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The frequency accuracy of optical atomic clocks has dramatically increased over the past 15 years, improving by more than two orders of magnitude from 16 digits of precision to 18 or even 19 digits of precision. Since around 2015 researchers from around the world began to consider a redefinition of the second that uses optical atomic clocks. Since then, the development of optical atomic clocks has progressed and the recent results demonstrated to detect the frequency change with 18 digits of precision.The National Institute of Information and Communications Technology (NICT) has developed the Sr optical lattice clock and optical ion clocks employing In+ and Ca+, as well as a Sr optical lattice clock that provides calibration data to BIPM. On the other hand, the centimeter-level uncertainty of site elevation has caused 10-18-level frequency uncertainties of optical frequency standards. Therefore, it is significantly important to understand frequency changes caused by solid-earth tides that often range from 10 to 20 cm in amplitude, by oceanic tidal loading, crustal deformations due to earthquakes, and ground movements with groundwater changes for the stable operation of optical atomic clocks.NICT, in collaboration with partners including the Geospatial Information Authority of Japan (GSI), has begun observations and data analysis to evaluate how these effects interact with optical atomic clocks. Since early 2021, NICT and GSI have been jointly conducting leveling surveys and relative gravimeter observations at NICT’s headquarters in Koganei. These observations reduce the contribution of gravitational redshift to the total uncertainty of the NICT-Sr1 optical lattice clock has been reduced to the 10-19 level.With the support of National Institute of Polar Research (NIPR), absolute gravity measurements were performed in the August 2019 and May 2022 to evaluate the effects of the 2011 March 11 Tohoku megaquake on coseismic vertical crustal movement. The obtained absolute gravity change between the two periods was -43.8 μGal. This matches the trend of GNSS result obtained by GSI, which show a vertical movement of up to 31.5 mm from August 2019 to May 2022, equivalent to about -10 μGal gravity change, even though the values do not agree precisely.We have introduced a Micro-g LaCoste's gPhoneX gravimeter for continuous gravity measurements near by the optical clocks in the end of 2021. The preliminary results over seven months detects stable gravity change due to solid-earth tide with about 22 μGal precision. In addition, we have started to investigate the temporal variation of the ground water level at Koganei. We are also monitoring vertical crustal movements by geodetic GNSS measurements. We will investigate uncertainties of optical clocks due to vertical movements caused by geodetic phenomena using continuous gravity and GNSS measurements.

The 13th Symposium on Polar Science/Ordinary sessions [OG] Polar geosciences, Wed. 16 Nov.

Understanding how evolutionary forces interact to drive patterns of selection and distribute genetic variation across a species' range is of great interest in ecology and evolution, especially in an era of global change. While theory predicts how and when populations at range margins are likely to undergo local adaptation, empirical evidence testing these models remains sparse. Here, we address this knowledge gap by investigating the relationship between selection, gene flow and genetic drift in the yellowtail clownfish, Amphiprion clarkii, from the core to the northern periphery of the species range. Analyses reveal low genetic diversity at the range edge, gene flow from the core to the edge and genomic signatures of local adaptation at 56 single nucleotide polymorphisms in 25 candidate genes, most of which are significantly correlated with minimum annual sea surface temperature. Several of these candidate genes play a role in functions that are upregulated during cold stress, including protein turnover, metabolism and translation. Our results illustrate how spatially divergent selection spanning the range core to the periphery can occur despite the potential for strong genetic drift at the range edge and moderate gene flow from the core populations.

The 11th Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 2 Dec.

The 11th Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Poster presentations, Mon. 16 Nov. - Fri. 18 Dec.

