Dietary variation in a predatory shrimp Macrobrachium hainanense (Palaemonidae) in Hong Kong forest streams

Abstract

Macrobrachium hainanense is a large palaemonid shrimp (total length > 7 cm), present at high densities (> 2 individuals m-2) in pools of low-order forested streams in Hong Kong, Southern China. The present study investigated the diet and trophic status of this shrimp in pools of Tai Po Kau Forest Stream (TPK) and Tai Shing Stream (TSS). Macrobrachium hainanense was a generalist predator that fed selectively on sedentary and slow-moving prey (snails, chironomid larvae and calamo-ceratid caddisflies) eating them disproportionately relative to their abundance in pool benthic communities. Shrimp diets varied by size of the individual, by season and by stream. The diet of large males was dominated by the thiarid snail Brotia hainanensis, and ovigerous females had guts that were half as full as large females although their diet was similar to other life stages. Gut fullness was reduced in shrimps that had molted recently. Over 80 % of the food items eaten by M. hainanense at TPK during the wet season were B. hainanensis, while the dry season diet included a greater number of taxa including tube-dwelling Chironomidae (31 %), Trichoptera (25 %) and Ephemeroptera (24 %). This pattern of seasonal variation was less noticeable at TSS than at TPK. TPK M. hainanense had less full guts (∼ 50 %) than those at TSS (∼ 70 %), but, at both sites, shrimps with empty guts were encountered only during the wet season. The predaceous nature of M. hainanense through all life stages is in contrast to the omnivorous or detritivorous habit of other Macrobrachium species. The results of δ15N and δ13C stable isotope analysis of samples from TPK confirmed the predatory trophic status of M. hainanense. All life stages had isotopic signatures that were higher than to sympatric predatory invertebrates (e. g. Odonata). Allochthonous detritus made no direct contribution to the assimilated tissue of M. hainanense.