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Kurihara T.,Seikai National Fisheries Research Institute | Yamada H.,Seikai National Fisheries Research Institute | Inoue K.,Okinawa Prefectural Fisheries and Ocean Research Center
Plankton and Benthos Research | Year: 2011

Giant clams are important species in tropical aquaculture. The larvae artificially hatched are often cleaned by running seawater containing the larvae through a coarse mesh, designed to catch contaminants larger than the larvae (e.g. fouling materials detached from adult clams); and then through a fine mesh, designed to catch the larvae and wash away smaller contaminants (e.g. bacteria). Such larval cleaning is assumed to improve the larval survival rate. We conducted experiments on how the cleaning improves the survival rate of larvae of the giant clam Tridacna crocea during the planktonic stage. The experiments revealed that larval cleaning improves the survival rate for larval densities of 0.3 to 9.8 indiv.mL -1. The experiments suggested that the survival rate at 0.5 indiv.mL -1, a typical larval density in hatcheries, would be 16.6% for the cleaned larvae, much higher than 1.4% for those not cleaned. Through larval cleaning, both bacteria and small T. crocea embryos showing maldevelopment were found to decrease in the water containing T. crocea larvae, which can explain in part the improved survival rate. © The Japanese Association of Benthology.


Kurihara T.,Seikai National Fisheries Research Institute | Fuseya R.,Seikai National Fisheries Research Institute | Katoh M.,Seikai National Fisheries Research Institute | Inoue K.,Okinawa Prefectural Fisheries and Ocean Research Center
Plankton and Benthos Research | Year: 2010

Giant clams are simultaneous hermaphrodites and are assumed to ejaculate first and, after completely stopping ejaculation, release eggs. In the seed production method aimed at preventing self-fertilization, each adult clam is induced to ejaculate in a tank and then release eggs in another tank. Giant clams, however, have recently been suggested to continue ejaculation for a period after the beginning of egg release. The overlap between ejaculation and egg release might lead to self-fertilization in the tank used for egg release, especially for the eggs released just at the beginning of spawning. We examined the possibility of such self-fertilization for the giant clam Tridacna crocea and obtained three results. (1) In observations with the naked eye in a laboratory, 2 of 38 T. crocea simultaneously ejaculated and released eggs. (2) In a laboratory experiment, 1.5 to 80.0% of eggs released from each adult clam developed into D-shaped larvae without artificial cross-fertilization. Such development occurred more frequently for the eggs released earlier from each adult clam than for the eggs released later from the clam. (3) In observations at a hatchery, 2 to 94% of the eggs released from 4 of 5 adults were found to develop into D-shaped larvae without artificial cross-fertilization. The three results suggest that at least some T. crocea adults continue ejaculation for a period after starting spawning eggs, which causes self-fertilization. © The Japanese Association of Benthology.


Maeda T.,Tokyo University of Marine Science and Technology | Maeda T.,Japan Agency for Marine - Earth Science and Technology | Hirose E.,University of Ryukyus | Chikaraishi Y.,Japan Agency for Marine - Earth Science and Technology | And 13 more authors.
PLoS ONE | Year: 2012

The sea slug Plakobranchus ocellatus (Sacoglossa, Gastropoda) retains photosynthetically active chloroplasts from ingested algae (functional kleptoplasts) in the epithelial cells of its digestive gland for up to 10 months. While its feeding behavior has not been observed in natural habitats, two hypotheses have been proposed: 1) adult P. ocellatus uses kleptoplasts to obtain photosynthates and nutritionally behaves as a photoautotroph without replenishing the kleptoplasts; or 2) it behaves as a mixotroph (photoautotroph and herbivorous consumer) and replenishes kleptoplasts continually or periodically. To address the question of which hypothesis is more likely, we examined the source algae for kleptoplasts and temporal changes in kleptoplast composition and nutritional contribution. By characterizing the temporal diversity of P. ocellatus kleptoplasts using rbcL sequences, we found that P. ocellatus harvests kleptoplasts from at least 8 different siphonous green algal species, that kleptoplasts from more than one species are present in each individual sea slug, and that the kleptoplast composition differs temporally. These results suggest that wild P. ocellatus often feed on multiple species of siphonous algae from which they continually obtain fresh chloroplasts. By estimating the trophic position of wild and starved P. ocellatus using the stable nitrogen isotopic composition of amino acids, we showed that despite the abundance of kleptoplasts, their photosynthates do not contribute greatly to the nutrition of wild P. ocellatus, but that kleptoplast photosynthates form a significant source of nutrition for starved sea slugs. The herbivorous nature of wild P. ocellatus is consistent with insights from molecular analyses indicating that kleptoplasts are frequently replenished from ingested algae, leading to the conclusion that natural populations of P. ocellatus do not rely on photosynthesis but mainly on the digestion of ingested algae. © 2012 Maeda et al.


