Summary. Darwin hypothesized that sexes in a species should be similar unless sexual selection, fecundity selection, or resource partitioning has driven them apart. Male dwarfism has evolved multiple times in a range of animals, raising questions about factors that drive such extreme size dimorphism. Ghiselin noted that dwarf males are more common among smaller marine animals, and especially among sedentary and sessile species living at low densities, where mates are difficult to find, or in deep-sea environments with limited energy sources. These benefits of male dwarfism apply well to Osedax (Annelida: Siboglinidae), bone-eating marine worms. Osedax males, notable for extreme sexual size dimorphism (SSD), are developmentally arrested larvae that produce sperm from yolk reserves. Harems of dwarf males reside in the lumen of the tube surrounding a female. Herein, we describe Osedax priapus n. sp., a species that deviates remarkably by producing males that anchor into, and feed on, bone via symbiont-containing 'roots,' just like female Osedax. Phylogenetic analyses revealed O. priapus n. sp. as a derived species, and the absence of dwarf males represents a character reversal for this genus. Some dwarf male features are retained due to functional and morphological constraints. Since O. priapus n. sp. males are anchored in bone, they possess an extensible trunk that allows them to roam across the bone to contact and inseminate females. Evolutionary and ecological implications of a loss of male dwarfism are discussed.
