Morphological covariates of the ontogenetic shift from nauplii to copepodite prey in larval fish.

Larval fish are active planktonic predators, with many species feeding initially on copepod nauplii and gradually shifting their selection to copepodites. This study evaluated whether it is possible to develop a general widely applicable empirical model to describe the transition from feeding on copepod nauplii to copepodites in relation to body length, maxilla length and eye diameter. The study also evaluated whether the switch to copepodites is linked to what prey are in the stomach or which copepodite species are replacing nauplii in the diet was also considered. The high degree of covariation among the three morphometric variables makes it difficult to establish statistical differences among predictive models. However, the highest overall fit to a logistic model and accuracy in the transition from nauplii to copepodites are achieved when eye diameter is used as a predictive variable. There are also fewer significant differences in the residuals among fish taxa in the case of eye diameter relative to the other morphometric variables. Fish taxa that shift their diet from copepod nauplii to prey on cyclopoid copepodites demonstrate a slower ontogenetic transition away from nauplii than taxa that shift to calanoid copepodites. Eye diameter may be a stronger predictor of the shift in diet because it contributes to most of the processes that make up the foraging sequence of larval fish prey-predator interactions.