Akihiko Shinomiya

Mating habits of the rockfishSebastes inermis

SynopsisUnderwater observations were conducted on the mating habits of rockfish,Sebastes inermis, southwest of Kyushu Island, Japan, in 1984–1986. The reproductive season of the fish lasted from October to January, when most males over 1 year of age established territories ranging from 12 to 70 m2. Home ranges of females overlapped with male territories. Males performed courtship behaviors when females approached the territories. Principal motor patterns of courtship were characterized by ‘lateral display’ and ‘rushing and turning’ movements. The mating pair formed about 30 min before copulation. As the mating pair ascended to 1.5–2.0 m above the bottom, the male suddenly coiled around the females body to copulate. In spite of frequent courtship behavior [14.8 (±10.7 SD)×15 min−1, December 1985] of males, only seven copulations were observed, occurring mostly at dusk in December. In December and January, more than half of the males had accumulated urine, some large volume of it. It was assumed that the male attracted the female by releasing substances via urine during the ‘lateral display’ movements, which were performed just in front of the female, near the snout. Other functional aspects of courtship behavior are discussed.

Bi-directional sex change in the gobiid fish Trimma sp.: does size-advantage exist?

Summary The size-advantage model predicts that a large dominant female should become male in fishes where large males monopolize mating opportunities with multiple females. We investigated the sexuality of the gobiid fish Trimma sp. to determine if a size advantage exists for this goby. Histological examination of the gonads showed that individuals functioned only as one sex at a time, even though the gonads contained both ovarian and testicular tissues. Experiments in aquaria demonstrated that bi-directional sex change occurred in Trimma sp. In natural populations, males were larger than females and the sex ratio was strongly biased towards females. This indicates that Trimma sp. may be polygynous and function primarily as a protogynous hermaphrodite. However, sex-size distributions largely overlapped and contained small males. Furthermore, in many three-female groups, the largest fish remained female and the smaller fish became male. The population density of Trimma sp. was about 12 times that of T okinawae; high population density is likely to allow the possibility that small males attain some degree of reproductive success, and this reproductive success selects for the small male strategy. This is in agreement with the predictions of the size-advantage model and a new version of the size-advantage hypothesis.

Mating system and protandrous sex change in the lizard flathead, Inegocia japonica (Platycephalidae)

Mating system and protandrous sex change of the lizard flathead Inegocia japonica were investigated off Nagashima, Kagoshima, Japan. The reproductive season of this species was estimated to be from June to September by a fluctuation of gonadsomatic index from monthly sampling data. The hermaphrodite fish, having testicular and ovarian parts, were collected from February to December, indicating that the sex change occurred in both reproductive and nonreproductive seasons. Females were larger than males and hermaphrodites. Moreover, five cases of the sex change from male to female were directly confirmed during the 1995–1997 field census in the behavioral observation area. Home ranges, where no aggressive interaction took place among any individuals, overlapped within and between sexes. Seven pair spawnings observed in the area were not of the same individual combination, and the size of pair fish was likely to be random. This species may thus have a promiscuous mating system, i.e., random pairing, which favors protandry as predicted in the size-advantage model.

Mating system of the lefteye flounder,Engyprosopon grandisquama

Underwater observations were conducted on the reproductive behavior and mating system of the lefteye flounder,Engyprosopon grandisquama, off Nagashima, southwest of Kyushu Island, Japan. Two types of males were found: large males, which defended territories against other large males, and small males, which did likewise but only against smaller males. Large males established territories which encompassed or ovelapped the home ranges of 1 or 2 cohabitant females. Territories of the small males, in which a smaller female often maintained a home range, overlapped those of large males. Pair spawning occurred around sunset. Mating of large males with cohabitant females was observed 36 times and that of a small male with a smaller female once. Pair formation was assorted by body size, paired males being larger than females in most cases. Thus, inE. grandisquama, gigamous large males were common, small males occurring within the formers erritorial boundaries mating with smaller females.

Bi-directional sex change and gonad structure in the gobiid fish Trimma yanagitai

Bi-directional sex change in the deep-water gobiid fish Trimma yanagitai was examined. The gonads of all individuals consisted of ovarian and testicular elements, and an accessory gonadal structure. In no gonads were both testicular and ovarian parts simultaneously active. Bi-directional sex changes occurred during the rearing experiments in aquaria under conditions of which there was co-existence of two males or plural females. The sex of individuals could be determined by their relative body size or social dominance: the largest individuals acting as male and the remainder as female.

