Original Paper
Title Local variations in shoot density, shoot morphology, and bite mark frequency of subtropical seagrasses in Japan
Authers Yoshitake TAKADA* and Osamu ABE*
Keywords Seagrass, density, size, structure, herbivory, biomass
Citation Bull. Fish. Res. Agen. No.4, 1-10, 2002
This study demonstrated local variations in seagrass species composition, biomass, shoot density, shoot morphology and the frequency of bite marks in seagrass leaves at coastal sites in the Yaeyama Islands, subtropical Japan. Seven seagrass species (Zostera japonica, Halodule pinifolia, H. uninervis, Cymodocea rotundata, C. serrulata, Thalassia hemprichii and Halophila ovalis) were sampled, and T.hemprichii was the most dominant. Total biomass did not vary between the four sampling sites, but there were considerable local variations in shoot density and shoot morphology of the larger three species (C.rotundata, C.serrulata and T. hemprichii). At the sites with a low shoot density (19-29 shoots per 625cm2), the shoots of T.hemprichii had a lower number of leaves, wider and longer leaves, and a longer "sheath + erect shoot" length than the shoots at the sites with a high shoot density (44-93 shoots). The frequencies of bite marks on leaves of C. rotundata and T. hemprichii varied between sites, but they did not correspond to local variations in the leaf length structure. The relation between the seagrass shoot morphology and the balance of production and consumption requires further clarification.

Received on January 22, 2002
Contribution No.A 17 from the Fisheries Research Agency
* Ishigaki Tropical Station, Seikai National Fisheries Research Institute, Fukai-Ota 148-446, Ishigaki, 907-0451, Japan

Title Effects of body size and temperature on the relationship between growth and ration of young Japanese flounder, based on laboratory experiments
Authers Masaaki FUKUDA*1, Hiroshi SAKO*2, Toshihiro SHIGETA *3, and Rena SHIBATA*3
Keywords Paralichthys olivaceus, energy model, food requirement, growth rate
Citation Bull. Fish. Res. Agen. No.4, 11-18, 2002
To derive an empiricial equation for the relationship between growth and ration of young Japanese flounder, Paralichthys olivaceus, the effects of fish size and temperature were studied. Juvenile flounder were reared at six ration levels using squid as food in the laboratory for 14 days. Feeding experiments were conducted for six different fish sizes (7.5-337g) at 20℃ and for 70g size at two more temperature conditions (15℃ and 26℃). Significant positive linear relationships were found between growth (g/day) and daily ration (kcal/day) for all size groups and for all temperature conditions. The net conversion efficiencies(b) and the maintenance rations(c) at 20℃ could be expressed as a power function of body weight(W) as follows: b=0.678W-0.095 and c=0.023W0.746. The maintenance rations(c) were correlated with the three temperatures, and the relationship was expressed as c=0.153e0.054T. The relationship between growth rate and daily ration at any size and at any temperature is described by the following equation:
R=0.007(e0.054T)W0.75 + 1.47W0.10G kcal/day,
where R=daily ration(kcal/day), T=temperature(℃), W=body weight(g) and G=growth(g/day).

Received on January 24, 2002
Contribution No.A 18 from the Fisheries Research Agency
*1 Coastal Fisheries and Aquaculture Division, National Research Institute of Fisheries Science, Nagai 6-31-1, Yokosuka, Kanagawa 238-0316, Japan
*2 Research Promotion and Development Division, Fisheries Research Agency, Fukuura, Kanazawa, Yokohama, 236-8648, Japan
*3 National Research Institute of Fisheries and Environment of Inland Sea, Ohono-cho 2-17-5, Saeki-gun, Hiroshima 739-0452, Japan

Title Comparison of larval shell morphology of two coastal turbinid snails, Turbo (Batillus) cornutus and Turbo (Marmarostoma) stenogyrus (Vetigastropoda: Turbinidae)
Authers Naoaki KONO*1 and Hiroshi YAMAKAWA*2
Keywords larval shell morphology, Turbinidae, SEM, Turbo (Batillus) cornutus, Turbo (Marmarostoma) stenogyrus
Citation Bull. Fish. Res. Agen. No.4, 19-24, 2002
Larval shell morphology was compared to determine criteria for the identification of larvae and juveniles of two coastal turbinid snails, Turbo (Batillus) cornutus and Turbo (Marmarostoma) stenogyrus: long axial length, short axial length, aperture diameter and surface sculpture. For all three morphological parameters of the larval shell, T.cornutus was smaller than T. stenogyrus, for example the mean long axial length being 267μm and 303μm, respectively. Several differences were detected between the two species in the surface sculpture of the larval shells based on scanning electron microscope observations. Distinction could be achieved between the two species even for snails in which the postlarval shell had partially covered the larval shell. The current results allow the differentiation for all life stages of the postlarval stages between T. cornutus and T. stenogyrus and therefore facilitate ecological studies of the postlarval stages.

