Predation on naked protozoan microzooplankton by fish larvae

Abstract

To evaluate the role of fish larvae as a link between the microbial loop and higher trophic levels, predation of protozoan zooplankton by young larvae was investigated. More than 400 individual fish larvae with total lengths of less than ca 10 mm in 52 different taxonomic groups were collected at different sampling times from several coastal regions, and the gut contents of larvae were examined under epifluorescence microscopy after staining with DAPI. Among numerous fragments of copepod nauplii, many flagellate-like cells with a size of 5 μm and ciliate-like cells with a size of 20 to 30 μm were frequently recognized. The number of protozoan cells varied significantly from one larva to another. Some individuals had more than 60 protozoa, while others contained none at all. The amount of protists contained in the gut of larvae depended on the fish species and did not show any trend with the body or mouth sizes of larvae, nor the sampling site or season. Fish taxa were divided into 3 groups depending on the amount of protists in the gut: 'abundant', 'moderate', and 'none'. The Acanthopterygli group contained the highest concentration of protozoa. Results of the present study suggested that fish larvae of some taxonomical groups were important predators of protozoa and may be an important link between the microbial loop and the grazing food chain.