title: The different types of sperm morphology and behavior within a single species: Why do sperm of squid sneaker males form a cluster?
abstract: Some coastal squids exhibit male dimorphism (large and small body size) that is linked to mating behaviors. Large "consort" males compete with other, rival males to copulate with a female, and thereby transfer their spermatophores to her internal site around the oviduct. Small "sneaker" males rush to a single female or copulating pair and transfer spermatophores to her external body surface around the seminal receptacle near the mouth. We previously found that in Loligo bleekeri, sneaker sperm are ~50% longer than consort sperm, and only the sneaker sperm, once ejaculated from the spermatophore (sperm mass), form a cluster because of chemoattraction toward their own respiratory CO2. Here, we report that sperm clusters are able to move en masse. Because a fraction of ejaculated sperm from a sneaker's spermatophore are eventually located in the female's seminal receptacle, we hypothesize that sperm clustering facilitates collective migration to the seminal receptacle or an egg micropyle. Sperm clustering is regarded as a cooperative behavior that may have evolved by sperm competition and/or physical and physiological constraints imposed by male mating tactics.
authors.affiliation: Oki Marine Biological Station; Education and Research Center for Biological Resources; Shimane University; Oki, Shimane, Japan.
authors.forename: Noritaka
authors.initials: N
authors.lastname: Hirohashi
authors.affiliation: Atmosphere and Ocean Research Institute; University of Tokyo; Kashiwa, Chiba, Japan.
authors.forename: Yoko
authors.initials: Y
authors.lastname: Iwata
keywords: alternative reproductive tactics; chemotaxis; sperm cooperation; sperm evolution; squid