A sea surprise: some corals dine vegetation.

Ocean dwellers would drive a chef nuts. Certain sponges, once considered the picture of happy herbivores, are actually crustacean-trapping carnivores, researchers found recently (SN: 2/4/95, p.69). Now, scientists say that some species of coral, long considered carnivores, prefer the vegetable plate.

"A . . . soft coral [Dendronephthya hemprichi] from the Red Sea feeds almost exclusively on phytoplankton, a mode of nutrition so far unknown for corals," report Katharina E. Fabricius of the University of Munich in Germany and her colleagues. Most corals survive on zooplankton, which is composed of tiny animals.

The scientists wondered how the common D. hemprichi manages to grow so well. Most corals play host to algae, which provide them with carbon, but D. hemprichi does not. So the researchers assessed the food preferences of this coral in their laboratory, they report in the April 7 Science.

They found that D. hemprichi consumed large quantities of phytoplankton fairly quickly. Indeed, phytoplankton supplied about twice as much carbon in D. hemprichi's diet as zooplankton, which made up less than 1 percent of the food in its gut, says coauthor Yehuda Benayahu of Tel Aviv University in Ramat Aviv, Israel. The scientists now know of three other coral species that dine primarily on phytoplankton, they report.

Corals trap their food on branched tentacles. Soft corals have closely spaced pinnules, or branches, on their tentacles that make them well suited for capturing phytoplankton, which are smaller than zooplankton, the authors note. What's more, soft corals have relatively poorly developed stinging cells on their tentacles, which makes it more difficult for them to prey on animals.

The herbivorous corals probably evolved to take advantage of an abundant food source not tapped by other species, Benayahu says. Researchers now want to find out how widespread this vegetarian movement is among reef dwellers.

In an unpublished study, Benayahu and his colleagues find that D. hemprichi reproduces all year, "which is completely exceptional within the coral reef," he says.