Stable isotope analyses and demographic responses counter prospects of planktivory by Caridina (Decapoda: Atyidae) in Lake Victoria

Abstract

Caridina nilotica, a freshwater atyid prawn, is a vital component of the Lake Victoria ecosystem. Despite its important role in the food web leading to Nile perch, the diet of Caridina is not well understood. Caridina freshly collected from the inshore littoral and offshore plankton of Lake Victoria were cultured individually under laboratory conditions on (A) decomposing hydrophytes, (B) living hydrophytes, (C) planktonic algae, (D) zooplankton and (E) 35-µm filtered lake water (a 'starvation' control). Inter-moult intervals (IMI, days), size-standardized moult intervals (MI, days mm⁻¹), per moult growth increments (PMI, mm) and survivorship (%) were monitored daily for up to 5 weeks. Significant effects of both food type and shrimp source on MI were revealed by ANOVA. MI increased progressively from treatment A to D, and was shorter in offshore than littoral shrimps. Food influence on IMI was confirmed by ANCOVA. PMI values were close to the limits of detection, but were generally in line with MI responses. PMI values were marginal in treatments A and B, and negligible or negative in treatments D and especially E. Survivorship values, although confounded by non-dietary factors, were generally consistent with dietary influences on MI, although values obtained for treatment E were inconsistently high for true starvation. Disparate responses between inshore and offshore shrimps hint at possible ecotypic differentiation, or perhaps the existence of cryptic species. Stable isotope analyses (SIA, δ¹³C and δ¹⁵N signatures) of cultured shrimps were further consistent with their utilization of food type A but not D. SIA signatures of feral shrimps maintained in situ in enclosure bags with three separate potential fresh hydrophyte food sources (Vossia cuspidata, Cyperus papyrus, and Eichhornia crassipes) reflected Caridina's probable dietary reliance on decomposed organic matter with accompanying bacterial exudates. Collections of feral shrimps from various locations yielded parallel SIA results. No support for zooplanktivory by shrimps occupying either inshore littoral/benthic or offshore planktonic habitats is provided by the δ¹⁵N signatures obtained from our data, which support Caridina's primary role as a detritivore.