Koji Omori

Bioconcentration and biomagnification of polybrominated diphenyl ethers (PBDEs) through lower-trophic-level coastal marine food web.

Bivalves, crabs, fishes, seawater, and sediment collected from the inner part of Tokyo Bay, Japan, were measured for 20 polybrominated diphenyl ether (PBDE) and 5 polychlorinated biphenyl (PCB) congeners. To determine the trophic levels of the organisms, carbon and nitrogen stable isotope ratios (delta(13)C and delta(15)N) were also measured. Bioconcentration factors of PBDE and PCB congeners increased as the octanol-water partition coefficient (K(ow)) rose to log K(ow)=7, above which they decreased again. Biomagnification of PCBs and several PBDE congeners (BDE47, 99, 100, 153 and 154) up the trophic ladder was confirmed by a positive correlation between their concentrations and delta(15)N. Other PBDE congeners showed a negative or no correlation, suggesting their biotransformation through metabolism. The more hydrophobic congeners of both PBDEs (Br=2-6) and PCBs (Cl=6-9) were biomagnified more. It thus appears that PBDEs are less biomagnified than PCBs.

Starvation of drifting goby larvae due to retention of free embryos in upstream reaches

The embryonic drift pattern of amphidromous Rhinogobius species varies along river courses. It peaks soon after sunset in reaches with gentle gradient on the plains, whereas it occurs throughout the day in steep reaches. Examination of the size of the sagittal otolith and notochord length indicated that the age of embryos drifting in steep reaches varied widely, ranging from 0 to 7 days. Many had exhausted their yolk. We attributed the all-day drift in steep reaches to retention of drifting embryos in slack waters or eddies near the banks. In contrast, most embryos drifting in reaches with gentle gradients were recently-hatched individuals. We estimated that in normal or low river flow most embryos from the upstream reaches perish through starvation before they reach the sea.

Life History of the Caridean Shrimp Periclimenes Ornatus Bruce Associated with a Sea Anemone in Southwest Japan

ABSTRACT The life history of Periclimenes ornatus, an obligate symbiont of the sea anemone Parasicyonis maxima, was studied in southwest Japan. Ovigerous females occurred year-round, and the proportion of such females was high from May-November. Most of the settlement of juveniles took place from August―October. Females had higher growth rates and attained larger sizes at sexual maturity than males. The eggs of P. ornatus were of about average dimensions among caridean shrimps, but the brood size was relatively large. Most juveniles that settled in August and early September attained sexual maturity and reproduced within the year (the first generation), whereas juveniles that settled later did not reach maturity within the year. The immature shrimps that had overwintered reached maturity in the next reproductive season (the second generation). A fraction of the first generation survived into the next reproductive season and, together with the second generation, was the source of the following generations.

Life histories of sympatric mud-flat crabs, Helice japonica and H. tridens (Decapoda: Grapsidae), in a Japanese estuary

ABSTRACT Life-history traits of two mud-flat crabs, Helice japonica and H. tridens, were examined on the basis of monthly sampling at 13 stations from April 1986 to March 1987 in the Shigenobu River estuary, Shikoku Island, southwest Japan. Helice japonica and H. tridens co-occur at high densities in estuaries and salt marshes throughout western Japan, but their main habitats do not overlap in the Shigenobu River estuary. Helice japonica was found in the midintertidal, while H. tridens occurred from the midintertidal up to the high intertidal. The density of H. japonica was higher than H. tridens, though the habitat of H. japonica was limited to the lower part of the estuary. The size-frequency distributions of both species were complicated and the generation structures of them could not be elucidated, with the result that growth curves and survivorship curves of both species could not be established. Helice japonica matures more quickly, is smaller at first reproduction, has a shorter reproductive period, smaller number of broods, slightly smaller eggs, and a larger brood size. Moreover, only H. tridens shows a semilunar or lunar reproductive cycle, which may increase the reproductive rate at low population density. The two closely related species inhabited an estuary and had different life histories. In the present study, an example of the possibility of the diverse patterns of life-history traits under the constraint of a single set of environmental factors is provided.

Sources and Exchange of Particulate Organic Matter in an Estuarine Mangrove Ecosystem of Xuan Thuy National Park, Vietnam

The spatio-temporal variations in stable isotope signatures (δ13C and δ15N) and C/N ratios of particulate organic matter (POM), and physicochemical parameters in a creek water column were examined in an estuarine mangrove ecosystem of Xuan Thuy National Park, Vietnam. The objective was to examine the factors influencing creek water properties, and the sources and exchange of POM in this important mangrove ecosystem. The diel and seasonal variations in water temperature, flow velocity, pH, dissolved oxygen, and salinity demonstrated that tidal level, season, and biological factors affected the creek water properties. Mangroves had relatively low δ15N and very low δ13C values, with respective average values of 1.5 ± 0.9‰ and −28.1 ± 1.4‰. The low mangrove leaf δ15N indicated minor anthropogenic nitrogen loading to the mangrove forests. A significant positive correlation between POM–δ13C and salinity along the axis of Ba Lat Estuary, Red River, indicated that marine phytoplankton (δ13C value, −21.4 ± 0.5‰) was the predominant source of POM at the estuary mouth. Based on the co-variation of δ13C and C/N ratios, marine phytoplankton and mangrove detritus were predominant in POM of major creeks and small creeks, respectively. During the diurnal tidal cycle, the dynamics of POM were affected by sources of organic matter, tidal energy, and seasonal factors. The contribution of mangrove detritus to POM reached a maximum at the low tide and was enhanced during the rainy season, whereas marine phytoplankton contribution was highest at high tide.

