Kindai University clarify mechanisms behind Selenanthias sex change from female to male

A research group consisting of Master's Students Rei Nomura (at the time of the research, currently at the Yamaguchi Prefectural Fisheries Research Center) and Takeru Uechi (at the time of the research, currently at Marino Research) and Associate Professor Yusuke Miyazaki of the Graduate School of Agriculture, Professor Yasuhisa Kobayashi of the Department of Fisheries in the Graduate School of Agriculture at Kindai University, Chief Researcher Kei Miyamoto of the Okinawa Churashima Foundation, and Curator Hidetoshi Wada of the Kanagawa Prefectural Museum of Natural History, has clarified that fish belonging to the genus Selenanthias in the family Anthiadinae, whose collection was difficult and whose reproductive patterns were unknown, undergo a sex change from female to male.

While it is known that some fishes undergo a sex change where their sex alters as they grow, detailed research examples are biased toward species that are commonly used for food. There has been almost no research on fish species like those of the genus Selenanthias that inhabit deep sea zones and are not used for food. This time, they have also clarified the differences between the sexes in the external characteristics of the genus Selenanthias, and it is expected to advance the understanding of fish taxonomy and fish reproductive patterns in the future. The results were published online in Ichthyological Research.

Targeting Selenanthias that inhabit the mesophotic zone (water depth of about 40 to 130 meters) and the rariphotic zone (water depth of about 130 to 300 meters), the research group conducted collections by boat fishing in the waters near such as Sagami Bay, the Kii Peninsula, and Okinawa Island. They performed observations of external morphology and gonadal tissue of the collected fish, and conducted a phylogenetic analysis based on the COI region of mitochondrial DNA.

As a result, a clear difference in coloration was recognized. While male Selenanthias has a black spot on the anal fin, a yellow spot on the head, and pearlescent scales on the body surface, the female has white margins on the opercle and preopercle. It was also found that the body length and the filamentously elongated soft rays of the pelvic fin, anal fin, and caudal fin are longer in males. These are considered to be sexual dimorphism in external morphology. On the other hand, multiple specimens of individuals showing a body color and morphology intermediate between males and females were also obtained.

When examining the gonadal tissue in detail, testes were observed in the male-type external morphology, and ovaries were observed in the female-type external morphology. Among the individuals that showed an intermediate body color and morphology between males and females, bisexual gonads in which ovarian tissue and testicular tissue exist simultaneously were observed in four individuals.

Through a similar approach for Selenanthias sp., the existence of differences between sexes common to Selenanthias was clarified. Although individuals possessing bisexual gonads were not observed in Selenanthias sp. due to a shortage of the number of tested individuals, it was suggested that they undergo protogynous sex change similarly to Selenanthias.

Furthermore, from the analysis of gonadal development and the gonadosomatic index (GSI) of Selenanthias, the possibility was shown that they spawn from winter to early summer (around December to June), and that sex change occurs during or at the end of that period.

Through genetic analysis, it was shown that the male-type and female-type external morphologies are each included within the same species, confirming that the differences in body color and morphology are a "difference between sexes" rather than "different species." In addition, based on the result that individuals showing an intermediate body color and morphology between males and females were intermediate in size between males and females, it can be said it was confirmed that Selenanthias undergoes a protogynous type of sex change. This demonstrated protogynous sex change in this genus for the first time.

This achievement significantly advances the understanding of the mating system in Anthiadinae fishes adapted to deep waters and is expected to contribute to the future development of fish taxonomy and the understanding of fish mating systems.

Miyazaki said, "Although Selenanthias was previously considered a rare fish, it has become clear that a substantial number of specimens can be obtained through catching small fish in deep waters called the mesophotic and rariphotic zones, which has been popular recently among young ichthyologists. In this study, an integrated approach was realized through careful specimen observation by Mr. Nomura during his student days and the support of multiple joint researchers. In addition, by conducting boat fishing surveys at a fixed point every month throughout the year, we were able to obtain specimens in the middle of a sex change, which also led to the estimation of the spawning period. The rare species Selenanthias sp. is still quite difficult to obtain, but both species are beautiful and have high value as ornamental fish (although they are small fish, they are delicious to eat), and we expect that it will become possible to rear them stably in aquariums."

Journal Information
Publication: Ichthyological Research
Title: Sexual dimorphism and protogeny in two species of Selenanthias (Serranidae: Anthiadinae) collected from Japanese waters
DOI: 10.1007/s10228-026-01070-1