't able to study seafloor ecology until scuba gear was made available in the 1950s, are just now noticing them.

Dixon and Schroeter note that catastrophic epidemics provide an unusual opportunity to study the population dynamics of marine animals and the effects on an ecological system of removing one species. The researchers suspect, for example, that the drop in the number of starfish--which are known to have a taste for white sea urchins--may be related to an observed increase in urchins, which in turn have increased their grazing of kelp forests off northern San Diego County. But the link between starfish and urchin populations is still hypothetical at best.