We studied the feeding behavior of the nauplii and adult females of the marine cyclopoid copepod Oithona davisae in the laboratory. Functional response experiments showed that O. davisae can feed on a broad size range of prey but that high clearance rates only occur in a narrow prey size range. Neither the nauplii nor the females were able to feed on Nannochloropsis oculata (2.5 mm), but . 4 mm prey were ingested. The highest clearance rates occurred when the nauplii and females were offered the dinoflagellate Oxyrrhis marina and the ciliate Strombidium sulcatum, respectively. O. davisae females preyed on Acartia grani nauplii but not on nauplii of their own species. Optimal prey : predator size ratios were similar for the nauplii and the females (, 10% of the predator's body length) and were higher than those reported for suspension-feeding calanoid copepods (2-6%). The finding that the nauplii and females of O. davisae feed on relatively larger prey appears to be a consequence of their strict ambush-feeding behavior, which constrains feeding activity to prey large enough to create hydromechanical signals above the detection threshold. Very high weight-specific ingestion rates (. 150% d 21 ) were obtained when O. davisae fed on relatively large prey. Such high daily rations are much higher than those that can be calculated indirectly from egg production. Size measurements of the mouth of O. davisae females indicate that those prey resulting in extreme feeding rates were too large to be swallowed completely and suggest the presence of sloppy feeding in Oithona.

Planktonic copepods are a key group in the marine pelagic ecosystem, linking primary production with upper trophic levels. Their abundance and population dynamics are constrained by the life history tradeoffs associated with resource availability, reproduction and predation pressure. The tradeoffs associated with the ageing process and its underlying biological mechanisms are, however, poorly known. Our study shows that ageing in copepods involves a deterioration of their vital rates and a rise in mortality associated with an increase in oxidative damage (lipid peroxidation); the activity of the cell-repair enzymatic machinery also increases with age. This increase in oxidative damage is associated with an increase in the relative content of the fatty acid 22:6(n-3), an essential component of cell membranes that increases their susceptibility to peroxidation. Moreover, we show that caloric (food) restriction in marine copepods reduces their age-specific mortality rates, and extends the lifespan of females and their reproductive period. Given the overall low production of the oceans, this can be a strategy, at least in certain copepod species, to enhance their chances to reproduce in a nutritionally dilute, temporally and spatially patchy environment.

We studied the phenotypic response to temperature of the marine copepod Paracartia grani at the organismal and cellular levels. First, the acute (2 days) survival, feeding and reproductive performances at 6–35°C were determined. Survival was very high up to ca. 30°C and then dropped, whereas feeding and fecundity peaked at 23–27°C. An acclimation response developed after longer exposures (7 days), resulting in a decline of the biological rate processes. As a consequence, Q10 coefficients dropped from 2.6 to 1.6, and from 2.7 to 1.7 for ingestion and egg production, respectively. Due to the similarity in feeding and egg production thermal responses, gross-growth efficiencies did not vary with temperature. Respiration rates were less sensitive (lower Q10) and showed an opposite pattern, probably influenced by starvation during the incubations. The acclimation response observed in the organismal rate processes was accompanied by changes in body stoichiometry and in the antioxidant defense and cell-repair mechanisms. Predictions of direct effects of temperature on copepod performance should consider the reduction of Q10 coefficients due to the acclimation response. Copepod population dynamic models often use high Q10 values and may overestimate thermal effects.