Copepod swarms: Attributes and role in coral reef ecosystems

Abstract

Monospecific aggregations of copepods were observed and sampled from eleven coral reefs, two sea grass habitats, and several marine lakes in the Central and Northern Great Barrier Reef Provinces of Australia and in Palau, Western Caroline Islands. Acartia australis, Acartia bispinosa, Oithona oculata, and Centropages orsinii regularly occurred at swarm densities of from 500,000·m −3 ( A. australis ) to 1,500,000·m −3 ( O. oculata ). During the day copepods swarmed over white sand on the lagoon side of windward reefs, adjacent to or downstream from isolated coral outcrops, with size of swarm dependent on size and shape of the nearby coral head. Swarms 1–2 m in diameter were common; they occasionally exceeded 30 m 3 . Seasonally these large swarms coalesced and carpeted the lagoon floor with copepods. In Palau O. oculata also swarmed near the surface in a continuous linear band in the solution notch that undercuts the limestone islands. Acartia bispinosa swarmed at the surface in open water in the marine lakes of Palau and over sea grass beds in the lagoon at low water. Swarming probably serves several adaptive functions, but protection from predators appears of major importance. Visual behavior and orientation to surrounding objects maintained swarm integrity, as did mechanical winnowing by currents. At night, swarms dispersed throughout the water column and swarming species sometimes dominated the nocturnal plankton within the reef lagoon. Previous investigations of coral reef zooplankton have not sampled swarming copepods properly and prior estimates of food available to the reef community must be re‐evaluated.