Copepod feeding quantified by planar laser imaging of gut fluorescence

Erdem M. Karaköylü, Peter J.S. Franks, Yuji Tanaka, Paul L. D. Roberts, Jules S. Jaffe
Limnol. Oceanogr. Methods (XXXX) X:XXX-XXX

We present a new method for quantifying the feeding of individual copepods, using a planar sheet of laser 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 x 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 mid-gut 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.