This study aims to investigate solar radiation pressure acting on the spherical geodetic satellites, Ajisai, LAGEOS-1, and LAGEOS-2. The solar radiation pressure coefficients ( C R ) are derived in two independent ways: (a) through precise orbit determination (POD) using satellite laser ranging (SLR) data, and (b) through modeling using the optical properties of the satellite surface material. The average C R value of Ajisai (1.039), as calculated from the time series of C R POD estimates every 15 days, is consistently smaller than those of LAGEOS-1 (1.140) and LAGEOS-2 (1.103). This difference can be explained by the fact that the surface of Ajisai is mostly covered by mirrors. The Ajisai C R values estimated by POD show remarkable semi-annual variation, which disagrees with the results of a previous study (Sengoku et al., 1995) predicting that the C R of Ajisai varies almost annually. We attribute this semi-annual variation to two physical reasons: the non-spherical additional cross-sectional area due to the “attached fitting ring” and the low reflectivity of the surface material in the polar regions. Furthermore, the solar radiation pressure acting on Ajisai varies annually in a direction perpendicular to the sun-satellite vector. Finally, the two independent C R values of Ajisai agree more when we assume a total solar irradiance of 1361 W/m2, whereas the value 1367 W/m2 has been commonly used in POD.

Patchy habitats often enhance species coexistence and, consequently, abundance of each species. The present study examined two indicators of potential habitats for anemonefish Amphiprion frenatus: total area of dark-colored patch reefs that are detectable on an aerial image with image analysis software, and total area of tall patch reefs (>1.5 m in height) that are detectable on stereoscopic aerial images with a stereoscope. Relationships between patch reef area and anemonefish abundance, as estimated by number of host anemone Entacmaea quadricolor, were investigated at Shiraho Reef, Ishigaki Island, Japan. Total number of anemones was not correlated with total area of tall patch reefs but was highly correlated with total area of dark-colored patch reefs. Underwater observations confirmed that upper surface of tall patch reefs often involved bare substratum, whereas dark-colored patch reefs did not include this substratum. Because reef area approximately reflected reef volume within shallow back reefs, total area of dark-colored patch reefs that are detectable by aerial photography will reflect total volume of reef fish habitat. The present study suggests that aerial photography with a drone can provide a useful predictor of reef fish abundance.

The Tenth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 4 Dec. / 3F Seminar room, National Institute of Polar Research

The Tenth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 4 Dec. / 3F Seminar room, National Institute of Polar Research

Geological reconstruction of the retreat history of the East Antarctic Ice Sheet (EAIS) since the Last Glacial Maximum (LGM) is essential for understanding the response of the ice sheet to global climatic change and the mechanisms of retreat, including a possible abrupt melting event. Such information is key for constraining climatic and ice-sheet models that are used to predict future Antarctic Ice Sheet AIS melting. However, data required to make a detailed reconstruction of the history of the EAIS involving changes in its thickness and lateral extent since the LGM remain sparse. Here, we present a new detailed ice-sheet history for the southern Soya Coast, Lutzow-Holm Bay, East Antarctica, based on geomorphological observations and surface exposure ages. Our results demonstrate that the ice sheet completely covered the highest peak of Skarvsnes (400 m a.s.l.) prior to ∼9 ka and retreated eastward by at least 10 km during the Early to Mid-Holocene (ca. 9 to 5 ka). The timing of the abrupt ice-sheet thinning and retreat is consistent with the intrusion of modified Circumpolar Deep Water (mCDW) into deep submarine valleys in Lutzow-Holm Bay, as inferred from fossil foraminifera records of marine sediment cores. Thus, we propose that the mechanism of the abrupt thinning and retreat of the EAIS along the southern Soya Coast was marine ice-sheet instability caused by mCDW intrusion into deep submarine valleys. Such abrupt ice-sheet thinning and retreat with similar magnitude and timing have also been reported from Enderby Land, East Antarctica. Our findings suggest that abrupt thinning and retreat as a consequence of marine ice-sheet instability and intrusion of mCDW during the Early to Mid-Holocene may have led to rapid ice-surface lowering of hundreds of meters in East Antarctica.

