Mating system and protandrous sex change in “Magochi” Platycephalus sp. 2 (Platycephalidae)

Abstract

In this study we investigated sex change and the mating system of a platycephalid fish, Platycephalus sp. 2 (Japanese name: Magochi), found in Japanese waters. Among 202 specimens from Tateyama, Chiba, Japan, obtained in 2014 and 2015, histological observation of the gonads distinguished 103 females, 97 males and two hermaphrodites (individual with both ovarian and testicular parts). Females were significantly larger than males. In a rearing experiment conducted with six males and three females, only the smallest male changed sex to female, thereby revealing protandry in this species. In field observations, seven males and eight females appeared throughout the study period. During pairing, a male would follow a female on the sandy bottom. The mating system of this species has been regarded as random mating based on temporal pair bonding, no territoriality, and no size-assortative mating, which correspond well with the size-advantage model. In a previous study Platycephalus sp. 2 was regarded as a gonochore, with a higher growth rate for females than males. Thus, females should be larger than males. In the present study, there was no significant difference in age between the sexes, as estimated from the number of opaque zones in sagitta otoliths. Overall, these findings indicate that there are primary females and males without sex change. Thus, protandrous and gonochoristic individuals may co-exist in populations of Platycephalus sp. 2.