THE COMMENSALISTIC ASSOCIATION OF NANOCLADIUS (DIPTERA: CHIRONOMIDAE) ON PTERONARCYS BILOBA (PLECOPTERA: PTERONARCYIDAE): INSIGHTS FROM STABLE-ISOTOPE ANALYSIS.

Doucett, R.R.(1), Giberson, D.J.(2) & Power, G.(1)(1) Department of Biology, University of Waterloo, Waterloo, ON, Canada (2) Department of Biology, University of Prince Edward Island, Charlottetown, PEI, Canada.

Nymphs of Pteronarcys biloba, found to be hosts to chironomid larvae [Nanocladius (Plecopteracoluthus) undescribed sp., nr. branchicolus], were collected from Catamaran Brook, New Brunswick, between May and November 1997, for stable-carbon and nitrogen-isotope analysis. Nanocladius (P .) sp. had mean (1 SD) dð13C (-27.7 1.0 per mil) and dð15N (4.9 0.6 per mil) values that were isotopically more enriched than the stable C and N isotope ratios of P. biloba (-29.0 1.2 per mil and 1.6 0.7 per mil, respecti vely) Average fractionation factors for carbon and nitrogen (i.e. 1.5 per mil and 3.6 per mil, respectively) suggest a parasitic relationship between Nanocladius (P .) sp. and P. biloba, confirming an association hypothesized in a previous study. No statistical differences were found among stable C and N isotope ratios of the plecopteran shredder in 4 stream reaches from headwaters to mouth (ANOVA, p > 0.05), despite differences in the riparian canopy. Stable isotope ratios of parasitized stoneflies were not significantly different from those of non-parasitized individuals (t-test, p > 0.05). However, other chironomids, including free-living Nanocladius sp., were isotopically distinct from parasitic forms (t-test, p < 0.05). These findings highlight the usefulness of stable isotope technology to distinguish between phoretic and parasitic relationships among aquatic organisms.