Life history characteristics of the invasive freshwater gammarids Dikerogammarus villosus and Echinogammarus ischnus in the river Main and the Main-Donau canal

Abstract

The life history characteristics of two invasive gammarids, Dikerogammarus villosus and Echinogammarus ischnus, from 2 sites in the river Main and one in the Main-Donau canal were studied monthly from May to October 2000 and one additional time in April 2002. The species coexisted, but D. villosus was much more abundant at upstream sites whereas E. ischnus prevailed at the downstream site. Females of D. villosus produced big clutches; mean clutch size during the period of reproduction ranged between 29 and 136 and maximum clutch size was 188 eggs per ovigerous female. Volume of stage 2 eggs was between 0.07 and 0.08 mm 3 . Echinogammarus ischnus females produced smaller clutches than D. villosus females but larger eggs. Mean clutch size in this species varied between 14 and 27 and maximum clutch size was 51 eggs per ovigerous female. Volume of stage 2 eggs varied between 0.1 and 0.2 mm 3 . The calculated fecundity index (weight-specific number of eggs per clutch) was higher in D. villosus than in E. ischnus (0.62-1.37 vs. 0.44-0.60, respectively) while the reproductive effort (egg volume times fecundity index) was fairly the same in both species (0.05-0.12). Ovigerous females and precopula pairs were absent in September/October indicating the reproductive diapause. The results point to different reproductive strategies in these species. Large eggs (probably supplied with reserves) suggest advantage of E. ischnus when food is scarce while big clutches with small eggs suggest reproductive superiority of D. villosus under conditions of abundant food.