Nanami A.,Seikai National Fisheries Research Institute | Sato T.,Seikai National Fisheries Research Institute | Ohta I.,Okinawa Prefectural Fisheries and Ocean Research Center | Akita Y.,Okinawa Prefectural Fisheries and Ocean Research Center | Suzuki N.,Seikai National Fisheries Research Institute
Ichthyological Research | Year: 2013

Epinephelus ongus forms spawning aggregations at specific reef sites at Okinawa. The aim of the present study was to describe the spawning behavior of E. ongus based upon underwater video imagery. Epinephelus ongus spawning is usually paired with courting males and females rushing upwards towards the surface during the spawning ascent, with a subsequent release of eggs at the apex of the ascent. In two of four spawning events we observed, however, the ascending pair was joined by additional males. Fish eggs collected at the site were identified as those of E. ongus and confirmed that this species spawns mainly at night during a slack tide. This study is the first documentation of E. ongus spawning behavior in the field. © 2013 The Ichthyological Society of Japan.


Uehara M.,University of Ryukyus | Uehara M.,Okinawa Prefectural Fisheries and Ocean Research Center | Kashiwagi F.,University of Ryukyus | Imai H.,University of Ryukyus | Tachihara K.,University of Ryukyus
Ichthyological Research | Year: 2011

The age, growth, reproductive condition, and occurrence of natural hybrids of two Nematalosa species around Okinawa Island were examined using 128 specimens obtained from April 2003 to June 2004. Standard length (SL) reached approximately 150-210 mm within the first 2 years, and then remained stagnant. The maximum age for both sexes was ca. 5 years old. Maturity sizes and ages were estimated to be at least 173.2 mm SL and 2 years old for females and 192.6 mm SL and 3 years old for males. Spawnable individuals were mainly observed from January to March based on histological observations of gonads. Natural hybrids appeared at all sampling sites except for the Haneji Inlet and were dominant at Makiminato (in south-central Okinawa Island). Their incidence was also quite high (66.9%) in the Makiminato population, when compared with records for other marine fishes around Japan. In Okinawa Island, these shallow areas are rapidly decreasing in size because of recent reclamation and land exploitation. Hybrid production may be caused by not only the reproductive biology and sympatric distributions of the parent species but also recent environmental changes. © 2011 The Ichthyological Society of Japan.


Uehara M.,University of Ryukyus | Uehara M.,Okinawa Prefectural Fisheries and Ocean Research Center | Tachihara K.,University of Ryukyus
Ichthyological Research | Year: 2012

The reproductive cycle and spawning characteristics of the Japanese gizzard shad (Nematalosa japonica) were examined histologically using specimens collected around Okinawa Island in southwestern Japan. The observed lengths at 50 % maturity were approximately 170 mm in standard length (SL) for females and 160 mm SL for males, which corresponded approximately to 3-year-old fish. The spawning season was estimated to take place from January to May, peaking from February to April, which closely coincided with the coldest water temperatures of the year. The lipidosomatic index of both sexes decreased from January onward and maintained low values until July, presumably as the fish expended energy to reproduce. Spawning intervals were estimated to occur between 1. 3 and 1. 8 days from February to April. Fecundity (F) for females tested using 69 ovaries was related to SL: F = 1. 313 × 10 -4 × SL 3. 966. Histological observations and a fish market survey suggested that the species spawned in sandy muddy areas (<20 m) based on the presence of mature individuals. The implications of our results toward conservation and management of N. japonica are discussed. © 2012 The Ichthyological Society of Japan.


Nishizawa H.,Kyoto University | Naito Y.,Hokkaido University | Suganuma H.,Everlasting Nature of Asia | Abe O.,Seikai National Fisheries Research Institute | And 13 more authors.
Marine Biology | Year: 2013

