The copepod Acartia tonsa is adapted to high food concentrations which it encounters in estuaries and upwelled waters. It cannot obtain sufficient food for reproduction on the middle and outer shelf, where food concentrations are usually low, because it decreases clearance rates when concentrations of Thalassiosira weissflogii fall below 0.25 mm3 1-' In comparison, the offshore copepod Paracalanus sp. continues to increase its clearance rate when food levels are below the abovementioned concentration. Several factors are thought to be responsible for this reduction of clearance rates of A. tonsa feeding on T. weissflogii: ( l ) The proportion of time during which water is transported towards the copepod decreases with decreasing food concentration. (2) The efficiency of capturing food particles decreases below 22 pg C I-' (= 0.28 mm3 I-' of T. weissflogb]. (3) A. tonsa does not seem to reroute phytoplankton cells individually towards its median, and therefore cannot use a, hypothesized, increased sensitivity of its chemoreceptors at low chlorophyll concentrations to increase clearance rate.

Particles of varying food quality but similar size were offered singly and in mixtures to copepodid Stage V of Eucalanus pileatus to determine the rates at which calanoid copepods ingest naturally occurring particulate matter. The sequence of ingestion rate was: living phytoplankton cells > dead phytoplankton cells > fecal pellets > polystyrene spheres. Cinematographic observations revealed that both long range and short range chemoreception contributed to these differences. Living phytoplankton cells were ingested at rates 1.2 to 3 times higher than similarly-sized non-living food particles. Non-food particles offered alone were not ingested. In mixtures, the rate at which non-food particles were ingested was a function of the feeding rate on food particles. At environmental concentrations the presence of non-food particles does not appear to reduce feeding rates on food particles.

The goals of this and following studies were to describe how nauplii of related calanoid copepods gather and ingest phytoplankton cells, and to compare their feeding behavior with that of copepodids and adult females of the same species. Nauplii of the calanoids Eucalanus pileatus and E. crassus draw particles towards themselves creating a feeding current. They actively capture diatoms > 10 pm width with oriented movements of their second antennae and mandibles. The cells are displaced toward the median posterior of the mouth and then are moved anteriorly for ingestion. The nauplii gather, actively capture, and ingest particles using 2 pairs of appendages, whereas copepodids and females use at least 4 of their 5 pairs of appendages (second antennae, maxillipeds, first and second maxillae) to accomplish the same task. These nauplii are not able to passively capture small cells efficiently like copepodids and females because they lack a fixture similar to the second maxillae. Gathering and ingestion by late nauplii of E. crassus and E. pileatus require together an average of 183 ms for a cell of Thalassiosira weissflogii ( l 2 pm width) and 1.17 S fox Rhizosolenia alata (150 to 500 pm length). Although na.uplii of related species show little difference in appendage morphology, they differ markedly in feeding and swimming behavior. Their behavior is partly reflected in the behavior of copepodids and adult females.

No abstract