Between a whale bone and the deep blue sea: the provenance of dwarf males in whale bone-eating tubeworms.

When researchers first caught a glimpse of the lush carpet of pink tubeworms covering the scattered bones of a dead grey whale 2900 m below the surface of Monterey Bay, the excitement onboard the Western Flyer (the mother ship of the remotely operated vehicle the Tiburon) must have been electrifying. The discovery of a new genus and several species of whale bone-eating Osedax tubeworms (Annelida, Siboglinidae) a mere 6 years ago from the deep sea was itself noteworthy. But what the researchers peering into the video monitors aboard the Western Flyer could not have known at that moment was that in the gelatinous tubes of those worms clung even more peculiar forms: harems of tiny, paedomorphic males of Osedax, numbering in the hundreds at times. Whereas female tubeworms bore into the marrow of whale bones (possibly via enzymes from their endosymbiotic bacteria), the dwarf males secondarily colonize the tubes of the resident females. The number of males in a female's tube increases over time in a curvilinear fashion. Dwarf males are known from all Osedax species examined to date, yet the origin of the males was an open question. In this issue, Vrijenhoek et al. provide compelling evidence that dwarf males found in the tubes of female Osedax worms are derived from a common larval pool and are unlikely to be the sons of host females or the progeny of females in the local genetic neighbourhood. This study provides an important foundation for future work on the ecology and evolution of extreme male dwarfism in Osedax and sexual size dimorphism more generally.