Parasitic Association of Nanocladius (Diptera:Chironomidae) and Pteronarcys biloba (Plecoptera:Pteronarcyidae): Insights from Stable-Isotope Analysis

Abstract

Nymphs of Pteronarcys biloba Newman and attached chironomid larvae (Nanocladius [Plecopteracoluthus] undescribed sp., nr. branchicolus) were collected from Catamaran Brook, New Brunswick, in May and November 1997, for stable-carbon- and stable-nitrogen-isotope analysis. Nanocladius (P.) sp. had mean (±1 SD) δ ¹³C and δ ¹⁵N values of -27.7 ± 1.0‰ and 4.9 ± 0.6‰, respectively, whereas those of P. biloba were -28.4 ± 1.0‰ and 1.3 ± 0.7‰, respectively. Nanocladius (P.) sp. always had more positive δ ¹³C and δ ¹⁵N values than P. biloba, and average fractionation factors (isotope differences between symbiont and host) were +1.2‰ and +3.5‰ for carbon and nitrogen, respectively. These results confirm a parasitic relationship between Nanocladius (P.) sp. and P. biloba. No statistical differences were found among the δ ¹³C values of the plecopteran shredder in 4 stream reaches from headwaters to mouth, and δ ¹³C values were similar to those of their expected leaf litter diet. Pteronarcids from the headwater site (Upper Reach) were not parasitized and had distinct δ ¹⁵N values from those at downstream sites. However, stable-isotope ratios of parasitized stoneflies were not significantly different from those of non-parasitized individuals at the 3 other study locations. Nanocladius (P.) sp. were more enriched in ¹⁵N than other chironomid genera in Catamaran Brook, including Ablabesmyia, a chironomid with predatory feeding habits, and fell within the range of other known invertebrate predators. These findings highlight the usefulness of stable-isotope technology to distinguish between phoresy and parasitism in ectosymbiotic relationships among aquatic organisms.