Copepod feeding quantified by planar laser imaging of gut fluorescence

Abstract

We present a new method for quantifying the feeding of individual copepods, using a planar sheet of laser light to stimulate the fluorescence of phytoplankton ingested by the copepod. The fluorescence is imaged with a sensitive CCD camera, giving two‐dimensional images of the copepod's gut with 20 × 20 µm spatial resolution. Using tethered copepods, we have obtained > 3 h long time series of copepod gut fluorescence with images every 15–20 s. The same individual copepod can be used for multiple experiments, obviating the problems of individual variability as a source of error. Initial data reveal two distinct patterns of variability as material moves through two functionally different gut compartments. These patterns reflect processes occurring in each compartment. The upper (anterior) mid‐gut shows higher variability and less repeatability than the posterior midgut where undigested material is aggregated into a fecal pellet and evacuated at regular intervals. Variability in the upper mid‐gut is likely due to factors such as intermittence of feeding and relatively complex mixing dynamics. In the posterior mid‐gut, mixing dynamics are much simpler, and the variability of the upper compartment is integrated over the time scale of pellet formation.