Threshold dimorphism in ejaculate characteristics in the squid Loligo bleekeri

Male dimorphism has been thought to correlate with alternative reproductive behaviors. Alternative reproductive behaviors promote asymmetry in sperm competition, and the differences in fertilization success could promote adaptations in ejaculate characteristics in relation to each reproductive behavior. Using allometric analysis, we show that ejaculate dimorphism clearly exists in males of the squid Loligo bleekeri, a cephalopod species with body size-related alternative mating behaviors. A morphological switch point was detected for internal characters: larger individuals produced discontinuously longer spermatophores than did smaller individuals, although no switch point was detected for external characteristics (fin length, fin width, head width, mantle width, tentacle length and hectocotylus length) except for bimodal body size. This clear internal switch point could be an adaptation to the characteristic alternative mating behaviors of loliginid squid, in which males use different mating tactics to pass spermatophores to different sperm storage sites in and on the females. Our results indicated that alternative reproductive behaviors can result in morphological adjustment in internal characteristics.