The effects of colonization by Zebra mussels, Dreissena polymorpha, on fitness in two Anisopteran species: Hagenius brevistylus and Didymops transversa

Abstract

The colonization of two Anisopteran larvae (Hagenius brevistylus and Didymops transversa) by Dreissena polymorpha was studied in Douglas Lake after the recent mussel invasion from the Lake Huron watershed. Our objectives included measuring the frequency of the zebra mussels on larvae and exuviae, and measuring fitness indirectly by means of “righting” tests, or how long it took the D. transversa larvae to “right” itself after being placed upside down. We hypothesized that larvae with zebra mussels attached would show a decrease in fitness because of the increased energy costs. Because we found a low frequency of larvae with mussels, colonization was induced. Our results indicated that as the number of zebra mussels per larva increased, so did “righting” time. “Righting” time also increased as the ratio of mussel weight to larva weight increases. None of the H. brevistylus were able to right themselves when observed. There was a significant difference between the average “righting” time for D. transversa larvae before after colonization. The larvae with mussels had a higher “righting” time than larvae without mussel attachment, supporting our hypothesis. Because the majority of our larvae were induced with zebra mussels, they had a higher ratio of mussel weight to larval weight than is found in naturally occurring populations in Douglas Lake from previous studies. This indicates that although a large load of zebra mussels may decrease fitness, the occurrence is unusual and may not have an effect on large populations in Lakes.