Observations of the feeding activity of copepod populations have revealed a wide range of responses both between and within species. Feeding may be continuous, with no change in gut fullness on a diel period, or feeding may occur on a cyclical, diel pattern. A diel pattern may be seen in both non-migratory copepods and in the absence of vertical stratification of chlorophyll a. Recently, researchers have demonstrated that nocturnal grazing in Acartia tonsa was timed to lighting conditions. However, while light may cue a feeding response, light conditions alone cannot explain an energetic advantage for cyclical patterns in feeding. One hypothesis is suggested Calanus finmarchicus, a vertical migrator, where a cyclical pattern is seen in gut cell development during feeding under experimental conditions. These results indicate that during the course of feeding certain cell types are depleted, and require a non-feeding period to regenerate. As a vertical migrator, C. finmarchicus has a natural period during the daylight hours in which to allow this regeneration. This research will investigate differences that might exist in non-migrating copepods, exposed to continuous food. This project will use morphometric analyses of the gut cells of the nearshore copepod Acartia tonsa over a 24-h period in the laboratory. Analyses will be conducted at both the light and electron-microscope level. Laboratory conditions will simulated natural light and food conditions, and these results will be compared to samples taken from the field. Patterns in physiological rates (digestive enzyme activity, glutamate dehydrogenase activity, and respiration rates) and gut fullness will be determined concurrently.
