Size dependent sex allocation in a simultaneous hermaphrodite parasite

Most models of sex allocation distinguish between sequential and simultaneo us hermaphrodites, although an intermediate sexual pattern, size-dependent sex allocation, is widespread in plants. Here we investigated sex allocatio n in a simultaneous hermaphrodite animal, the tapeworm Schistocephalus soli dus, in which adult size is highly variable. Sex allocation was determined using stereological techniques, which allow measuring somatic and reproduct ive tissues in a common currency, namely volume. We investigated the relati onships between individual volume and allocation to different reproductive tissues using an allometric model. One measure of female allocation, yolk g land volume, increased more than proportionally with individual volume. Thi s is in contrast to the measure of male allocation, testis volume, which sh owed a strong tendency to increase less than proportionally with individual volume. Together these patterns led to sex allocation being strongly relat ed to individual volume, with large individuals being more biased towards f emale allocation. We discuss these findings in the light of current ideas a bout size-dependent sex allocation in, primarily, plants and try to extend them to simultaneous hermaphrodite animals.