The adaptive significance of a lunar or semi-lunar reproductive cycle in marine animals

Abstract Many marine invertebrates and fishes have a lunar or semi-lunar reproductive cycle. There are several possible explanations for the advantages of reproductive rhythmicity. Such rhythmicity may (1) increase the rate of mating; (2) increase the rate of fertilization; (3) help to retain free-living larvae in nursery sites; (4) help to spread free-living larvae over some adult habitats; (5) decrease predator pressure on free-living larvae; or (6) increase reproductive success by the direct influence of lunar cyclic environmental effects such as moonlight. In this study, the adaptive significance of reproductive rhythmicity in increasing the reproductive rate, is examined by analyzing the model which has three major controlling parameters, i.e. population density, sex ratio, and the duration of breeding interval. The objective function, that is adaptiveness, of per capita oviposition number per unit time is calculated under the non-cyclic or the cyclic reproductions. It appears, from the analysis of the model, that the cyclic type is accompanied by a higher rate of oviposition that the non-cyclic one under conditions of high sex ratio (female > male) and/or of low population size. In particular, under conditions of low population size, the cyclic reproduction decreases the lower limit of population growth and, thereby, reduces the probability of extinction of the population.

The limitations to organic loading on a bottom of a coastal ecosystem

Abstract Artificial organic loading of a coastal ecosystem with materials such as urban sewage and aquacultural waste must be confined within the assimilative ability of the ecosystem, which is defined as the capacity for oxygenic degradation of organic matter at the bottom of the system. Suitably restricted loading should allow the sustainable use of a coastal ecosystem. Bed sediments at eight sites in Uwajima Bay, Japan, were sampled with a cylindrical core sampler and were incubated under constant temperature for the measurement of their rates of oxygen uptake. The rate of oxygen uptake was maximal for a particular concentration of sulphide in the substratum, which may have some proportional relationship to the extent of organic loading. The concentration of sulphide at the peak of oxygen uptake ranged from 0.02–0.06 mg S g −1 among sampling sites. A positive relationship between the density of several kinds of benthos and the rate of oxygen uptake by the substratum was also found by multivariate analysis. A model of a coastal ecosystem was constructed, and the effects of environmental factors, such as temperature, depth and vertical mixing rate, on the upper limit of organic loading were investigated. The estimated concentration of reduced substance, as an indicator of the upper limit of organic loading, did not vary, but the upper limit itself varied widely depending on environmental factors.

Spatial distribution and corresponding determining factors of metal concentrations in surface sediments of Beppu Bay, southwest Japan.

This study determined the factors contributing to the spatial distribution of 14 metal concentrations in the surface sediments of Beppu Bay on the basis of comparisons of the organic geochemical properties and environmental parameters through principal component analysis (PCA) and redundancy analysis (RDA). The results of PCA and RDA showed that the concentrations of V, Cr, Co, and As were closely related to the distances between the sampling sites and the Oita River. This indicated that these metals originated from the rivers drainage area. The Mn, Cu, Mo, and Cd concentrations were related to the water depth. These results indicated that the Mo, Cd, and Cu deposition processes were controlled by oxygen depletion, and that these elements accumulated in the deeper parts of the bay under anoxic conditions.

Tracing anthropogenic inputs to production in the Seto Inland Sea, Japan--a stable isotope approach.

The Seto Inland Sea (SIS) receives waste runoff from ∼24% of Japans total population, yet it is also important in regional fisheries, recreation and commerce. During August 2006 we measured carbon and nitrogen stable isotopes of particulate organic matter (POM) and zooplankton across urban population gradients of the SIS. Results showed a consistent trend of increasing δ(15)N in POM and zooplankton from the western to eastern subsystems of the SIS, corresponding to increasing population load. Principal components analysis of environmental variables indicated high positive loadings of δ(15)N and δ(13)C with high chlorophyll-a and surface water temperatures, and negative loadings of low salinities related to inputs from large rivers and high urban development in the eastern SIS. Anthropogenic nitrogen was therefore readily integrated into the SIS food web from primary production to copepods, which are a critical food source for many commercially important fishes.

A monogamous pipefish has the same type of ovary as observed in monogamous seahorses

Syngnathid fish (pipefish and seahorses) are unique among teleost fish in that their ovary consists of a rolled sheet with germinal ridge(s) on the dorsal side running along the entire length of the sheet. A distinct difference is seen in the ovarian structure between polygamous Syngnathus pipefish and monogamous seahorses (Hippocampus spp.), the former having one germinal ridge and the latter with two ridges. This study examined the ovarian structure and the mode of egg production in a monogamous pipefish Corythoichthys haematopterus. The ovary of C. haematopterus had two germinal ridges like that observed in monogamous seahorses. There were two distinct groups of follicles in the ovary, one being a cohort of extremely small follicles and the other a cohort of follicles developing and increasing in size with the passage of time. We suggest that the ovarian structure and the mode of egg production in this pipefish are adaptations to monogamy.