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.

To investigate the role of light in controlling copepod grazlng behavlor, feeding responses of adult female Acartia tonsa Dana were determined under light conditions similar to those in natural, subsurface waters. Measured feeding responses were then related to feeding behavior under similar light conditions in nature. Copepods were adapted to a range of light intensities before being offered food (Thalassiosira weissflogii, 0.5 mm3 1 -l ) . Thirty min later gut contents were measured by gut fluorescence of chlorophyll a and pheopigment a. The grazing index was inversely related to the light intensity at which the copepods were adapted. The most dramatic change in grazing occurred at light levels close to those found in the copepod's natural habitat during twilight. An endogenous nocturnal feeding rhythm was found in a separate experiment. A 96 h field study in the Newport River estuary, North Carolina, USA, revealed a nocturnal feeding pattern in A. tonsa and a negative correlation between feeding and light intensity during daylight hours. No significant correlations were found between copepod feeding and in situ concentrations of chlorophyll a, pheopigment a and combined pigments. These results challenge Gauld's (1953) hypothesis that nocturnal grazing results from nocturnal vertical migration into a food-rich layer; they offer a reasonable explanation of how light may time grazing. Results indicate that migration and feeding may be independent behaviors separately controlled by light cues.

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Phototactic behavior of red drum Sciaenops ocellatus larvae (age = 1 to 17 d posthatch, size = 2.5 to 7.5 mm standard length) was examined. Numerical percent of test larvae phototactically responding per sample was determined after dark and light adaptation upon exposure to different stimulus light intensities and compared with dark or light controls. The relative spectral composition In these experiments was similar to that measured at mid-afternoon at 1 m depth in a coastal channel where red drum larvae naturally occur. Day 1 larvae were not photoresponsive; all tested older larvae showed positive phototaxis at higher stimulus intensities [> ca 10-4 (Day 16) to 10-' (Day 3) pE m-2 S-'] and negative phototaxis at lower light levels [as low as ca 10-S (Day 3) to 10-' (Day 5) pE m-' S-']. The light range for positive phototaxis generally increased with larval age, as did the percent positive response when tested at the same light levels. These results indicate an ontogenetic increase in photoresponsiveness, beginning as early as Day 3 posthatch. Light-adapted larvae as young as Day 4 exhibited a lower response than dark-adapted larvae at the same stimulus intensities, indicating dark and light adaptation. Estimated intensity thresholds for negative phototaxis were compared with nearbottom daylight intensities in 2 coastal channels where red drum larvae occur. Results indicate there is sufficient daylight available throughout the water column in the field for photoperception by red drum larvae. Using an estimate from the literature of a 3 to 4 log unit higher threshold for feeding than for negative phototaxis, we conclude that natural intensities at depth are adequate for visual predation during daylight hours.