Model Evaluation of Linear Gut Evacuation in the Larval Radiated Shanny Using a Combination of Laboratory and Field Data

Abstract

The combination of gut contents and evacuation rate is an important tool with which to determine the in situ feeding rates of many organisms. Traditionally used equations of gut evacuation models, however, have made comparisons among models difficult. For consistency, we changed the notation of the linear gut evacuation model to provide a rate-constant parameter (r) similar to the one commonly used in the exponential model. In a previous analysis using examples from the literature, we demonstrated that prolonged retention of food after a true linear pattern of gut evacuation may be mistakenly identified as an exponential pattern. We explored the linear gut evacuation rate model empirically and in more detail using the radiated shanny Ulvaria subbifurcata as the model species. Rate constants based on field data were similar to those determined in the laboratory. The basic assumption of nonfeeding during darkness was met, indicating that foraging was primarily based on visual cues. As predicted by our model, both laboratory and field data confirmed that gut evacuation was linear and characterized by a rate constant (i.e., instantaneous rate of gut evacuation) that was independent of initial gut fullness. Rate constants were, however, influenced by fish size and in some cases by the size of food particles. Analysis of triplicate time courses of gut evacuation in the field showed that larger larvae evacuated food at a relatively higher rate than did smaller larvae.