Ethological observations on foraging behavior of the ctenophore Leucothea sp. in the open sea1

Of all the Leucothea seen during daylight dives in the Catalina Channel, 45% were between the depths of 18 and 22 m, usually feeding at the thermocline on a layer of copepods, primarily Clausocalanus. Undisturbed individuals were observed and video recorded for periods up to 40 min. The ctenophores actively selected specific depths for foraging and spent about 60% of their time foraging horizontally in the water column, 20% of their time swimming rapidly either directly upward or downward, and 20% of their time immobile and neutrally buoyant. The extent and importance of long-distance horizontal foraging activity of zooplankton and the relationship of individual behavior of zooplankton to patch dynamics has been underestimated. The discipline of ethology is concerned with the natural behavior of individual animals in their own environments (Eisner and Wilson 1975). Although an ethological approach to understanding the behavior of wild animals is now routine for most of the environments on the surface of the earth, the use of ethological techniques in the open sea is rare. At sea, as distinct from all other habitats, it is generally impossible to return day after day, and many times not even hour after hour, and encounter the same individual animal or even the same species that was present previously. Students of the natural behavior of oceanic animals therefore have been concerned instead almost exclusively with the statistical behavior of populations of animals in space and time. As a consequence, investigations of statistically defined phenomena such as vertical diurnal migrations of plankton populations or horizontal migrations of populations of fish dominate the oceanic behavioral literature. Yet in selected instances behavioral investigations of individual oceanic animals have been possible. For example, the research by Carey and Robison (198 1) utilized acoustic tagging techniques to follow highly motile individual swordfish for many days at sea, and Klimley and Brown (1983) repeatedly located and photographed schooling individual hammerhead sharks near a specific