Abstract

Within the Annelida phylum, the Histriobdellidae Family groups freshwater polychaetes sharing an ancient Gondwanic relationship; very little is known about their life histories. Here we describe the life cycle of Stratiodrilus aeglaphilus, an epibiont of the crab Aegla laevis, which inhabits rivers of central Chile. All its life stages were found associated with A. laevis. The presence of a chitinous penis and its larger size permit distinguishing males from females. The observation of larvae/embryos at different development stages allowed us determining the absence of a free-swimming larvae stage in this species; recently hatched juveniles have movements similar to those of adults. S. aeglaphilus shows direct development; different embryo development stages could be observed in the coxal base of the Aegla pereiopods, where all females migrate for oviposition. The life cycle described here is similar to the cycle of other marine organisms such as molluscs and crustaceans. These organisms have adapted to limnic systems by changing their life cycles and showing an absence of planktonic larvae that ensures higher survival in these environments.