Quasi-monandry with a single-male predominating female sperm storage in pair-forming squid, Thysanoteuthis major

Monogamous animals often engage in long-term pair bonding. The oceanic diamond squid, Thysanoteuthis spp . are unique among cephalopods in that way they are often found in pairs consisting of one male and one female. In this study, we investigated the extent of mating fidelity in female Thysanoteuthis major in comparison to the highly polyandrous oceanic squid, Todarodes pacificus. Fragment length analysis was carried out using newly developed microsatellite markers. We found that in T. major , most females are polyandrous and each seminal receptacle (SR) contained a mixture of sperm genotypes with the mean of 6 different alleles. Additionally, all SRs within a female exhibited similar patterns of allele peaks with usually two major alleles, suggesting that there is a single male predominating sperm storage. In contrast, T. pacificus exhibited numerous alleles within SR and displayed different allele patterns between SRs. In T. major , rare alleles tended to appear in SRs locating at the most ventral side. These results suggest that T. major females are generally polyandrous and store sperm from males using two different strategies: one involving pair bonding, and the other through extra-pair copulation. We discuss a possible correlation between long-term pair bonding and a single-male predominance for sperm storage in this species.