EVIDENCE FOR STRATEGIC EGG-PRODUCTION IN A HERMAPHRODITIC CESTODE

The cestode Schistocephalus solidus is a simultaneous hermaphrodite th at grows in 2 intermediate hosts and reproduces rapidly within a few d ays in the gut of a bird. Reproduction takes place by self- or cross-f ertilization. Here, it was tested whether egg production differs betwe en S. solidus that reproduce alone and those that are allowed to repro duce in pairs. Egg production in an in vitro system was found to depen d on the cestodes' social situation. When kept alone, larger cestodes produced larger eggs. This was not so when kept in pairs - the differe nce between these 2 reproductive modes being highly significant in thi s respect. Further experiments revealed that, within the first 3 days, these hermaphrodites produced a larger total egg mass when kept alone than when kept in pairs. This was also reflected by the energy conten ts of the cestodes after this time-span: selfers had used up more ener gy than paired worms. Furthermore, S. solidus appeared to adjust its i nvestment per egg depending on whether the offspring will be the resul t of self- or cross-fertilization. Selfers produced larger numbers of eggs, but these eggs were smaller and contained even smaller embryos p er given egg size than eggs of potentially outbreeding cestodes. Selfe d eggs reached lower hatching rates. Although this is to be expected f rom inbreeding depression it may also be an effect of the reduced mate rnal investment per egg. The observed phenotypic plasticity in the rep roduction of S. solidus is discussed within 4 evolutionary frameworks: local mate competition adjusted for hermaphrodites, the hermaphrodite 's dilemma, bet-hedging, and sib-competition.