Impact of salinity on the grazing rate of a cladocera (Latonopsis australis) in a large tropical estuarine system.

Cladocerans are ecologically important as active grazers at the secondary trophic level, and they are economically important in aquaculture as potential live feed for many commercially valuable fishes. This study deals with the effect of salinity on grazing of the rare cladocera Latonopsis australis. The experimental specimens were collected from the lower reaches of the Kochi backwaters, the largest estuarine system along the west coast of India, during the Pre-Southwest Monsoon (May 2015), and their cultures developed in the laboratory. Eight experimental salinity treatments (freshwater, 2, 4, 6, 8, 10, 12 and 14 salinity) with three life phases of the specimens (phase I: neonates with a developed digestive tract, phase II: adults carrying egg and phase III: adults carrying developing neonates) were carried out to measure their grazing rates. Two different approaches, namely individual-specific and biovolume-specific grazing measurements, were adopted in the study. The results showed a significant influence of salinity on the size and grazing rates of L. australis, irrespective of their life phases. Filtration and ingestion rates of the specimens also varied significantly between different life phases with the lowest rates in phase III. Irrespective of the life phases, grazing rates were the highest in freshwater conditions, which decreased to 25-84% in medium salinity (4-8) and 65-93% in high salinity (10-14). Further, the study demonstrated that size/biovolume/weight-based quantification of cladocera grazing provides a more precise method for the estimation of feeding rates compared to the individual-basic approach. This study emerges as the first attempt to quantify the grazing measurements of a cladocera from Indian waters.