On dwarf males in the deep-sea acorn barnacle Bathylasma hirsutum (Thoracica: Bathylasmatidae)

The sessile lifestyle of thoracican barnacles has driven the evolution of a mating behavior that maximizes reproductive success by increasing the reach and number of potential mating partners. While most species are simultaneous hermaphrodites, possessing both male and female reproductive structures, some have evolved an androdioecious system in which hermaphrodites and dwarf males coexist. These dwarf males allocate more resources to male function than growth and typically settle near the orifice of their conspecifics. Environmental factors such as food availability and the number of potential mates influence mating group size in barnacle populations, which is generally lower in deep-sea habitats compared to nutrient-rich shallow-water environments. Evolution is likely to favor the presence of dwarf males in deep-sea barnacles to facilitate mating success in such challenging conditions. The genus Bathylasma Newman & Ross, 1971 currently comprises four extant species, two of which are known to exhibit an androdioecious sexual system. Bathylasma hirsutum (Hoek, 1883) is the sole representative of this genus at northern latitudes, inhabiting hard-bottom habitats down to 1829 meters depth. This study reports on two specimens from the Reykjanes Ridge axis, where minute individuals were found apically attached in the tergal furrow, hypothesized to function as dwarf males. This suggests B. hirsutum as the third species within the genus to exhibit an androdioecious sexual system. These findings offer new insights into the reproductive diversity and sex allocation strategies of deep-sea thoracican barnacles, for which a complete understanding has yet to be achieved.