A comparative in-vitro-test on the digestibility of live prey for fish larvae under specific consideration of trypsin

When evaluating live prey for the successful rearing of fish larvae, general availability and nutritional quality of the prey are mainly in the major focus. However, the digestibility of prey items is also crucial as it directly affects the accessibility of the nutrients inside the prey item. Harpacticoid copepods, as well as nematodes, are considered as potential live feed for fish larvae but their digestibility has not yet been identified. Therefore, a comparative in-vitro evaluation of the digestibility of several prey organisms for larval fish was conducted under specific consideration of the efficiency of the proteolytic enzyme trypsin. This endoprotease plays a major role in the digestion process in the early stages of marine species. Common (Artemia sp., Brachionus plicatilis) and candidate prey organisms for commercial larval rearing (Acartia tonsa (Calanoida), Tachidius discipes, Tisbe sp. (both Harpacticoida), Panagrolaimus sp. (Nematoda)) were exposed to a trypsin solution. Photos of trypsin-treated prey organisms were taken and compared with controls to quantify the effect of trypsin on the inner body disintegration. Additionally, the effects of the larval ring muscles in the gut and the pharyngeal teeth were imitated by mechanical treatment. While Artemia sp. showed the highest digestibility, Panagrolaimus sp. was the opposite with very low degradation. The calanoid copepod A. tonsa and the harpacticoid copepod Tisbe sp. were more digestible than T. discipes and the nematode Panagrolaimus sp.