Crown-of-thorns no Johnny-come-lately.

Crown-of-thorns no Johnny-come-lately

The crown-of-thorns starfish made a big splash in the mid-1960s--and another in the early 1980s--when unusually large numbers of the species devastated live coral on Australia's Great Barrier Reef. Because scientists had detected no starfish scourges in Australia before 1962, some blamed pollution, dredging or harvesting of starfish predators. But new evidence indicates the crown-of-thorns and its population surges may go back to the reef's beginning.

Australian and U.S. researchers have turned up spines of Acanthaster planci in ancient reef sediment and dated them to 8,000 years ago, near the time the reef began to form. Moreover, the variable abundance of spines in old sediment suggests a long history of population booms and busts,s ays A.J. Timothy Jull of the University of Arizona in Tucson. "The current increase in the starfish population isn't necessarily due to some man-made event," he says.

Jull used radioactive carbon-14 dating to determine the age of bits of starfish skeleton. His co-workers at the James Cook University of North Queensland in Townsville and the Australian National University in Canberra used a different carbon-14 dating method to correlate reef-sediment depth with its age.

Surface sand at sites of A. planci outbreaks contains abundant starfish spines and skeletal elements called ossicles. At a reef spared of starfish invasions, sand contains virtually no ossicles or spines.

Spines and ossicles also abound in deeper, older sediments, revealing the crown-of-thorns as a longtime resident, the team reports in the Aug. 25 SCIENCE. Jull says the fluctuating numbers of spines and ossicles found in dated sediments indicate crown-of-thorns populations have periodically surged throughout most of the reef's existence.

But sediment mixing by burrowing animals and the possibility that spines "float" upward in sediment lead geochemist Ellen R.M. Druffel of Woods Hole (Mass.) Oceanographic Institution to question the suggestion of ancient outbreaks. While Druffel says "the major premise is probably correct, that in the past these Acanthaster were abundant," she argues that the periodic waxing and waning has yet to be proved.

Jull remains confident of both assertions. "We dated the bits of starfish themselves," he says, "so even if the sediment is totally churned up, you still have bits of starfish going back 8,000 years, showing that these starfish have always been there." And the ages of individual spines found in undisturbed sediments, he says, closely match the dates of those sediments.