Prey selection by Euphausia lucens (Hansen) and feeding behaviour in response to a mixed algal and animal diet

Abstract

Prey selection by adult Euphausia lucens (Hansen) was investigated in the laboratory using different developmental stages of Calanus agulhensis De Decker et al. (nauplius I - copepodite V) and larval stages of Euphausia lucens (nauplius - calyptosis III). In addition, adult copepods of other species ( Calanus Centropages, Oithona, Paracalanus, Metridia and Paracartia) and Artemia nauplii of different sizes were used as prey. Prey size and swimming behaviour were thought to influence prey selection. Euphausia lucens preyed preferentially on intermediate-sized prey (0.8–1.0 mm total length), selecting slow continous swimmers in preference to fast intermittent swimmers. Predation rates were highest on 1-mm Artemia nauplii and Euphausia lucens nauplii, owing to their limited escape responses, and lowest on adult Calanus, which have a refuge in size as well as faster escape responses. Calanus CIIs (0.95 mm total length) were the most heavily selected developmental stage of Calanus, owing to their optimal size and relatively slow swimming speed. Adult Metridia lucens Boeek were also ingested at higher rates than other prey of a similar size, which was attributed to their continous, spiral swimming motion. Experiments with different combinations of Artemia nauplii and diatoms as food revealed that ingestion rates on Artemia nauplii were significantly depressed by increasing concentrations of diatoms ( Thalassiosira weissflogii (Grunow & Van Heurck, Fryxell & Hasle) and vice versa. However, the preference for Artemia nauplii (α ARI), as indicated by Chesson's measure, did not change significantly with relative changes in the abundance of the alternative ( Thalassiosira weissflogii) food source indicating the Euphausia lucens does not switch from herbivory to carnivory, but rather shifts its diet to reflect the changing proportions of prey types. Clearance rates on Artemia nauplii were approximately five times higher than on diatoms in the same experiment, suggesting that Euphausia lucens uses a separate mechanism (such as chemo- or mechanoreception) to capture prey, as opposed to filtering phytoplankton cells.