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Nakamura Y.,Kochi University | Hirota K.,Kochi University | Shibuno T.,National Research Institute of Aquaculture | Watanabe Y.,University of Tokyo
Marine Biology | Year: 2012

Seagrass beds are often considered to be important nurseries for coral reef fish, yet the effectiveness of these nursery functions (refuge and food availability) at different juvenile stages is poorly understood. To understand how the demands of juvenile fish on seagrass nursery functions determines the timing of ontogenetic habitat shifts from seagrass beds to coral reefs, we conducted visual transect survey and field tethering and caging experiments on three different sizes of the coral reef fish Pacific yellowtail emperor (Lethrinus atkinsoni) during its juvenile tenure in seagrass beds at Ishigaki Island, southern Japan. The study showed that although the number of individual L. atkinsoni juveniles decreased by >90 % during their stay in the seagrass nursery, the shelter and/or food availability functions of the nursery, at least for a juvenile size of approximately 5 cm total length (TL), provided the best survival and growth option. The timing of ontogenetic migration to coral reefs of larger fish (>8 cm TL) was attributed to foraging efficiency for larger food items in different habitats. Overall, the function of the seagrass bed nursery changed with juvenile body size, with marginally higher survival and significantly greater growth rates during early juvenile stages in seagrass beds compared to coral reefs. This would contribute to the enhancement in the number of individuals eventually recruited to adult populations. © 2012 Springer-Verlag. Source


O'Rorke R.,University of Auckland | Jeffs A.G.,University of Auckland | Fitzgibbon Q.,University of Tasmania | Chow S.,National Research Institute of Aquaculture | Lavery S.,University of Auckland
Journal of Experimental Marine Biology and Ecology | Year: 2013

Better understanding the diet of small marine predators such as the planktonic larvae of spiny lobsters is important for our awareness of interactions within marine assemblages and for species commercialisation. In DNA-based diet studies of small organisms there is a risk that any DNA contaminating the outside of an organism will be detected and falsely assumed to originate from the gut. Experiments with terrestrial predators have overcome the problem of exogenous contamination by treating the exterior of the predator with bleach (sodium hypochlorite). However, the use of bleach is a risky strategy when treating either a rare predator or aquatic predators, which are generally more permeable than terrestrial animals. Many plankton studies have not reported how they dealt with exogenous contamination, or do not use a control during PCR to detect false positives due to exogenous contamination. One approach is to wash the predator with MilliQ filtered water or ethanol and to use the final wash as a PCR template to detect residual DNA. In the present study we report that washing has variable success at removing exogenous contaminants and that using the final wash as a control for exogenous contamination consistently fails. Based on our results we recommend using DNA extracted from a swab of the exterior of the predator as a control for exogenous contamination. We also report on the benefit of using a novel syringe technique to obtain gut content that minimises contact with the predator surface, and therefore the risk of exogenous contamination. © 2013 Elsevier B.V. Source


Kobayashi T.,University of Shizuoka | Kobayashi T.,Ehime University | Kobayashi T.,National Research Institute of Aquaculture
International Journal of Developmental Biology | Year: 2010

To clarify the sexually dimorphic mechanisms of gonadal sex differentiation, we established an in vitro culture system for gonadal sex differentiation using the teleost fish Oreochromis niloticus. In vivo, the entry of germ cells into meiosis occurs around 35 days after hatching (dah) in XX gonads, whereas in XY gonads, meiotic cells became differentiated around 85 dah. In our in vitro culture system using gonads from young fry at 23 dah, the meiotic cells in the XX gonads appeared after 21 days of culture. In contrast, in the XY gonads, no meiotic cells were detected after 21 days. These results indicate that germ cell differentiation in this culture system progresses in a manner similar to that in vivo. To identify the gene products that are involved in the entry of germ cells into meiosis or in the arrest of germ cells at the gonial stage of gonadal sex differentiation, we performed subtractive hybridization screening with this in vitro culture system. From the screening process, we identified the female-related gene, FR-3, which is a homolog of zebrafish nanos-related gene (nos). The nos gene was expressed after gonadal formation around 35 dah in XX gonads, but not in XY gonads. In situ hybridization indicated that nos is expressed in oogenic meiotic cells, but not in spermatogenic meiotic cells. Further examination revealed that nos was expressed in oogenic meiotic cells after gonadal formation, specifically in teleost fish. Together, nos may be also involved in oogenic meiosis, with the exception of primordial germ cell migration. © 2009 UBC Press. Source


Tosaka R.,Hokkaido University | Todo T.,Hokkaido University | Kazeto Y.,National Research Institute of Aquaculture | Mark Lokman P.,University of Otago | And 3 more authors.
General and Comparative Endocrinology | Year: 2010

In order to elucidate how androgens may mediate their effects on ovarian growth, we investigated the mRNA levels of two subtypes of androgen receptor (ara and arb) in the ovary of feminized Japanese eel (Anguilla japonica) during artificially induced ovarian development by quantitative real-time reverse transcriptase polymerase chain reaction and in situ hybridization. Ara mRNA levels were high from the late oil droplet stage to the late vitellogenic stage, whereas arb mRNA levels were high from the late oil droplet stage to the midvitellogenic stage. Both ar mRNAs were predominantly observed in the follicle cells and the epithelial cells of the ovigerous lamellae in all stages. In the oil droplet stage, oogonia exhibited intense signals for ar mRNAs. There was no obvious difference in localization pattern between ara and arb in all ovaries examined, irrespective of maturational stage. It was difficult to identify the follicle cell types that were positive for ar mRNA during ovarian development. Only in post-ovulatory follicles could theca and granulosa cells be clearly identified, and ar signals were observed in both layers. The predominant localization of ar mRNA in the follicle cells suggests that androgens play important roles in oocyte growth by acting on these cells in this species. We have shown the expression profile and localization of ar mRNA during ovarian development for the first time in an oviparous vertebrate. © 2010 Elsevier Inc. Source


Egg size is a critical life-history trait in which maternal investment is optimized to maximize maternal fitness. The adaptive significance of variable egg size among spawning groups of Ayu (Plecoglossus altivelis) landlocked in the Lake Biwa system was examined through field investigations and rearing experiments. Observed egg size variations were explained by the water temperature around spawning grounds established near the mouths of inlet streams. Two typical streams with different incubation temperatures showed similar maternal body sizes and hatchling sizes, but eggs attached to the stream bed were larger in the colder stream. An experiment that used eggs from a single clutch showed that a smaller hatchling size was obtained with a lower incubation temperature, indicating that the effect of differences in egg size on hatchling size can be canceled out by variations in incubation temperature. In general, larvae that are less than a certain threshold of effective body size are not expected to be assured of early success among conspecifics competing for foods. It is proposed that environments in which the incubation temperature varies favor variability in egg size to ensure that sufficient food is accessible to larvae. © 2011 The Ichthyological Society of Japan. Source

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