Laboratory study of the grass shrimp feeding preferences

Grass shrimps are very common in various aquatic habitats, and are generally considered omnivorous. But, despite the numerous studies on interspecific trophic interactions, few studies have evaluated their feeding strategy, especially by experimental approaches. In this study, we determined, in the absence of both intra and interspecific competitive interactions, the feeding preferences of the freshwater grass shrimp Palaemonetes antennarius for the most common potential resources in its habitat. Plant and animal items were offered as food to each adult shrimp in multiple-choice laboratory experiments. Resource selection was determined under conditions of low and high food availability and related to the shrimp body size. The role of microbial colonisation in favouring detritivory was also investigated. Results of these experiments showed that P. antennarius consumed only animal items, mostly preferring the isopod Proasellus coxalis. In addition, palatability of the unselected plant remains and non-living prey was not improved by the microbial enrichment. Shrimps also resorted to cannibalism at low ration. By contrast, at high ration they specialised on a narrower range of resources, directing more their feeding efforts towards isopods. The preference ranking of the selected food types was inversely related to the average mass of the prey and varied with the shrimp body size. In particular, the number of live isopods in the diet increased exponentially with the shrimp mass at food high ration, which resulted not to be limiting. In conclusion, food availability influences the food niche of P. antennarius whose feeding strategy in the laboratory is facultative specialisation on dead and live animals rather than omnivory. Ease of handling and/or capture seems to play a role in the food preference ranking.