To clarify the role of estrogens in the onset of sex change in fish, estrogen levels were artificially reduced in hermaphroditic and gonochoristic female fish via the treatment with aromatase inhibitors (AIs). AI treatments caused depletion in blood estrogen levels and induced complete sex change from a female to a male in protogynous three-spotted wrasse and the honeycomb grouper. Opposite-direction sex change of the protandrous yellowtail anemonefish was also induced by AI treatments. Not only in hermaphrodites, AI treatments induced testicular differentiation and, in certain circumstances, a complete sex change in the developing ovaries of the gonochoristic fish: tilapia, medaka, zebrafish, carp and golden rabbitfish. We demonstrated that estrogen depletion induces the female to male sex change in both hermaphroditic and gonochoristic fish. Results suggested that some germ cells in the ovaries of both hermaphroditic and gonochoristic fish maintain sexual bipotentiality, which is the ability to differentiate into both female and male germ cells, throughout the life and that the fate of these germ cells' differentiation depends on the level of endogenous estrogen. Higher levels of circulating estrogen maintain femaleness, while lower levels force their differentiation into males. These studies contribute to the progress of aquaculture.

This review provides a brief summary of our field research on the behavioral ecology of coral reef fishes conducted at Sesoko Station, University of the Ryukyus, Okinawa, Japan. We have continued observations and experiments on fish behavior using SCUBA or by snorkeling on the fringing reefs of Sesoko Island since 1982. The results of our four main research subjects, that is, mimicry, parental care and mating systems, bidirectional sex change, and mate choice and alternative mating tactics, all of which have been major subjects of behavioral ecology, are summarized with references and historical information. The titles and speakers of papers, presented at two international meetings organized by the author and held at the Sesoko Station in 1991 and 2004, and field studies conducted on fishes in Sesoko Island by other researchers, including graduate students from various universities, are introduced. The Sesoko Station has been providing excellent facilities for field studies of coral reef fishes and has contributed greatly to the development of the behavioral ecology of fishes.

To better understand the eco-physiological characteristics of reproduction in the coral reef small honeycomb grouper, Epinephelus merra, we investigated their spawning migration using biotelemetry, in addition to reproductive physiology analysis. Histological observations indicated that final ovarian maturation was not completed in honeycomb grouper collected from the coral reef pond (CRP), even during their spawning season. Additionally, our visual observations revealed that fish numbers decreased in the CRP after a full moon, which is thought to be their spawning time, suggesting their spawning migration. Next, we investigated the migration of honeycomb grouper during their spawning season using biotelemetry. Our investigations indicated that honeycomb grouper migrated from the inside of the CRP to the outside after a full moon, and then back to the inside again a few days later. These results strongly suggested that honeycomb grouper migrate to spawning sites located outside of the CRP, attain final ovarian maturation, and spawn after a full moon in the spawning season.

Sesoko Station, Okinawa, has been the site of many significant advances in coral reproductive research and it continues to be a preferred destination for both Japanese and international researchers. Consequently, there are decades of spawning observations, which we present and explore here with the aim of making it easier to predict when species spawn at Sesoko Station. The data include over 700 spawning observations from 87 species of reef-building hermatypic corals. Almost all spawning occurred between dusk and dawn, with most spawning activity concentrated in the 2 to 4 hours after sunset. Some phylogenetic patterns were evident: most Acropora species spawn on or around the 6^th full moon after December 21^st (the northern hemisphere winter solstice); spawning in common species of merulinids and Porites appears to be concentrated around the 7^th full moon and spawning in the fungiids around the 8^th and subsequent full moons. The night of peak spawning with respect to the night of the full moon varied considerably among years in common Acropora species, but was dependent on the calendar date of the full moon in May or June. Therefore, despite an extended spawning season of over three months and considerable variation in the calendar date of spawning in many species among years, the month and night of spawning are reasonably predictable for many species enhancing the value of Sesoko Station as a site for coral reproductive research.

We investigated the effect of five levels of pCO₂-adjusted seawater (300 μatm [pre-industrial] to 1200 μatm [near-future]) on calcification rates of six coral species: Acropora nasuta, A. tenuis, Montipora digitata, Pocillopora damicornis, Porites cylindrica, and Galaxea fascicularis, which are common species in the subtropical Ryukyu Archipelago, Japan. In most species, declines in calcification rates were pCO₂-dependent as previously reported. Responses to lower pH seawater significantly differed among species in common-garden tanks. Corals showed both linear and non-linear responses to a wide range of seawater pCO₂, which could be attributable to physiological differences in inter-species. Our results are consistent with previous studies and suggest that the responses of corals to ocean acidification can vary among species.

Ocean acidification poses a severe threat to corals; declines in carbonate ion concentrations caused by increasing atmospheric CO₂ partial pressure (pCO₂) can severely impact coral calcification. Thus, there is an urgent need to understand the impacts of near-future ocean acidification on corals. In this study, we compared the effects of seawater at present and near-future pCO₂ (approximately +200 μatm) levels on skeletal weights of new coral recruits. Experiments were carried out using precisely pCO₂-controlled aquaria supplying stable pCO₂-controlled seawater in a flow-through system. Our results show that skeletal weights of new coral recruits decreased significantly at +200 μatm pCO₂, which is expected to be reached within this century if ocean acidification continues at the present pace.

