Caridina spp. (Atyidae) are widespread and abundant omnivorous shrimps in tropical Asian streams. Spatial and seasonal variation in C and N stable isotope signatures of 2 species (Caridina cantonensis and C. serrata) and their potential food sources (leaf litter, fine particulate organic matter [FPOM], and periphyton) were investigated in 2 shaded and 2 unshaded Hong Kong streams. The objectives were to identify food sources used by shrimps and to determine whether food use changed according to riparian shading, season, developmental stage, and sex. Isotopic signatures of different species of leaf litter were similar to each other and distinct from FPOM signatures in all streams, whereas periphyton δ13C and δ15N signatures were similar to FPOM signatures. FPOM and periphyton showed seasonal variation in δ13C and δ15N signatures in all streams, but signatures of leaf litter showed relatively minor variation. δ13C signatures of FPOM were 7 to 12‰ (shaded streams) and 5 to 7‰ (unshaded streams) higher than those of leaf litter during the dry season, and 5 to 8‰ (shaded streams) and 4 to 7‰ (unshaded streams) higher during the wet than the dry season. δ15N signatures of FPOM were almost 2× those of leaf litter in all streams and seasons. Periphyton was generally ≤6‰ more 13C-enriched and ≤4‰ 15N-depleted than FPOM. Seasonal variation in δ13C signatures of shrimps and their food sources were consistent among streams, and were less 15N-depleted during the dry season. Dual-isotope multiple-source mixing models indicated that FPOM and periphyton were the main foods of adult (male and female) and juvenile C. cantonensis and C. serrata. Leaf litter contributed <10% to the biomass of C. cantonensis in unshaded streams and 10 to 20% to the biomass of both species in shaded streams. Periphyton contributed >60% to the biomass of C. cantonensis in unshaded streams in the wet season, with the proportion slightly higher during the dry season. In shaded streams, periphyton contributed 35 to 60% to Caridina spp. biomass. Our results indicate that the Hong Kong atyids feed as omnivores, with herbivory as a primary feeding mode supplemented by collection of FPOM and limited direct consumption of allochthonous leaf litter.
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