Inequality in the distribution of 137Cs contamination within freshwater fish bodies and its affecting factors.

Contamination of freshwater fishes with 137Cs remains as a serious problem in Japan, nearly 10 years after the Fukushima nuclear power plant accident, but there is limited information on the distribution of 137Cs contamination in fish bodies. The 137Cs distribution can be used for the estimation of internal radiation exposure through the consumption of fish and for the dose estimation of fish themselves. In this study, the 137Cs distribution in the bodies of 8 freshwater fish species was investigated as percentages of total body burden for fish inhabiting Lake Inba. Fish samples were caught in stake nets placed close to the shore approximately once a month. After the measurement of body length and fresh weight, the radioactivities of 137Cs in muscle, internal organs, spawn, milt and bone were assayed using high-purity germanium detectors. Analysis of all fish samples showed that the 137Cs distribution was highest in muscle (54 ± 12%), followed by internal organs (7.8 ± 4.6%), spawn (7.4 ± 5.4%), milt (3.2 ± 2.1%) and bone (1.2 ± 0.58%). Among fish species, the highest proportion of 137Cs in muscle was detected in largemouth bass (71 ± 1 3%), followed by snakehead (69 ± 14%), channel catfish (63 ± 17%), common carp (62 ± 14%), barbel steed (58 ± 6.5%), silver carp (57 ± 7.7%), bluegill (53 ± 4.7%), and crucian carp (50 ± 10%). These results suggested that the 137Cs in muscle was likely to be high in piscivorous fishes compared to omnivorous fishes, especially crucian carp. The proportion of 137Cs in muscle of crucian carp was not explained either by body length or fresh weight. However, a positive correlation was found between the proportion of 137Cs in muscle and the condition factor which was an indicator of nutritional status calculated from a length-weight relationship. This correlation implied that more 137Cs accumulated in muscle tissue of a fish species with high nutritional status. This is the first study to show that condition factor is more important than body length and wet weight in explaining the high proportion of 137Cs in muscle tissues, at least for crucian carp.

Molecular Evidence for the Expansion of the Asian Cryptic Invader Hediste diadroma (Nereididae: Annelida) into the Northeast Pacific Habitats of the Native H. limnicola.

We used previously established molecular methods to determine how far the Asian invader nereidid worm Hediste diadroma has spread into northeast Pacific estuaries that are inhabited by the native congener H. limnicola. Further, we analyzed the mitochondrial DNA of 702 Hediste specimens collected from 27 estuaries along 1,350 km of coastline in Washington, Oregon, and California, USA, to distinguish between the morphologically indistinguishable immature stages of these two species. In total, 377 specimens were identified as the invader H. diadroma and 325 were identified as the native H. limnicola. The invader H. diadroma was dominant at many sites in Puget Sound, Washington, and in the Columbia River estuary, Washington, and Oregon, suggesting that this species initially invaded estuaries in Washington or northern Oregon. In contrast, the native H. limnicola was dominant at intertidal sites in California and at subtidal sites in the Columbia River estuary. We also analyzed a partial nucleotide sequence from the mitochondrial cytochrome oxidase subunit I gene of H. diadroma in specimens collected from seven sites in the US and 11 sites in Japan, which showed no marked geographic differentiation between 18 US and 31 Japanese haplotypes. This finding suggests that H. diadroma have been introduced repeatedly into US estuaries from many regions in Japan.

Genetic population structure of local populations of the endangered saltmarsh sesarmid crab Clistocoeloma sinense in Japan.

During recent decades, over 40% of Japanese estuarine tidal flats have been lost due to coastal developments. Local populations of the saltmarsh sesarmid crab Clistocoeloma sinense, designated as an endangered species due to the limited suitable saltmarsh habitat available, have decreased accordingly, being now represented as small remnant populations. Several such populations in Tokyo Bay, have been recognised as representing distributional limits of the species. To clarify the genetic diversity and connectivity among local coastal populations of Japanese Clistocoeloma sinense, including those in Tokyo Bay, mitochondrial DNA analyses were conducted in the hope of providing fundamental information for future conservation studies and an understanding of metapopulation dynamics through larval dispersal among local populations. All of the populations sampled indicated low levels of genetic diversity, which may have resulted from recent population bottlenecks or founder events. However, the results also revealed clear genetic differentiation between two enclosed-water populations in Tokyo Bay and Ise-Mikawa Bay, suggesting the existence of a barrier to larval transport between these two water bodies. Since the maintenance of genetic connectivity is a requirement of local population stability, the preservation of extant habitats and restoration of saltmarshes along the coast of Japan may be the most effective measures for conservation of this endangered species.

Phylogenetic relationships between the tideland snails Batillaria flectosiphonata in the Ryukyu Islands and B. multiformis in the Japanese Islands.

Phylogenetic relationships between two sibling species of Japanese tideland snails, namely, Batillaria multiformis from the Japanese Islands and B. flectosiphonata from the Ryukyu Islands, were analyzed on the basis of the nucleotide sequences of the mitochondrial gene for cytochrome oxidase I. Populations of B. multiformis were genetically distinct from those of B. flectosiphonata with the exception of a population from Amami-Oshima Island, which corresponded to the boundary between the distributions of these two species. Individuals with the mitochondrial gene of B. multiformis and those with the mitochondrial gene of B. flectosiphonata were collected from the same tidal flat on Amami-Oshima Island. All the snails with the mitochondrial gene of B. multiformis could be divided into two genetically distinct groups but there was no geographical structure to the distribution of these two groups. Individual populations of B. flectosiphonata in the Amami, Okinawa, Miyako and Yaeyama insular groups each consisted exclusively of a unique set of haplotypes, with the exception of a population at a northern site on Okinawajima Island, which included a few individuals with sequences related to those of individuals in the Amami insular group. All individuals from South Ryukyu formed a well-supported monophyletic group, while the monophyly of individuals from Central Ryukyu was not supported. The monophyly of B. multiformis was clearly demonstrated but there was no evidence to support that of B. flectosiphonata. Batillaria multiformis might have been derived from immigrants from the Ryukyu Islands, which became isolated and diverged genetically on the Japanese Islands.