Diatoms Are Copepod Contraceptive

For the ocean's tiny grazing animals, the microscopic algae called diatoms are a favorite food--and like most delicacies, they are turning out to be hazardous. A new study shows that at least two diatom species make compounds that reduce hatching rates when eaten by tiny shrimplike animals called copepods. When diatoms bloom, they can impede copepod reproduction and may even disrupt the marine food chain, the study suggests.

Ecologist Adrianna Ianora and neurobiologist Antonio Miralto of the Naples Zoological Station first noticed a diatom-related drop in copepod hatching rates in laboratory studies. Then in 1997 and 1998, they and their colleagues sampled copepods in the Adriatic Sea during diatom blooms in winter, when the copepods feast primarily on diatoms, and during the summer, when diatom numbers are down and copepods eat a more mixed diet that includes other algae. The copepods produced more eggs during the blooms, but fewer than one quarter of those eggs hatched. In contrast, 90% of the eggs collected in June hatched.

Back in the laboratory, the group isolated three compounds called aldehydes produced by the diatoms. Even at very low concentrations, they report in today's Nature, the aldehydes inhibited hatching of copepod eggs.

Other researchers have shown that some terrestrial plants make compounds that inhibit reproduction in herbivores, but "this type of interaction was not known for marine environments," says Ianora. "The bottom line is these diatoms are negative for the copepods," agrees Gustav Paffenhofer, an oceanographer at the Skidaway Institute of Oceanography in Savannah, Georgia. But he notes that others have not seen as dramatic a decrease in copepod reproductive success during diatom blooms elsewhere and warns that the Adriatic's heavy pollution may have enhanced the effect.