Coral reef ecosystems extend beyond the standard SCUBA diving depth. These ecosystems found between approximately 30 and over 150 m are referred to as mesophotic coral ecosystems (MCEs). Mesophotic coral communities have already been reported from Okinawa over 50 years ago. However, since then little additional knowledge has been obtained on the distribution and diversity of mesophotic corals in the region. Here, we provide an overview of the coral communities observed at selected sites in the region around Sesoko Island. Using photo-quadrats, we surveyed coral communities and specifically the contribution of the dominant coral genera to the total coral cover between different sites at depths ranging from 30 to 80 m. We found that deep coral communities around Sesoko Island hold a high potential biodiversity and need to be taken into account into coastal management plans.

Cultures of larger benthic symbiont-bearing foraminifers have been used for studying growth and reproduction by long term investigations taking several months. For getting results that can be interpreted as ‘natural growth', the culture medium must represent the same environmental conditions like the sea at sampling time. Growth of Nummulites venosus specimens was tested where about 40 chambers have been built in the natural environment at the time of sampling. After exposition to cultures with the same environmental conditions as those at the sampling stations, growth in the culture was compared with ‘natural growth' of the initial part. This was done calculating deviations of empirical data obtained in the culture from the Gompertz function established for the cell part constructed in the sea. Three reaction types could be detected: 1) strong (negative) deviations just after exposure to the culture; 2) continuing natural growth in the culture, starting deviation in later chambers; 3) throughout continuing natural growth. The chamber building rate, calculated by the Michaelis-Menten function as the second parameter determining growth, differs significantly between growth in the nature and culture, where the velocity of chamber building rates decrease immediately after exposure into the cultures. It must be concluded, that the culture medium, although approximating natural conditions rather perfectly, always impact growth, especially in the chamber building rate. Therefore, studies checking influences of environmental factors using cultures must be carefully interpreted.

Zalonema sesoskoensis n. sp. is described from shallow subtidal sediments of Sesoko Island. The new species is characterised by annulated cuticle with minute spines, male with ventral and lateral alae, circle of eight subcephalic setae between cephalic setae and anterior edge of amphideal fovea, amphideal fovea with 3.5 turns in male and 2.5 turns in females, buccal cavity with dorsal tooth and minute ventral tooth located further posteriorly in females than in male, spicules 1.1-1.7 cloacal body diameters long, and male with seven precloacal supplements, posteriormost supplement cup-shaped, cuticularized and conspicuous, other supplements small, pore-like. Zalonema sesokoensis n. sp. is the first Zalonema species to be described from Japanese waters. There are now 11 species in the genus worldwide.

Allopodion ryukyuensis is a gall-inducing copepod of Montipora coral that has been collected from the coral reefs of Sesoko Island, Okinawa, Japan. The current study was conducted to elucidate its geographic distribution along the Ryukyu Archipelago from Yonaguni Island to Amami Island, including Green Island, Taiwan. The result shows that A. ryukyuensis was found from Montipora collected from all study sites.

The ultrastructure of a symbiotic dinoflagellate Zooxanthella nutricula within the radiolarian Didymocyrtis tetrathalamus collected off the northwestern coast of Okinawa Island, Japan was examined in the symbiotic state and the cultured motile stage. In hospite, Z. nutricula had a coccoid morphology, and the typical dinoflagellate structures, i.e., cingulum, sulcus, and flagella were lost. The coccoid non-motile cells were surrounded by a perialgal envelope of the host radiolarian cytoplasm, and they possessed a dinokaryon with condensed chromosomes and mitochondria with tubular cristae. The chloroplast was located at the periphery of the cell and had one or two pyrenoids. In culture, however, the motile cells exhibited a typical cell shape of peridinioid dinoflagellates. The cell covering consisted of a plasma membrane and amphiesmal vesicles containing the thecal plates whose arrangement was congruent to the order Peridiniales. The chloroplast was enclosed by three membranes and the pyrenoid was the double-stalked type. Pusules constructed by tubules with invaginations were present, but eyespot and trichocysts were lacked. Symbiotic dinoflagellates, such as Zooxanthella, Symbiodinium, and Amphidinium, have historically been referred to using the general term ‘zooxanthella(e)' because of their similar appearances as yellow-brown spherical minute cells under light microscopy. However, the motile cells of Z. nutricula exhibited a typical peridinioid morphology and thecal plate arrangement that obviously differed from those of the other Symbiodiniaceae genera and Amphidinium. Considering the ultrastructural features and the currently available sequence database, it was suggested that Z. nutricula is a symbiotic dinoflagellate that lives specifically in holoplanktonic protist radiolarians.

Knowledge of environmental factors is crucial in understanding biological and ecological processes. Yet information on the environment around Sesoko Island, Okinawa, Japan, one of the main locations for coral reef research in Japan, remains scarce. Data of air and sea surface temperature (SST), wind velocity, wave height, and frequency of typhoons have been manually recorded at Sesoko Station, Tropical Biosphere Research Station, the University of the Ryukyus from September 1990 to November 2021. Here we describe the seasonal and long-term trends in these environmental variables at Sesoko Island. Some of the key findings were that the air temperature and SST fluctuated by ~9-12°C throughout the year. A rise in air temperature and SST between 1990 and 2021 was observed in the winter and autumn season, respectively. The Degree Heating Week (DHW) based on the in-situ data reflected the bleaching observations around Sesoko Station. The DHW exceeded the critical bleaching level of 8°C-week in 1998 and the significant bleaching level of 4°C-week in 2001, 2016, and 2017. Weak southerly winds were dominant in summers, while stronger northeasterly winds were dominant in winters. The frequency of winds between 3.4 to 7.9 m/s and northeastern winds have increased through time. Typhoons generally occur between May and October, and the frequency of typhoons has not increased over the past 30 years. Wave heights never exceeded 0.5 m and were highest between July and September. These findings will provide a reliable baseline of the environment at Sesoko Island for further ecological studies.