Alternative Male Mating Tactics in Hypoptychus dybowskii (Gasterosteiformes): Territoriality, Body Size and Nuptial Colouration

Abstract

Reproductive behaviour of Hypoptychus dybowskii was studied in the field and aquarium. Two male mating tactics were found. Some males with remarkable nuptial colouration established territories around sargasso weeds, while the remainder schooled. A territorial male began courtship in response to the approach of a gravid female. Several schooling (non-territorial) males followed the gravid female and were often attacked by the territorial male. Egg masses, comprising a mean of 32 eggs of about 2mm in diameter, were coiled around the branching points of sargasso weeds in the male's territory. Just after spawning by a female, both the territorial male and several non-territorial males rushed to the spawning site to fertilize the eggs; the territorial male, situated nearest to the female, rushed first. Females were never observed to spawn with non-territorial males only. The territorial male frequently picked at the eggs so as to attach them firmly together around the sargasso weeds, for about 30min after spawning and occasionally thereafter, but never conducted further paternal care such as fanning. Territorial males were not always larger than others, and schooling males in the aquarium suddenly became territorial or vice versa, with rapid changes in body colouration. It is therefore suggested that males alternate two mating tactics depending on their physiological condition, or as a mixed strategy at the individual level.