The Ninth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 5 Dec. / Entrance Hall (1st floor), National Institute of Polar Research

The Ninth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 5 Dec. / Entrance Hall (1st floor), National Institute of Polar Research

The Ninth Symposium on Polar Science/Special session: [S] Antarctic ice-ocean interaction ～observation, reconstruction, and modeling～, Tue. 4 Dec. / Entrance Hall (1st floor) at National Institute of Polar Research

Because fish have a high dispersal ability, an understanding coral reef fish metacommunity structure is vital for effective conservation. Coral reefs provide patchy habitat of various sizes and scales. We examined the species–area relationship (SAR) of damselfish (Pomacentridae) assemblages over 81 environmentally homogenous patch reefs ranging 0.07–45.4 m2 with low coral cover. Patch reefs were located in the shallow back reef ( 1/2500) color aerial photograph used as a fine-scale seascape map. To assess the effects of three-dimensional meso-scale rugosity on species richness, we assumed that all reefs had a cylindrical shape and examined species by volume (area × height) relationships (SVR). Patch reef volume was a better determinant of species richness than area, and the regression functions of SVR provided better estimates of patch reef species richness. Neither the observed SVRs nor SARs, however, could be explained by a random placement model alone. Our results suggest that several small reefs are likely to have higher species richness than a single large reef of equivalent area in the shallow back reef where large patch reefs are flat. Thus, total patch reef volume (area × height) better reflects meso-scale rugosity and is a useful indicator of total species richness relative to the total amount of essential habitat in shallow back reefs.

In local reef fish communities, species richness increases with increasing reef area. At Ishigaki Island, Japan, species richness is much lower on large reefs in the shallow back reef than that expected from random placement model simulations (RPMS). As three aggressive territorial herbivorous damselfish, Stegastes nigricans, Stegastes lividus and Hemiglyphidodon plagiometopon, coexist only on such large reefs, we focused on these species and examined patterns of their distribution and abundance on 84 patch reefs of various sizes (area and height). We also examined their aggressive intra- and interspecific behavioural interactions and habitat use on the two large reefs (the largest complex patch reef and the large flat patch reef) among the 84 patch reefs. While the abundance of both S. lividus and H. plagiometopon was highly correlated with patch reef area, that of S. nigricans was closely correlated with patch reef height. For S. nigricans and S. lividus, interspecific interactions occurred significantly more frequently than intraspecific interactions on the large flat patch reef. However, there was no significant difference in frequencies of the two interaction types on the largest complex patch reef, where they three-dimensionally segregated conspecific territories. This study suggested that reef height as well as reef area influence the distribution and abundance of these territorial herbivorous damselfish in the shallow back reef. As large patch reefs cannot be tall allometrically in shallow back reefs, relatively flat patch reefs may not have the high species richness expected from RPMS based on reef area.

I examined why anemonefishes, Amphiprion ocellaris, Amphiprion frenatus, and Amphiprion perideraion, which inhabit single host anemones, are monogamous and protandrous. Because they live in small groups (≤6 individuals) with a size hierarchy, they might have the environmental potential for polygyny. If the dominant fish were male, he could monopolize mating opportunities, and subordinates should be female. Female fecundity increases with body size, so that group body size composition largely influences his reproductive success. First, I developed an optimization model to predict the body size composition based on the carrying capacity of the host (C) and a fixed body size ratio (γ) or difference (δ) between individuals adjacent in rank. The γ and δ were assumed to be necessary for subordinates to avoid fatal eviction from the group. The model using δ and C could predict the body size composition of the 3 species. Next, I incorporated nonlinear female fecundity functions into the model to evaluate whether polygyny or monogamy was the better mating system for the dominant fish. I examined relationships between gonad weight and body size of A. frenatus and A. perideraion to estimate the functions. Assuming large δs and the nonlinear functions, dominant fish could have higher reproductive success in monogamous mating systems. The model also indicated that where C is limited, a larger δ resulted in one large, one small, and several very small subordinate individuals. This combination of body size composition model and nonlinear female fecundity functions can explain the function of protandry and monogamy under limited shelter space.

