The overlapping reproductive traits of the two male mating types of the oval squid Sepioteuthis lessoniana

Abstract

The oval squid Sepioteuthis lessoniana is polyandrous, and a female mates with several males during a spawning season. This has resulted in two alternative male mating tactics in this species. The larger consort males usually use the male-parallel (MP) mating posture to put spermatophores at the opening of the oviduct; the smaller sneaker males predominantly use the male-upturned (MU) mating posture to place spermatophores around the female's buccal membrane. Since squid eggs are released from the oviduct, MP mating is expected to have a higher fertilization rate than MU mating. Given the fact that these two male mating tactics are largely dependent on the body size of the male relative to the female, it is unclear how the smaller males are able to reproductively compete with the larger males. On analyzing spermatophore and sperm morphology from populations of adult male squids, spermatophore size was found to be positively correlated with mantle length of these squids, but sperm length was negatively correlated with squid mantle length. More importantly, no distinct size dimorphism exists in relation to either spermatophore morphology or sperm morphology. The overlap in size distributions of the spermatophores and sperms of the male squid thus implies that medium-sized males may have flexible reproductive strategies, switching between MP and MU mating tactics depending on the context of the mating event and/or female choice. This presumed behavioral plasticity might increase the reproductive success of these males and maintain the dynamic stability of two alternative mating types in this polyandrous species.