Intraspecific life-history variation in Erpobdella octoculata ( Hirudinea: Erpobdellidae). II. Testing theory on the evolution of semelparity and iteroparity.

A population of Erpobdella octoculata from the Blane Water was predominantly annual and had a high fecundity; another, from the Allander Water, had low fecundity and a life-span of = or >2yr. Leeches at the latter site bred repeatedly; those from the Blane Water site were semelparous. All leeches appeared to pay the costs of reproduction after cocoon release. There was a negative, linear relationship between reproductive investment (n) and post-reproductive survival of the parents (S_a). The implications of a linear trade-off between S_a and n for life histories of E. octoculata are discussed. Using the 'adaptive landscape technique', the optimal life history for both populations was semelparity, and therefore members of the Blane Water population were optimal but those of the Allander Water population were not. Differences in fecundity between populations appeared to have a genetic basis but fecundity was also modulated by environmental, possibly trophic, conditions.-from Authors