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.

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.

Aspects of the behavior of pelagic euphausiids are reviewed, emphasizing the behavioral biology of Antarctic krill (Euphausia superba). Euphausia superba makes an abrupt behavioral transition after the last larval molt from an individualistic, planktonic lifestyle to that of a highly social, nektonic juvenile. Then and throughout the rest of its life, most aspects of krill behavior are expressed within the context of highly organized, polarized schools. A krill school is an integrated whole that displays properties greater than the sum of its parts. Emergent attributes of the aggregate evolve in response to a suite of positive and negative selective factors that act on entire schools as well as individuals. We evaluate the individual advantages and disadvantages of living within large schools of conspecific euphausiids primarily for foraging, antipredator behavior, and epidemiology. We review the history of research on krill behavior, comment on the current state of knowledge, and suggest new avenues for future investigations.

The physical geography of the limestone islands of Palau permits permanent water-column stratification in 13 tropical sea-level marine lakes, each with unique watercolumn physics, chemistry, and biology. Embayments and lagoons amid the coral became isolated as marine lakes after Miocene uplifting. Surface mixing of lake water by wind is reduced by jungle-covered karst ridges. Surface tidal exchange through fissures in fenes trated karst is slow while midwater exchange through submarine tunnels is fast, but both produce damped, delayed tides with modest seawater exchange from the barrier-reef lagoon. Topographic protection from wind, heavy regular rain throughout the year, with precipitation exceeding evaporation, and modest tidal exchange produce stratified water columns with brackish waters above permanently anoxic saline hypolimnia. Permanent lake stratification is documented for 18 years; sediment cores (by others) show stratifica tion for >100 years, and recent constant sea level implies ecosystem stability for thousands of years. Therefore, the marine lakes in Palau are small, closed, simple ecosystems that do not change over time-steady-state chemostats permitting replicate field measurements of biological and physical attributes from day to day, month to month, or decade to decade.[

Although coral reefs are widely viewed as oases in an oceanic desert, we demonstrate that the oceans around coral reefs in Palau are not deserts devoid of food, nor are coral reefs oases isolated from the surrounding sea. On flood tides, oceanic zooplankton advects onto the Palau barrier reef, where it is consumed by a wall of planktivorous fish. Similarly, on ebb tides lagoon zooplankton is consumed as it advects across the reef flat. Coral reef planktivores ingest zooplankton imported from both the sea and the lagoon, but coral reef fish also export meroplanktonic fish eggs that disperse offshore. Every day about 30 species of reef fishes, some forming spawning aggregations (parrotfish and surgeonfish), spawn at the forereef when the tide turns, releasing clouds of buoyant eggs and sperm into the ebb-tide waters that flow off the reef flat. Fish eggs disperse seaward, but remain entrained within several kilometers of the reef in a tidal, neritic water mass, separated from oceanic currents by tidal fronts. Flood tides return fish embryos and offshore zooplankton to the reef. The flux of materials between oceanic, neritic, and lagoon waters near coral reefs must be reexamined. KEY WORDS: Coral reef zooplankton · Fish spawning · Egg dispersal · PalauResale or republication not permitted without written consent of the publisher