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.