Persistence of a highly monoandrous mating system despite an extremely male-biased operational sex ratio in the firefly squid Watasenia scintillans

Unlike most cephalopods, the firefly squid Watasenia scintillans is highly monoandrous. We investigated this unusual phenomenon by considering the rare occurrence of polyandrous mating and the extremely male-biased operational sex ratio (OSR) at the beginning of the mating season. Theoretically, male-biased OSR can intensify competition for mates, leading to increased polyandry. We estimated OSR (male/female) as 32.7 and 9.1 at the beginning and end of the mating period, respectively. Next, we estimated the rate of polyandry in a population on a weekly basis, based on our new finding that the rate of polyandry can be estimated stochastically from the total number of spermatangia attached to one of the two seminal receptacles within a female. We found that the female squid sustains their highly monoandrous mating regime despite the fact that the OSR is extremely male-biased. Furthermore, there were no significant changes in the polyandry rate during the reproductive season. These results suggest that the squid mating system was not influenced by seasonal changes in OSR. Here, we discuss the evolutionary mechanism of how monoandry persists in this species.