Reproductive strategies in Dendrocoelum lacteum (Turbellaria) — comparisons between Swedish and British populations

Summary

Traits of the reproduction biology of Dendrocoelum lacteum in Sweden and Great Britain were compared on the basis that genotypical differences between two populations reflect important differences in the selective advantage of these properties in relation to prevailing circumstances. Swedish triclads reproduce earlier, during a more limited time of the year, exhibit higher output of cocoons and hatchlings, as well as higher fertility than British ones. Further, hatching time and sterility percentages of Swedish cocoons are considerably lower. This was attributed to earlier temperature rise and higher densities of Asellus aquaticus, its main food.

Swedish triclads invest proportionally more energy on reproduction at the expense of other functions, therefore suffering from higher intraspecific competition. This leads to annual death of all adults in Sweden, whereas in Britain a considerable part postpone their breeding to next year; a tendency of iteroparity. Swedish triclads are considered to be more of r-strategists than are the British populations. Reproduction time is regulated by temperature regime, whereas the quantitative output is mainly set by food intake and age of the animal.