Dimorphic male squid show differential gonadal and ejaculate expenditure

Abstract

Under intense sexual selection, males less successful in fighting and mate guarding often adopt alternative reproductive tactics. In many species, males employing alternative tactics show not only behavioral differences, but also divergences in morphology and physiology, a phenomenon called intrasexual male dimorphism. Herein, we investigated intrasexual male dimorphism in the loliginid squid Doryteuthis plei, associated with alternative mating tactics. We show that small males (sneakers) have spermatophores with discontinuously smaller sperm mass and longer spermatozoa than large males (consorts). Moreover, sneakers produce club-like spermatangia, whereas consorts produce hook-like spermatangia, each type of spermatangia associated with a different female storage site and adoption of a distinct mating position. We also show that dimorphic males have different gonadal investment, sneakers showing higher increment rates in testis mass and higher investment in spermatophoric complex than consorts. Under the complex squid mating system, with two distinct fertilization environments, we hypothesize that sneakers may maximize their reproductive success by both investing more in gonad growth and partitioning ejaculates into extra mating opportunities, whereas consorts may benefit from investment in somatic growth. Squids may be the first example of animals that, even with distinct sperm storage sites, fit the general predictions of sneaks and guards theoretical models.