The leeches of Rutland Water

Seven species of leeches have been recorded in Rutland Water between September 1977 and October 1979 by monthly samples taken by Ekman grab.The preliminary analysis of leeches from the four study sites of the reservoir (46 grab samples) showed that Helobdella stagnalis was the most abundant species with Erpobdella octoculata and Glossiphonia complanata (cf. Bullock et al. 1982). The life cycle of H. stagnalis was elucidated from these samples by dividing the animals into weight classes (2 mg live weight) and revealed that this species has two breeding seasons. The overwintering population reproduces in June. The juveniles grow rapidly and produce the next generation in August and September. This life cycle is similar to that reported in Europe and Canada.The population density ranged from 32 m^–2 to 90 m^–2, being generally higher during Summer and Autumn after breeding, but low in Winter and Spring. The population biomass increased from June until October each year because of high growth and then declined through the Winter. There are marked variations in density and biomass between the two years of the study period, which probably reflects the dramatic change in benthic abundance and distribution found in many new impoundments.In order to calculate the energy budget, the respiration rate of H. stagnalis has been studied by using a Gilson respirometer operated at three temperatures 5 °C, 10 °C and 15 °C, and the respiration rate of individual H. stagnalis increased logarithmically with weight. The relationship between O₂ consumption and dry weight was examined using linear regression (log/log) for the three temperatures. There are significant differences between the three regressions with slopes of 0.54, 0.69, and 0.73 at 5 °C, 10 °C and 15 °C respectively. The repiratory coefficient (Q₁₀) for the range 5 °C to 15 °C was almost constant for all class weights.The food of the leech was determined qualitatively by exposing potential prey organisms to it, using all the common zoobenthos of Rutland Water. H. stagnalis fed extensively on oligochaetes (Tubificidea). Food consumption was estimated at the three different test temperatures for different weight classes. Similar studies have been undertaken on E. octoculata and G. complanata.