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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. Source


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. Source


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. Source


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. Source


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. Source

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