Diel Movements of Resident and Transient Zooplankters Above Lagoon Reefs at Enewetak Atoll, Marshall Islands.

摘要

Of those zooplankters above reefs on the lagoon shelf atEnewetak Atoll at some time during the diel cycle, the vast majority of thoselarger than about 1.5 mm were there only at night. Many of these larger formswere local residents that by day sheltered in or near shelf substrata, or in swarmsclose to these substrata, and at night made purposeful forays above the shelf.Many others, however, were transients from the deeper regions of the lagoon, orfrom the open sea outside the atoll, and these were above the shelf at night bychance. The residents included various polychaetes, cypridinacean ostracods,copepods, mysids, tan aids, isopods, amphipods, and carideans. The transientswere mostly holoplankters that included halocyprid ostracods, calanoidcopepods, euphausids, and chaetognaths. Both residents and transients wereabove the shelf at night as a result of diel vertical migrations. The residents wereadapted to stay within reach of their diurnal habitats while in the nocturnal watercolumn, often by avoiding currents, and so were readily able to return to thosehabitats at dawn. The open-water transients, however, lacked such adaptations,and, as a result, probably many were stranded in the shallows above the shelfat dawn, unable to return to their daytime depths and probably vulnerable toplanktivorous fishes.