Dimorphic sperm transfer traits for alternative mating tactics in loliginid squid.

Animal species usually have a single sperm-storage site in the female body, but females of the squid Heterololigo bleekeri possess two distinct sperm-storage sites simultaneously. Use of two sperm-storage sites correlates with alternative male mating behaviours: large consorts guard females and place spermatophores inside the oviduct just before spawning, whereas small ‘sneaker’ males place spermatophores on the membrane around the female’s mouth within the arm crown, where a seminal receptacle is present. Previous work showed that permatophore and sperm morphology diverge between consort and sneaker males. Here we show novel dichotomous adaptations in the sperm-transfer strategy of males of this squid, associated with the use of two distinct sperm-storage sites on females. The spermatangia ejaculated from spermatophores were clearly dimorphic: all spermatangia ejaculated from spermatophores smaller than 12 mm from small sneaker males were drop-shaped, whereas all spermatangia from spermatophores longer than 12 mm from larger consort males were rope-shaped. In addition, the drop shaped spermatangia were distinct in having a spine on their base, which might reduce the risk of it being shed from the female’s body surface. Our findings suggest that existence of alternative sperm storage sites, and related biological and environmental factors, lead to the evolution of divergent sperm transfer strategies.