PREDATORY FEEDING ECOLOGY OF EUCHAETA ELONGATA ESTERLY, A MARINE PLANKTONIC COPEPOD

The predatory feeding ecology of Euchaeta elongata, an abundant planktonic copepod in Dabob Bay, Washington, was characterized to evaluate its importance as a source of mortality for several potential copepod prey species. In the laboratory, the exclusively carnivorous adult females exhibited nighttime predation on active prey. Feeding rates increased with prey concentration, reaching saturation levels on the intermediate size class of copepod prey. Ingestion efficiency (prey wholly eaten/prey attacked) varied with predator stage, predator hunger state, and prey concentration. Size of copepod prey strongly influenced diet selectivity. Adult female E. elongata (4.2 mm prosome length) exhibited highest predation rates on the intermediate size classes (950 (mu)m prosome length) of adult copepod prey and, similarly, on intermediate size copepodid stages of Calanus pacificus. The prosome length of primary prey was 70% the length of the second basipodal segment of the maxilliped for CIV, CV, and the adult female of E. elongata. In both single and multispecies experiments, predation rates of E. elongata appeared to be invariably focussed on prey having prosome lengths of 600-1000 (mu)m. Because E. elongata can consume adults of small copepod prey and the young copepodid stages of large copepod prey, it exerts mortality on the entire copepod assemblage.;In Dabob Bay, variations in predatory feeding activity of E. elongata were closely related to their diel vertical migrations. The number of females with ingested prey began to increase soon after sunset when E. elongata entered surface layers where prey of the preferred size reside. A periodicity in the composition of E. elongata's diet occurred in concordance with its nocturnal upward migration and the assemblage of zooplankton prey encountered during the vertical excursion. The abundance of prey seemed to regulate the portion of the predator population that migrated. An in situ feeding rate, estimated from fecal pellet production rates, indicated that within a nighttime feeding interval, a migrant female was able to obtain its maximum ration in Dabob Bay. Yet in order to feed at this rate, prey must be patchily distributed.