クマノミ類が共生するイソギンチャクは，“宿主イソギンチャク”と呼ばれ，クマノミ類にとっての隠れ場所として重要な役割を果たしている。一方，“宿主イソギンチャク”にとってのクマノミ類の役割については，これまで曖昧な点が多かった。本稿では，まず，褐虫藻に注目した最近の研究をまとめ，クマノミ類との共生がイソギンチャクにもたらす利益について明確にした。このことにより，あるイソギンチャク1個体の着底後初期の生存と成長には，クマノミ類との潜在的共生種数が多いほど有利であろうと推察された。“宿主イソギンチャク”には，定着したクマノミ類のほとんど全てが繁殖場所として利用する種（4種）のほかに，定着したクマノミ類の中の特定の種のみが繁殖に利用する種（2種），ほとんど全てが繁殖場所には利用しない種（4種）があり，クマノミ類にとってのイソギンチャクの価値に格差が存在する。イソギンチャク1種にとってのクマノミ類の潜在的共生種数はそこで繁殖が確認されたクマノミ類の種数と極めて強く相関しており，クマノミ類が価値の高いイソギンチャクを選択的に利用する傾向が示唆される。“宿主イソギンチャク”は，クマノミ類との関係性，特に双方にとっての価値の高低に注目すると4タイプに類型化されるが，タイプ間で観察例数に大きな違いが見られ，地理的分布が重複する221例の組合せのうち，実在するのは37.1%（82例）であった。これまでの研究を概観することにより，実在する組合せには，1）“宿主イソギンチャク”とクマノミ類の地理的分布の重複のほかに，2）生息場所の一致，3）クマノミ類による宿主選択性，さらに，4）宿主をめぐるクマノミ類の種間競争が大きく影響していることが理解できる。クマノミ類よりも寿命が長く，浮遊幼生期間も長い“宿主イソギンチャク”の地理的分布には，暖流の影響が大きいことが文献情報の分析から明らかになった。“宿主イソギンチャク”の分類は日本では混乱しているが，多少の問題点があったとしても，世界で幅広く用いられている分類に従うことにより，日本近海で報告されているイソギンチャクとクマノミ類の分布や生態，関係性について，他海域で得られた知見との比較分析が可能となる。

Small patch reefs can harbor many reef fishes because most fishes have a drifting larval phase to randomly disperse over patchy habitats. We examined the species–area relationship (SAR) of damselfish (Pomacentridae) assemblages over 84 small patch reefs (0.05–45.4 m2) using an enlarged section of a high-resolution color aerial photograph as a field map (1/2500) in a shallow coral reef shore zone (

Habitat loss due to land reclamation often occurs in sandy coral reef shore zones. The giant sea anemone Stichodactyla gigantea, which harbors the false clown anemonefish Amphiprion ocellaris, both of which are potentially flagship species, inhabit these places. To assess habitat quality for S. gigantea, we examined correlative associations between the number and the body size of S. gigantea and the amount of habitat types in fine-scale seascape composition quantified from an enlarged section of a high-resolution (1/2,500) color aerial photograph of the shallow shore zone of Shiraho Reef, Ishigaki Island, Japan. This study confirmed that anemones were most abundant at the edges of dense seagrass beds characterized by shallow sandy bottoms, rock beds, and sparse seagrass beds, while they were less abundant in coral patch reefs. However, anemones inhabiting coral patch reefs were significantly larger and their rate of disappearance over 3 years was lower than those inhabiting other habitats. This suggests that coral patch reefs may be more suitable habitats supporting larger animals and greater persistence of S. gigantea. The visual census techniques applied here, combined with aerial photography and image-analysis software, may be useful as a simple analytical tool for local assessment of suitable habitats for relatively small-bodied marine fauna in shallow-water seascapes.

Habitat use by the resident coral reef anemonefish, Amphiprion frenatus, was examined in the complex coral reef landscape of Shiraho Reef, Ishigaki Island, Okinawa, Japan, using an enlarged color aerial photograph processed using image analysis software as an accurate field map. The anemonefish inhabit assemblages of the host sea anemone, Entacmaea quadricolor (clonal type), which inhabit various patch reefs in the back reef moat. We located all patch reefs inhabited by the host in the map based on snorkel observations: 297 anemonefish were found in 93 host assemblages in the study site of 2.9 ha. These patch reefs could be recognized by the reef colors, including the shadows (blackish color) in the photograph. Using image analysis software, the colors of the patch reefs were chosen and pixels with the same color values were regarded as potential habitat patches for the fish (PHPs). PHPs were 3D patch reefs (>0.5 m in height). Total areas (TA) and total perimeters (TP) of PHPs were measured in nine quadrats in the digitized aerial photograph. Host abundance and anemonefish abundance in a quadrat showed stronger correlations with the product of TA and TP of PHPs than TA alone. A site with numerous large 3D patch reefs (≥0.75 m2 in situ) can be a better habitat for the fish than other sites consisting of several huge 3D patch reefs of the same total area. The methodology applied here may be useful for assessing the quality of habitats for small resident animals in shallow subtidal reefs.

