CONTRASTING MORTALITY IN YOUNG FRESH-WATER LEECHES AND TRICLADS

A guild of leeches and triclads coexist and are the most numerous inve rtebrate predators on the stony shores of productive British lakes. Po pulations of all species are food-limited. Mortality of recruited youn g is considerably higher in leech than in triclad populations, and thi s paper investigates reasons for this. In particular, the feeding succ ess of young leeches and triclads in relation to prey species, prey si ze, prey condition (alive or crushed), spatial heterogeneity (with or without the presence of stones or gravel), and the presence or absence of other young or adults predators (leeches or triclads) of the same or different species are investigated in the laboratory. Feeding succe ss by young leeches and triclads on crushed prey without the presence of stones was high, but declined dramatically in leeches but not tricl ads when stones were present. Young leeches and triclads were inept at capturing live prey, of a small or large size, with the exception of soft-bodied prey such as oligochaetes. Feeding success by young predat ors on live prey was not increased by the presence of other young pred ators of the same or different species. With only a few exceptions, th e presence of adult leeches, and to a much lesser extent adult triclad s, increased the feeding success, growth and survival of young leeches and triclads. It is concluded that the high mortality of young leeche s, compared to triclads, in field populations is due to their inabilit y to locate damaged food in an environment with spatial heterogeneity due to a poorly developed chemosensory system. High and low levels of juvenile mortality are accompanied by high and low reproductive rates in leech and triclad populations, respectively. It is unusual for a fo od limited population to have a high level of recruitment, but it is s peculated that the characteristically high reproductive output in para sitic leeches, from which predaceous leeches are derived or have affin ities, has been retained to counterbalance high juvenile mortality rat es.