Paint additive hammers coral.

Ocean corals around the world are ill or dead for reasons that remain mysterious. One of the first signs of sickness is bleaching, in which reef-building animals lose the symbiotic algae that give them color and nutrients (SN: 1/30/99, p. 72). New laboratory experiments indicate that one contributor to coral decline may be the paint additive tributyl tin (TBT).

For decades, ships throughout the world relied on TBT to limit the growth of drag-inducing barnacles, algae, and other organisms on their hulls. Although the United States is among nations that have banned the toxic additive, many foreign ships still ply the seas wearing TBT-impregnated skins, notes Jai Dwivedi of St. John's University in Jamaica, N.Y. He adds that such marine paints are designed to slowly leach the pesticidal additive from their hulls.

When those ships run aground on coral reefs, says Dwivedi, they leave paint debris. Reports of high concentrations of TBT in sediment 8 years after the 1992 grounding of a ship on Australia's Great Barrier Reef prompted his team to investigate the additive's potential impact on coral.

Dwivedi incubated staghorn coral (Acropora semoensis) and its symbiotic algae in aquarium water containing TBT concentrations from 5 to 100 nanograms per liter. Such amounts could easily be encountered at ship-grounding sites, he says.

All treated corals immediately showed a decline in their symbiotic algae. Aquarium tanks with low TBT concentrations lost 30 percent of their algae, while the A. semoensis in tanks with high TBT concentrations lost 80 percent of their algae.

Skin cells of individual corals quickly began sloughing off in the aquariums at all doses, but high TBT doses produced the most severe skin shedding. Underlying muscle cells in the animals also showed signs of damage. Within 12 hours of exposure, animals in tanks with even the lowest TBT concentrations were dead.

Dwivedi's team is now trying to identify biochemical changes in coral tissues chronically exposed to TBT. With such markers, scientists might discern whether coral illnesses such as bleaching are an early indication of TBT poisoning or some other threat.--J.R.