Krill do not feed by passive, continuous filtration but use area-intensive searching and various rapid feeding behaviors to exploit local high food concentrations. Chemicals alone at low concentrations, not particles, trigger feeding. Krill form dense schools that move rapidly and migrate primarily horizontally. Abrupt disruption of a school can trigger mass molting, and molts may act as decoys.

Observations by SCUBA divers on the distribution and biology of gelatinous zooplankton have stimulated speculations about the structure of tropical oceanic ecosystems. The gelatinous group represents one of four apparent strategies for survival in pelagic animals. Conventional plankton collection probably does not sample these organisms accurately due to their patchy distribution, fragility, and escape responses. Many gelatinous plankters filter feed using mucous structures; these mechanisms are important because of their efficiency in collecting particulate material, and because mucus is a source of organic aggregates in the sea. Such aggregates are often large, irregularly distributed, and of complex composition; these properties are rarely discerned by conventional sampling gear. The aggregates contribute considerable spatial heterogeneity to a seemingly homogeneous environment. An entire category of pelagic animals lives in association with these floating substrates. The diversity and trophic complexity of epipelagic plankton communities have been underestimated by previous investigators.

Eil Malk Jellyfish Lake in Palau is a 5-ha, 30-m deep, meromictic saline lake, stratified, with anoxic hypolimnion, damped, delayed tides, and no heat accumulation. The top 15 m are oxygenated, nutrient-poor but turbid, with high primary productivity and large planktonic populations of only five metazoans: the medusae Mastigias and Aurelia, the copepods Oithonu. and Ac~ocuZu~s, and silverside fish, Prunesus. At the chemocline a floating bacterial plate dominated by photosynthetic purple bacteria (Chromatium) degrades and solubilizes almost all particulate material and absorbs all sunlight. Below the plate the anoxic, clear water contains extremely high levels of NH,+, H,S, and PO,"-. Sediments are primarily undisturbed fish fecal pellets full of copepod exoskeletons.The lake is stable, aseasonal, and shows no obvious cycles of the populations.An unusual horizontal and vertical swimmingbehavior of Mastigias appears related to light and to the nutrient requirements of its symbiotic zooxanthellae.Other zooplankton show normal vertical diurnal migration patterns. Facultative schooling by Prunesus reflects the absence of predators in the lake.

The filtration rates of Pegea confederata, Salpa maxima, and Cyclosalpa a;ffinis were measured. Filtration rates increase exponentially with increasing length and body carbon. Filtration rates exceeding 100 ml min-' were recorded. The mucous net of P. confederata can retain particles at least as small as 0.7 P. There appears to be no change in filtration rate with changes in particle concentration. Filtration rate per microgram of carbon increases with increasing body weight of salps in contrast to ascidians, perhaps because salps use muscular pumping rather than ciliary currents to transport water.The filtration rates recorded suggest that the role of salps in the open ocean ecosystem may be more important than previously assumed.

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