Ecological studies of the common leeches of Rutland Water.

Three species of leeches (Helobdella stagnalis, Erpobdella octoculata and Glossiphonia complanata) were studied in Rutland Water (England) between September 1977 and October 1979. Samples were taken by Ekman grab at monthly intervals and by hand collecting from unit areas near the shore. For each sample, the number and size distribution was recorded of each species, and these data were used both to analyse the effect of depth and substratum on distribution and to provide base-line data on the life cycles and population structure for estimating the parameters of the annual energy budgets. The abundance and distribution were clearly related to water depth and to the type of substratum. Maximum densities were recorded from July to October, while the lowest densities were recorded in January to March. The life cycle of both H. stagnalis and G. complanata were predominantly annual, while E. octoculata took two years to complete its cycle. The respiration rates of the three specie using a Gilson respirometer operated at 5, 10 and 15C. Oxygen consumption increased with dry weight of leeches over all three temperatures, and in each species the three regression equations were significantly different from one another. The monthly respiratory metabolism was calculated for each species by applying the regression equation for the nearest temperature to the recorded population structure and from these values, annual respiration was determined. Food consumption was measured at 5, 10 and 15C and direct relationships between food consumption and both body weight and temperatures were found. No effect of day length was demonstrated. The apparent percentage assimilation was calculated, and was more than 90% in all three species. Monthly consumption was calculated by applying the appropriate regression equation of food consumption on body weight to the data for population structure at the nearest temperature to that in the field. This value was multiplied by the apparent assimilation efficiency to give the monthly assimilation rate and these values were summed to determine the annual assimilation. Annual production for each species was calculated using annual assimilation, minus annual respiratory metabolism. Problems encountered in this study have been discussed.