The protandrous anemonefish Amphiprion frenatus often forms a group consisting of a large female, a small male, and a smaller nonbreeder at an isolated single host anemone, where home ranges of subordinates were covered with the female's home range. Within the group, the dominant individuals suppress the growth of subordinates, resulting in large size differences. The spacing pattern and body size composition of A. frenatus on colonial hosts were investigated in Ishigaki Island, Okinawa, Japan. Six breeding pairs and 14 nonbreeders inhabited a colony of 157 anemones. Each pair maintained a territory in which pair members used different hosts. Nonbreeders had unstable home ranges on the outskirts of or in the pairs' territories. Body size differences between males and females in pairs and between males and nonbreeders were small. The small size differences in the colony of hosts are caused by reduced suppression of growth of subordinates by the dominant individuals. The total area of host anemones largely affects spacing pattern and social suppression of the anemonefish.

The distribution patterns of the leathery sea anemone, Heteractis crispa, which contains an algal endosymbiont (zooxanthellae) and anemonefish, were investigated in relation to size distribution on a shallow fringing reef (3.2 ha, 0–4 m depth) in Okinawa, Japan. Individual growth and movements were also examined. Large individuals (>1,000 cm2) inhabited reef edges up to a depth of 4 m, while small anemone (

Protandry in anemonefishes has been attributed to random pair formation, which results from their limited mobility after random recruitment to isolated host anemones. The recruitment and movement of the anemonefish Amphiprion frenatus were investigated in relation to its group structure and the spatial distribution of its host anemone on a coral reef, where it inhabits isolated single hosts and interhost movement is rare. A juvenile tended to be recruited to a host from which a former resident(s) had disappeared, indicating that larval recruitment is not random. After mass bleaching of corals during which many hosts died, a quarter of adults moved between hosts on average 42 m in 3 weeks, indicating that their mobility is potentially high. The few migrations under normal conditions were probably due to the low benefit of movements. Even after the movements, a female was much larger than her mate in a host, and a large female tended to pair with a large male. Although body size of females was positively correlated with their host size, that of males was not. It is suggested that the size of a single host does not allow the coexistence of two or more large fish, and the size composition of each pair is affected by the host size. Protandry in the monogamous fish may be attributed to the nonrandom pair formation in the limited space of a host.

Microhabitat use and feeding behavior of the rainbowfish Melanotaenia duboulayi (Castelnau) were investigated in a slow-flowing stream adjacent to riparian forest in south-eastern Queensland, Australia. Fish were more abundant in vegetated areas, but did not enter dense Vallisneria beds, where predators were observed. In sunny conditions shoals of juveniles occurred near the water surface feeding floating material on the surface, but larger fish tended to occur at the bottom near submerged vegetation, often utilizing the overhanging aquatic plant community as a refuge and food source. In the middle of the day, juveniles and small fish seemed to show behavioral thermoregulation at the surface in the warmest site. Under cloudy conditions, however, fish of all sizes preferred deeper water. The present study suggests that in still and sunny pools thermal change caused by sunlight influences the microhabitat choice of small fish. A field experiment using a kingfisher model implies that fish swimming at the surface could escape from aerial predators in sunlit conditions by responding to moving shadows, but could not do so under cloudy conditions.

