Zooplankton feeding ecology: contents of fecal pellets of the copepods Eucalanus pileatus and Paracalanus quasimodo from continental shelf waters of the Gulf of Mexico

Feeding patterns of adult female Eucalanus pileatus and Paracalanus quasimodo were examined by comparison of fecal pellet contents and ava~lable phytoplankton at 12 stations in northern Gulf of Mexico continental shelf waters. Pellets contained remains of a wide variety of various-sized solitary and chain-forming phytoplankters, tintinnids, crustaceans, amorphous detritus, and/or Mississippi River sediment. Dominant phytoplankters in fecal pellets largely corresponded to those dominant in the water at time of collection. There was, however, an apparent lack of ingestion of the most abundant phytoplankter (Ditylurn brightwellii) by P. quasimodo at one station. This diatom may be too large for ingestion by P. quasirnodo. At the same station, the larger E. pileatus did eat D. bdghtwellil. Phytoplankton species ingested changed markedly in accordance with changes in phytoplankton species available. Crustacean, tintinnid and detrital remains were present in pellets mainly when phytoplankton abundance was low. In the plume of the Mississippi River, riverine sediment was a major component of pellets. Comparison of the phytoplankters ingested by both copepods at the same stations revealed that E. pileatus and P. quasimodo have many food items, encompassing a broad array of particle sizes, in common. These results reveal that E. pileatusand P. quasirnodoare omnivorous and primarily opportunistic herbivores.