Daunting densities in undersea scum.

Each Dec. 31, people cram into New York City's Times Square to hail the coming of the new year. But this crowd pales in comparison to the one that lives in decaying surf grass off Southern California. Tides, currents, and wind wash grass and kelp into the underwater canyon there, where these plants form a green-brown mat, says Eric W. Vetter of the Scripps Institution of Oceanography.

While scuba diving, Vetter discovered teeming populations of leptostracans -- small, shrimplike invertebrates -- and amphipods in this detritus. His spring sampling indicates that some 690,000 leptostracans and 780,000 amphipods, of a total of 3 million crustaceans, crowd into each square meter. That degree of crowding far exceeds any known density of macroscopic animals, he notes.

The gut contents of fish hovering near these mats indicate that the tiny creatures, whose biomass exceeds 1 kilogram per square meter, are an important food supply, Vetter reports in the Nov. 3 NATURE. He suspects many coastal fish rely on similar nutritionally rich hot spots and hopes his find will prompt other researchers to investigate decaying grass mats.