Summary 1 According to meta-population models, a superior competitor and a superior disperser can coexist in a patchy environment. The two anemonefishes, a large aggressive Amphiprion clarkii Bennett and a small less-aggressive A. perideraion Bleeker, use the same host anemone Heteractis crispa Ehrenberg on a coral reef, Okinawa, Japan, where most of the hosts disappeared after the coral bleaching in 1998. Their microhabitat (host) use and coexistence, and the quality and quantity of microhabitats were investigated in 1988, 1989, 1999 and 2000 on the coral reef. Their interspecific interaction was also examined. 2 Before the habitat destruction, the two species coexisted. Although A. clarkii was behaviourally dominant over A. perideraion in a cohabiting group, A. perideraion was a superior competitor in terms of site displacement, because A. perideraion could displace a microhabitat. Adult A. clarkii emigrated from a cohabiting group probably due to the high cost of interactions with adult A. perideraion . Although it is easier to defend a small area for a larger species, sharing a host with adult A. perideraion may not pay for A. clarkii because A. clarkii needs a larger area. 3 A. clarkii was not only a superior disperser, which was able to find a vacated host, but also a pioneer species that was able to use newly settled small hosts. Larval A. clarkii settled on such a small host because they were able to move to larger hosts for future reproduction, while A. perideraion did not settle on a small host because of low mobility after settlement. Microhabitat (host) with various sizes might have promoted their coexistence. 4 After the habitat destruction, the superior competitor A. perideraion went extinct locally due probably to lack of small host utilization ability. The present study implies that the difference in body size between the two competitors plays an important role in their coexistence, because species with different body sizes can have different mobility and require different amounts of resources.

We investigated habitat use and diversity of waterbirds at one of the coastal and satellite lagoons around Lake Biwa, which is a registered wetland of the Ramsar Convention in Japan. To evaluate the importance for waterbirds of a lagoon around the freshwater lake, we conducted 26 censuses over 1 year on seven blocks with different landscape elements in a small lagoon. A total of 25 species were found and most of them (72%) belonged to a guild in which birds forage without diving. Species density (per ha) and diversity was high in blocks where Zizania latifolia Turcz and Phragmites australis L. reed beds existed. Not all of the waterbird species were feeding all the time, but were often resting in their preferred blocks, suggesting that they use the lagoon as both a refuge and a feeding site. Habitats with a structural reed bed community and shallow waters (

The effect of a larger anemonefish Amphiprion clarkii (Bennett) on the social and mating system of a smaller congener Amphiprion perideraion (Bleeker) was investigated on a coral reef at Okinawa, Japan, where both species use the same host sea anemone Heteractis crispa (Ehrenberg). The population of A. perideraion consisted of adult, subadult and juvenile groups. Adult groups, which consisted of one or two adults and a varying number of subadults and juveniles, usually did not share the host with A. clarkii. In contrast, subadult and juvenile groups, which included no adults, always cohabited with A. clarkii. In the heterospecific groups, subadult A. perideraion were able to mature histologically, and changed to female when they were the largest among conspecific members, although their reproduction was suppressed by A. clarkii. After all members of A. clarkii emigrated or disappeared from a heterospecific group, adult A. perideraion could begin breeding. I suggest that A. perideraion in both heterospecific and conspecific groups adopt a mating strategy that involves waiting for vacated breeding posts because of their low mobility and a low host density.

The monogamous and protandrous anemonefish,Amphiprion clarkii, utilizes only large host sea anemones as breeding sites. Under conditions of breeding site shortage, an important strategy for unmated fish may be to prepare themselves to enter quickly a breeding site from which a breeding male or female has recently disappeared. There are two types of subadults. Some juveniles become subadult males, which can change either to male or female afterwards, but others become subadult females, which can change only to female. Subadult females seem to acquire female posts more easily than subadult males. We develop an ESS model which evaluates the future reproductive success of subadult females and subadult males and test whether coexistence of subadult males and females can be interpreted as adaptive alternative tactics, using field data from two populations. The model predicts that if mortality does not differ between adult males and females the frequency for juveniles to become subadult females should be equal to the proportion of breeding post acquisition by subadult females among that by all subadults. The model also predicts that if mortality does not differ between subadult males and females, nor in adults, the frequency of subadult females among all subadults should be equal to the proportion of post acquisition. The model, with data from two study sites, suggests that alternative life-history pathways found inA. clarkii have evolved through adaptive tactics of breeding post acquisition.

