Intraspecific life-history variation in Erpobdella octoculata ( Hirudinea: Erpobdellidae). I. Field study.

Shorter life histories may be due to high organic loadings, resulting in increased food and hence growth rate (hypothesis 1), or hypoxia, resulting in differential age-specific mortality (hypothesis 2). The Blane Water population had a higher fecundity and shorter life span than the Allander Water population. The latter site was <1km downstream from a sewage outfall and consequently had a higher BOD and lower summer Po₂ than the Blane Water site. Despite such differences, the populations did not differ either in individual growth rate or size attained after 1yr; therefore, hypothesis 1 was rejected. Hypothesis 2 predicts that organic enrichment will produce selection for high reproductive investment at the earliest possibility, but an annual/semelparous life history was more prominent at the less polluted site. Selection for different levels of investment under the influence of age-specific mortality pressures may still apply.-from Authors