No abstract

Headlands, islands, and reefs generate complex three-dimensional secondary flows that result in physical and biological fronts. Mixing and diffusion processes near these reefs and headlands are quite different from these processes in the open sea, and classical advection-diffusion models that were developed for the open sea are not valid near shore. Topographically generated fronts affect the distribution of sediments, and they aggregate waterborne eggs, larvae, and plankton. This aggregation influences the distribution and density of benthic assemblages and of pelagic secondary and tertiary predators.

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.

Populations of jellyfish, Mastigias sp., landlocked in tropical marine lakes during the Holocene, show extreme genetic isolation (0.74 < ST < 1.00), founder effects (genetic diversity: 0.000 < < 0.001), rapid morphological evolution, and behavioral adaptation. These results demonstrate incipient speciation in what we propose may be modern analogues of Plio-Pleistocene populations isolated in ocean basins by glacially lowered sea level and counterparts to modern marine populations isolated on archipelagos and other distant shores. Geographic isolation in novel environments, even if geologically brief, may contribute much to marine biodiversity because evolutionary rates in marine plankton can rival the most rapid speciation seen for limnetic species, such as cichlids and sticklebacks. Marine lakes present situations rare in their clarity for studying evolution in marine taxa.founder effect ͉ island ͉ rate of evolution