Received on March 25, 2002
Contribution No.A 19 from the Fisheries Research Agency
*1 National Research Institute of Fisheries and Environment of Inland Sea, 2-17-5 Maruishi, Ohno, Saeki, Hiroshima 739-0452, Japan
*2 Tokyo University of Fisheries, Konan, Minato, Tokyo 108-8477, Japan

Title Total amount of carbon and nitrogen in Zostera marina in Hiroshima Bay, western Seto Inland Sea, Japan
Authers Toshinobu TERAWAKI*1, Hitoshi TAMAKI*2, Masaki NISHIMURA*2, Koji YOSHIKAWA*1, and Goro YOSHIDA*1
Keywords carbon, nitrogen, Zostera marina, Seto Inland Sea, Hiroshima Bay
Citation Bull. Fish. Res. Agen. No.4, 25-32, 2002
The total amount of carbon and nitrogen in Zostera marina was surveyed in Hiroshima Bay, Western Seto Inland Sea. Three study stations were selected in Zostera beds on sandy coasts of islands from the northern area to the mouth of the bay. Quantitative sampling using 100cm2 (25cm by 40cm) quadrate in triplicate were conducted by SCUBA divers at D.L. (based on Datum Level) 0~1m as the central depth of the vertical distribution of the Zostera beds at each study station. The standing crop of Z.marina during the spring luxuriant season were 0.20kg.d.w./m2(aboveground: 0.16kg.d.w./m2, belowground: 0.04kg.d.w./m2). The average carbon content in the total plant body of Z.marina was estimated at 32.3% for the aboveground and belowground parts, through the year. The average nitrogen content in the total plant body of Z.marina was estimated at 1.9% for the aboveground and belowground parts. The total amount of carbon and nitrogen in Z. marina at Hiroshima Bay was calculated as 164 tons and 10 tons based on the total area of Zostera beds of 254 ha and total plants biomass of 508 tons (dry weight).

Received on March 25, 2002
Contribution No.A 20 from Fisheries Research Agency
* 1 National Research Institute of Fisheries and Environment of Inland Sea, Maruishi, 2-17-5 Ohno, Saeki, Hiroshima 739-0452, Japan
* 2 Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan

Title The meiobenthic community on the tidal flat of Miyajima in Hiroshima Bay, with special reference to the biomass of free-living nematodes
Authers Mutsumi TSUJINO*1, Goro YOSHIDA*1and Takuji UCHIDA*2
Keywords meiobenthos, Nematoda, biomass
Citation Bull. Fish. Res. Agen. No.4, 33-39, 2002
Meiobenthos communities were surveyed to clarify the composition of animal groups and to estimate the production of major taxa inhabiting the tidal flat of coarse substrate at Miyajima (Itsukushima Island) in Hiroshima Bay.
Nematoda formed the most major constituent of the meiobenthos in numerical abundance, accounting for 78% of the total miobenthos on the annual average. The density of Nematoda was highest in April at 2,620±888×103 indiv./m2, and lowest in October at 780±307×103 indiv./m2. Copepoda was the next most important taxon; its density was low from spring to summer and highest in October at 218±114×103indiv./m2, showing converse seasonal pattern compared with Nematoda.
 Dry weight of Nematoda ranged on average from 0.32μg/indiv. in December to 0.82 μg/indiv. in May. The biomass of Nematoda, calculated from the average dry weight and the density, peaked in May, and ranged 0.28~1.33g dry wt./m2 or 0.08~0.40gC/m2 during the study period. The production of Nematoda was estimated to be 44.3g dry wt./m2・year or 13.5gC/m2・year.

