Alternative mating pattern in Enoploteuthis chunii is associated with polyandry and male-biased sex ratio

Although cephalopods are primarily polyandrous, genetic evidence revealed rare monogamy in <i>Watasenia scintillans</i> . Here, we studied the sister species <i>Enoploteuthis chunii</i> . We found that copulation began in early July, with egg spawning occurring in early August. The sex ratio was female-biased until late August, then shifted to male-biased. After early August, the average number of sperm sacs in females exceeded that of males. As the season progresses, females began storing sperm sacs in a cryptic sperm pocket on the right lateral trunk within the mantle. This behavior is associated with male-biased sex ratio. These results suggest polyandry in <i>E. chunii.</i>