First Evidence Of Male Dwarfism In Scale Worms: A New Species Of Polynoidae (Annelida) From Hermit Crab And Molluscan Shells

Extreme sexual size dimorphism is one of the most striking phenomena in evolutionary biology. While the origin has been well discussed and some causes have been suggested, the evolutionary history remains unclear. We found a new species of deep-sea scale worm (Annelida: Polynoidae) inside gastropod shells, either empty or occupied by hermit crabs collected at 140-306 m in depth, Mie Prefecture, Japan. This highly specialized habitat, together with the fact that it has never been found free-living, led us to consider the scale worm to be an obligate symbiont. The species is characterized by males being dwarf, with their minute bodies (ca. one-fourth the length of females) always riding on the dorsal side of females, being thus the first case of extreme sexual size dimorphism in scale worms. Based on a detailed morphological, histological, and molecular phylogenetic approach we are here describing a new species, Eunoe issunboushi sp. nov.