I Am Your Father: Investigating the Genetic Mating System in the Antarctic Sea Spider Nymphon australe

AbstractPycnogonids (sea spiders) are benthic invertebrates exhibiting unique reproductive strategies including paternal brood care by the male in many species. To date, the mating systems of brooding Antarctic sea spiders have yet to be investigated via molecular methods, despite their dominance and importance in the Southern Ocean. To better understand how sea spiders reproduce and maintain their natural populations in this region, we employed 3RAD-derived single nucleotide polymorphisms to investigate genetic mating systems of the abundant, and putatively circumpolar, Antarctic sea spider Nymphon australe. By analyzing single nucleotide polymorphisms in genomes of individual larvae taken from offspring-carrying males, we inferred paternal full- and half-sibships and parentage for specimens of N. australe collected from the eastern Antarctic continental shelf. Notably, N. australe exhibits a polygynandrous mating system where both males and females engage in multiple mating events. Male brood partitioning on their ovigerous legs varied, with some males partitioning offspring into clutches by female genotypes and others carrying progeny from multiple female genotypes in a single clutch. However, we found no evidence of cuckoldry, where males inadvertently raise offspring sired by other males, indicating that this species has a high assurance of paternity for the progeny carried by each individual male. These findings provide foundational insights into the genetic mating system of Antarctic sea spiders and contrast with the cuckoldry frequently observed in other male brood care systems. This contrast highlights some of the evolutionary pressures acting on reproductive strategies in polar environments.