In order to develop effective conservation strategies for endangered migratory species, the link between feeding and breeding grounds needs to be clarified. In this study, the genetic compositions of consecutive Japanese feeding aggregations of green turtles (Chelonia mydas) along the Kuroshio Current were examined by mixed-stock analyses of mitochondrial DNA control-region sequences. The results indicated that the southern feeding aggregation around Yaeyama (24.3°N, 124.0°E) was sourced from various Pacific rookeries in the Yaeyama, Ogasawara, Western Pacific, and Indian Oceans and Southeast Asia. Among northern feeding aggregations, the Ginoza (26.5°N, 128.0°E) aggregation was also sourced from the Western Pacific Ocean, but the Nomaike (31.4°N, 130.1°E), Muroto (33.2°N, 134.2°E), and Kanto (35.6°N, 140.5°E) aggregations were contributed mostly by the closer Ogasawara rookeries. The reduced contribution from tropical Pacific rookeries to northern feeding aggregations and the significant correlation between genetic differentiation and geographical distance matrices of feeding aggregations indicated that most hatchlings from these regions transported by the Kuroshio Current settle in upstream feeding grounds along the Japanese archipelago, implying that current flow influences the composition of feeding aggregations. Differences in the composition of relatively close neritic feeding aggregations have important conservation implications, for which both regional and multinational conservation strategies are needed. © 2013 Springer-Verlag Berlin Heidelberg.


Ebisawa A.,Okinawa Prefectural Fisheries and Ocean Research Center | Kanashiro K.,Okinawa Prefectural Fisheries and Ocean Research Center | Kanashiro K.,Okinawa Prefectural Sea Farming Center | Kiyan T.,Okinawa Prefectural Fisheries and Ocean Research Center
Fisheries Science | Year: 2010

The growth, sex ratio with age, and age at sexual maturation were determined based on sectioned otoliths in 257 specimens of the blackspot tuskfish Choerodon schoenleinii collected in waters off Ryukyu Island. Opaque rings observed by reflected light in the sectioned otoliths were found to form once a year from January to July. The three growth parameters of the von Bertalanffy growth equation were L∞ = 68. 1 (cm), k = 0.263, and t0 = -0. 023 (year). The age at which the sex ratio reached 50% by sexual transition was about 6.15 years, and the age at which 50% of females were sexually mature was approximately 2 years. The oldest specimen among the samples was 17 years old. © 2010 The Japanese Society of Fisheries Science.


Murata R.,University of Ryukyus | Karimata H.,Okinawa Prefectural Fisheries and Ocean Research Center | Alam M.A.,University of Ryukyus | Nakamura M.,University of Ryukyus | Nakamura M.,SCI Solution
Aquaculture Research | Year: 2010

The Malabar grouper Epinephelus malabaricus is an important candidate species for commercial aquaculture in tropical and subtropical areas. In nature, this species requires more than 10 years to change sex from female to male and have active spermatogenic tissues in the testis. Thus, it is essential to find a means of producing sperm for seed production. This is the first report of artificial sex change in underyearling E. malabaricus. Female E. malabaricus with immature ovaries at 144 days post-hatch (DPH) were fed a diet with 17α-methyltestosterone (MT) at 50 μg g-1 diet for 6 months. Sex change occurred in most of the treated fish, which had testis with all stages of spermatogenic germ cells including spermatozoa. In contrast, most of the control fish had immature ovaries. These results, which reveal that germ cells in the underyearling grouper have the ability to produce spermatozoa in response to exogenous androgen, demonstrate that sex change can be artificially induced during ovarian development. © 2009 Blackwell Publishing Ltd.


Kurihara T.,Seikai National Fisheries Research Institute | Yamada H.,Seikai National Fisheries Research Institute | Inoue K.,Okinawa Prefectural Fisheries and Ocean Research Center | Iwai K.,Okinawa Prefectural Sea Farming Center | Hatta M.,Ochanomizu University
PLoS ONE | Year: 2013

To survive the juvenile stage, giant clam juveniles need to establish a symbiotic relationship with the microalgae Symbiodinium occurring in the environment. The percentage of giant clam juveniles succeeding in symbiosis establishment ("symbiosis rate") is often low, which is problematic for seed producers. We investigated how and why symbiosis rates vary, depending on whether giant clam seeds are continuously reared in UV treated or non treated seawater. Results repeatedly demonstrated that symbiosis rates were lower for UV treated seawater than for non treated seawater. Symbiosis rates were also lower for autoclaved seawater and 0.2-μm filtered seawater than for non treated seawater. The decreased symbiosis rates in various sterilized seawater suggest the possibility that some factors helping symbiosis establishment in natural seawater are weakened owing to sterilization. The possible factors include vitality of giant clam seeds, since additional experiments revealed that survival rates of seeds reared alone without Symbiodinium were lower in sterilized seawater than in non treated seawater. In conclusion, UV treatment of seawater was found to lead to decreased symbiosis rates, which is due possibly to some adverse effects common to the various sterilization techniques and relates to the vitality of the giant clam seeds. © 2013 Kurihara et al.

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