Rogue algae may harm Mediterranean fish.

Since it got loose in the mid-1980s, an aquarium-derived variant of a tropical seaweed has been spreading over the bottom of the Mediterranean Sea like a dense shag carpet. A new study now shows that this alien form of Caulerpa taxifolia may do damage beyond usurping floor space. The alga alters the activity of a fish's chemical-detoxification enzymes, even in fish that don't eat the weed.

This alga is an abnormally large, fast-growing variant of a species found naturally in tropical waters. Unlike the natural Caulerpa, which grows in small clumps and would never survive the Mediterranean's cool winters, the weedy variant has been choking out native flora and fauna (SN: 7/4/98, p. 8). Lack of natural predators in its new home aids this Caulerpa's advance.

Scientists in Montpellier, France, recently teamed up to probe the Mediterranean alga's effect on finfish. They housed locally caught scorpion fish, Scorpaena porcus, in aquariums for 4 weeks. One group of fish had no exposure to the alga. Another group ate mussels that the researchers had injected with ground-up C. taxifolia. The third group shared a tank with the Caulerpa, but not being algae eaters, these fish didn't consume the weed.

At the end of the 4 weeks, the researchers killed the fish and extracted their cytochrome P-450 enzymes, which break down chemicals. Then, they incubated these enzymes with the hormone progesterone. Christian Larroque of INSERM, the French Institute of Health and Medical Research, uses such hormones to assay the activity of an animal's detoxification system.

Exposure to Caulerpa through ingesting the alga or merely sharing a tank altered the fish's P-450 enzyme activity, Larroque's team reports in the May 15 ENVIRONMENTAL SCIENCE & TECHNOLOGY. Enzymes from the exposed fish produced only 25 percent as much of the major breakdown product of progesterone as enzymes from unexposed fish did. On the other hand, the enzymes from exposed fish produced unusually high amounts of a second breakdown product.

"We now plan to cage fish with Caulerpa in the sea to see if they respond the same way," Larroque says.

He adds that it's too early to say how the Caulerpa triggered the changes. However, his team suspects the effects may trace to caulerpenyne, a toxin made by the weed. In preliminary tests, caulerpenyne indeed appeared to inhibit fish P-450 enzymes, Larroque told SCIENCE NEWS.

Though the observed enzyme changes may not be deleterious, they raise a concern that affected animals might not be able to quickly clear a variety of chemicals from the body before they cause toxicity, Larroque notes. It "adds to concerns about what a dastardly weed this is," says Caulerpa biologist James N. Norris of the Smithsonian Institution's National Museum of Natural History in Washington, D.C.

Last October, a group of scientists recommended a ban on the weed's importation into the United States (SN: 11/21/98, p. 332). In record time, the federal government complied. Last month, it listed the alien form of C. taxifolia under the Federal Noxious Weed Act--the first marine plant banned under this law.

Photos of the alien alga have been circulated to agents of the Animal and Plant Health Inspection Service (APHIS). From now on, explains Polly P. Lehtonen of APHIS in Riverdale, Md., any plant labeled as C. taxifolia or resembling it will be barred from entry to the United States, unless experts show it is the noninvasive tropical form of the species.