Predation by omnivorous copepods on the offspring of Calanus and Pseudocalanus

A. Sell, L. Madin, D. van Keuren, J. Austin and B. Sullivan

During three cruises (EN 322, EL 9905 and EN 352) we caught and maintained cultures of several species of abundant omnivorous copepods that we expected to be important as predators of early life stages of Calanus finmarchicus and Pseudocalanus spp. We also caught live adult females of the two latter species and kept them in culture for the production of eggs and nauplii to be used in predation experiments.

We ran a total of 20 experiments at ambient sea water temperature using deck incubations in a plankton wheel. Omnivorous copepods we used were Metridia lucens, Centropages typicus and Temora longicornis. Predators were fed ad libitum with phytoplankton from cultures (Thalassiosira weissflogii and Heterocapsa triquetra) over a period of at least 24 hrs before being used in experiments.

We established functional response curves describing prey density-dependent ingestion rates for the three predator species feeding on the eggs and nauplii of the broadcasting spawner Calanus and nauplii only of Pseudocalanus, because Pseudocalanus females carry their eggs in egg sacs. All predators fed on eggs as well as nauplii. Ingestion rates in several cases exceeded 10 prey * predator -1 * day -1. Saturation of ingestion rates occurred at higher densities of prey, but in some cases (eg. Centropages feeding on Calanus eggs) could not be reached even with densities of 170 prey L-1.

Predation rates were dependent on prey size and mobility, as well as on temperature.

Rates of predation on older naupliar stages were lower than on young stages. Nauplii of Pseudocalanus moved faster and were ingested at lower rates than similar-sized Calanus nauplii. Predation rates increase with increasing temperature for Centropages typicus, but not for Metridia lucens.