Received on June 3, 2002
Contribution No.A 21 from the Fisheries Research Agency
*1 National Research Institute of Fisheries and Environment of Inland Sea, 2-17-5 Maruishi, Ohno, Saeki, Hiroshima 739-0452, Japan
*2 Hokkaido National Research Institute, 116 Katsurakoi, Kushiro, Hokkaido 085-0802, Japan

Doctoral thesis
Title Evaluation of characteristics and preservation of Pacific oyster, Crassostrea gigas, in view of the genetic resources
Authers Hironori USUKI
Keywords Pacific oyster, Crassostrea gigas, Genetics, Heritability, Cryopreservation
Citation Bull. Fish. Res. Agen. No.4, 40-104, 2002
Differences in characteristics among local populations of Pacific oyster (Crassostrea gigas) in Japan, evaluation of several characteristics in their genetics and environments, estimation of heritability for improvement of economic traits, and preservation of sperm and larvae were studied on the basis of genetic resources.
 Growth, mortality and other characteristics of local populations(Hiroshima, Miyagi, Ariake and Goto) were evaluated in parental and progeny groups cultured in Hiroshima Bay. For both generations, the Miyagi population grew fastest and the Ariake and Goto populations grew slower. Though the mortality was not significantly different for progeny cultured in pocket mesh bags, the progeny from Goto showed significantly higher mortality among progenies cultured by the collector method. Progeny from Ariake showed significantly lower mortality among the other progenies when cultured in narrow (10mm) opening mesh bags and is thought to be the most tolerant to deterioration of living environment. Progeny of the Ariake population also had a spiny projection on their valves and showed a higher biomass rate (volume after narcotization/volume before narcotization).
 Heritabilities for growth and survival of larvae and adult C.gigas were estimated by nested design (3 male×5 female). The heritability for survival rate (h2s+d=0.85) between May and October of one-year-old oysters was larger than that for whole weight in October(h2s+d≦0.4). Selection for improvement of survival was thought to be more effective than growth. No significant correlations in growth and survival between larvae and adults suggested that selection of larvae must be ineffective.
 To estimate the influence of differences of spat locality and of culturing spot to characters of oyster, spats of Miyagi and Hiroshima populations were cultured simultaneously at four spots in Hiroshima Bay and three-way ANOVA were performed.
 Though spat locality and culturing spot both significantly influenced many characters, shell width/shell length, density of shell, water content of flesh and water content of shell were not influenced by spat locality. Color of the mantle edge and internal shell cavity rate were influenced by spat locality more intensely than culturing spot, then, shell width/shell length and water content of flesh were influenced by spot more intensely than locality. Intensities of influence of interactions between locality and spot were obviously different among characters of oysters.
 RFLP analysis of calmodulin intron3 for oysters collected at shores in Ariake Bay aroused the suspicion of natural hybridization between spermatozoa of C.gigas and ova of C.sikamea. Artificial crossbreeding between C.gigas and C.sikamea discriminated by RFLP of 16SrRNA gene revealed that the same type of calmodulin intron3 as oysters collected in Ariake bay could yield in spats. Moreover, 0.6~5.7% of ova of C.gigas inseminated by spermatozoa of C.sikamea had developed and a few spats had set.
 To determine an effective method for the cryopreservation of C.gigas sperm and larvae, diluents containing cryoprotectants, cooling methods and developmental stages were tested. For the sperm, the combination of a diluent containing 8% (v/v) dimethyl sulfoxide(DMSO), 50mM sucrose and 6mM reduced glutathione with cooling in liquid nitrogen vapor showed the highest relative larval yield (45.9%). Fertility with sperm cryopreserved at a concentration of 108 spermatozoa/200μL was equivalent to that with fresh sperm when ova were inseminated in a concentration of 103 spermatozoa per ovum. Although spermatozoa cryopreserved for 4 years showed lower viability (40.3~53.3%) than the short-term cryopreserved spermatozoa (60~70%), the ratio of normal D-shaped larvae and survival rate at 6 days after fertilization were 78.0 and 77.4%, respectively.
 For the larvae, using 1/4 concentration seawater containing 1.0M DMSO and 250mM trehalose as the cryopreservative suspension, and cooling to -35℃ at -1℃/min and kept at this temperature for 5 min before plunging into liquid nitrogen, a oyster spat could yield from cryopreserved trochophore larvae (just before making shell gland) after 8 months of cyopreservation. This indicates that it is possible to establish cryopreservation of the Pacific oyster larvae by further improving the technique.

Received on July 22, 2002
Contribution No.A 26 from the Fisheries Research Agency
* National Research Institute of Fisheries and Environment of Inland Sea, 2-17-5 Maruishi, Ohno, Saeki, Hiroshima 739-0452, Japan