Inter-group movement of females of the polygynous gobiid fish Trimma okinawae in relation to timing of protogynous sex change

Social conditions and function of inter-group movement of females of the polygynous goby, Trimma okinawae, have been studied at Akamizu Beach, Kagoshima, Japan. Some females moved from their original groups, where the male was still present, to other groups. Before the movement females sometimes temporarily visited the group into which they subsequently moved, suggesting they were able to assess social conditions during the visit. By moving, the females increased in size rank or escaped from similar-sized female competitors in their previous groups. Although the social ranks of the moving females in their original groups were lower than those of the resident females, the ratio of the number of females that changed sex to the number of females surviving at the end of the study did not differ for the two types of female. Inter-group movement of females may increase the probability of their changing sex to become a dominant male.

Two spawning seasons and mating system of the bastard halibut, Tarphops oligolepis

Abstract We investigated the breeding habits of the bastard halibut, Tarphops oligolepis, in the southwest of Kyushu Island, Japan. This fish was found to have two spawning seasons in a year (around July and November); individual fish spawned over the two spawning seasons. During the spawning seasons, males established territories. Home ranges of females overlapped with those of other females and with territories of plural males. The courtship partner changed during a day, and multiple matings among both sexes were observed during a spawning season, suggesting that T. oligolepis bred promiscuously. This is the first report of the mating system among Paralichthyidae.

On the Habitat of Nautilus pompilius in Tañon Strait (Philippines) and the Fiji Islands

Since 1981, we have been engaged in field studies of the habitat of Nautilus pompilius in the Philippines (1981 and 1982) and in the Fiji Islands (1982 and 1983) (Hayasaka et al., 1982; Hayasaka, 1983, 1985). The main purpose of these studies was to obtain basic data on the habitat of N. pompilius in the Philippines and in Fiji, which are at opposite ends of the vast distribution range this species. Although the overall project is still in progress, the results of study to date are summarized in this chapter.

Evolution of bidirectional sex change and gonochorism in fishes of the gobiid genera Trimma, Priolepis, and Trimmatom

Size-advantage and low-density models have been used to explain how mating systems favor hermaphroditism or gonochorism. However, these models do not indicate historical transitions in sexuality. Here, we investigate the evolution of bidirectional sex change and gonochorism by phylogenetic analysis using the mitochondrial gene of the gobiids Trimma (31 species), Priolepis (eight species), and Trimmatom (two species). Trimma and Priolepis formed a clade within the sister group Trimmatom. Gonadal histology and rearing experiments revealed that Trimma marinae, Trimma nasa, and Trimmatom spp. were gonochoric, whereas all other Trimma and Priolepis spp. were bidirectional sex changers or inferred ones. A maximum-likelihood reconstruction analysis demonstrated that the common ancestor of the three genera was gonochoristic. Bidirectional sex change probably evolved from gonochorism in a common ancestor of Trimma and Priolepis. As the gonads of bidirectional sex changers simultaneously contain mature ovarian and immature testicular components or vice versa, individuals are always potentially capable of functioning as females or males, respectively. Monogamy under low-density conditions may have been the ecological condition for the evolution of bidirectional sex change in a common ancestor. As T. marinae and T. nasa are a monophyletic group, gonochorism should have evolved from bidirectional sex change in a common ancestor.

Simultaneous polygyny of the gobiid fish Asterropteryx semipunctata in relation to mate availability

Nesting males of Asterropteryx semipunctata conducted spawning behavior with 2–6 females simultaneously. We carried out field observations on a rocky reef in Kagoshima, Japan, to examine the hypotheses that large males will show multi-female spawning behavior because of their mating advantage, and that simultaneous multi-female spawning will occur when the operational sex ratio (OSR; the ratio of receptive males to receptive females) becomes female-biased. Contrary to our prediction, neither the total number of multi-female spawnings during a spawning season nor mean number of spawning females at a time were correlated with nesting male sizes. This indicates that larger males often did not conduct multi-female spawnings. As predicted, the incidence of multi-female spawning followed the change in the OSR over time—as the OSR in the study area became biased toward females, the incidence of multi-female spawnings gradually increased. Our results suggest that mate availability affects mating patterns in A. semipunctata.