Social structure and interactions between the anemonefishes, Amphiprion clarkii and A. perideraion, which utilize the same host sea anemone Radianthus kuekenthali, were investigated on a coral reef of Okinawa Islands, Japan. In an 87 × 373 m2 study area, 98 sea anemones were inhabited by both species (32.5%), by only A. clarkii (48.9%), or by only A. perideraion (18.6%). A group of A. clarkii often occupied two or more individual hosts, and group members often interchanged. However, a group of A. perideraion usually used only one host and migration between groups was rare. The larger A. clarkii suppressed reproduction of A. perideraion in cohabiting groups, while A. perideraion suppressed settlement of Juvenile A. clarkii to its own hosts. Juvenile A. clarkii settled on small hosts as well as on large hosts, whereas juvenile A. perideraion settled only on large hosts. Coexistence appears to be possible in part by differences in settlement patterns between juveniles of the two anemonefishes.

Social structure, growth and reproductive experience of a protandrous anemonefish,Amphiprion frenatus, were investigated on a coral reef in Okinawa, Japan. In a 67 m × 334 m study area, 24 breeding groups, 10 nonbreeding groups and 2 groups of unknown breeding experience were found around isolated sea anemones. One group usually consisted of 2 or 3 fish. The female in a breeding group was larger than not only her mate but also all males in other breeding groups. The body size and gonadal state of the largest individual in a nonbreeding group were intermediate between the female and male in a breeding group. In both breeding and nonbreeding groups, the largest fish retarded growth of the second largest. After the disappearance or removal of females, their mates took more than 1.5 years to attain the minimum functional female size (about 75 mm in standard length). This delayed sex change can be attributed to strong growth suppression by the female.

The process of gonadal sex differentiation in Amphiprion clarkii was investigated for 2 years at Murote Beach, Shikoku Island, Japan. Six color phases were discriminated on the basis of the caudal fin coloration, which corresponded well to six gonadal phases. From changes of the color phases with growth, three life history pathways were detected: (1) subadult male → subadult female → adult female, (2) subadult male → adult male → adult female, (3) subadult male → adult male. Different pathways were due to the difference of timing among individuals in the development of ovarian tissues of the hermaphroditic gonads involving the atrophy of testicular tissues. Irreversible differentiation of ovarian tissues of the gonads occurred more frequently among nonbreeders (10 cases) than among breeders (4 cases). The second pathway, which has been thought the norm of tropical anemonefishes, was therefore not primary in this population. This can be attributed to ecological conditions: fish are able to move between host sea anemones and nonbreeders can escape from social suppression by adult pairs because of high population density of hosts.

In Japan, there was an unusually cool and rainy summer in 1993, and an unusually hot and dry summer in 1994. We investigated chemical species in the waters of Lake Biwa during these two years. Samples were collected vertically from stations off Omimaiko in the mesotrophic northern basin and at a dredged area in the eutrophic southern basin, and surface water samples were also collected from three other stations at about one month intervals. We also studied chemical species in precipitation collected at Otsu, on the southeast shore of the southern basin, and sediment cores collected from the southern basin. The features of the lake water were very dependent on the summer climate. In 1993, abundant rain and weak thermal stratification supported high nutrient concentrations in the epilimnion. In 1994, nutrients were depleted and a high transparency and low concentration of chlorophyll a were observed in the epilimnion of the northern basin. This is because the supply of nutrients was limited owing to little rain and a well-developed thermocline. However, a great deal of dissolved reactive phosphorus (DRP) occurred until late August in the southern basin. Local anoxic conditions were extensively developed in 1994, since a large amount of macroalgae grew and regressed in the southern basin owing to a high water temperature (30°C) and decline in the water-level. Presumably DRP was desorbed from ferromanganese oxides that were reduced in the anoxic sediments and the DRP found its way into the lake water. Although it has been reported that the concentration of phosphorus in the lake water had not significantly increased in the last two decades, phosphorus which had steadily accumulated in the sediments had a crucial effect on the quality of the lake water.