Toshio Furota

Life Cycle Studies on the Introduced Spider Crab Pyromaia Tuberculata (Lockington) (Brachyura: Majidae). I. Egg and Larval Stages

ABSTRACT The introduced spider crab Pyromaia tuberculata has recently colonized eutrophic bays in Japan. In Tokyo Bay, this crab annually recolonizes an area of the upper bay which is subjected to severe hypoxic and anoxic water and subsequent extinction of benthic animal populations every summer. This research was designed to clarify egg and larval growth characteristics of P. tuberculata, and to shed light on how this species is able to effectively colonize such eutrophic areas. Three sets of experiments, incubation time, egg growth, and larval development, were conducted. Incubation time decreased with rising temperature, ranging from 3 months at 8°C to 7 days at 26°C. Egg growth was classified in 4 stages by structure as noted from external observation, and eye spots appeared in the final stage (last quarter of the incubation period). The larvae passed through first and second zoeal and megalopal stages. The experimental results, combined with field observations, indicate that P. tuberculata is capable of breeding, incubating, hatching, and metamorphosing to first crab throughout the year in Tokyo Bay. These reproductive and larval growth characteristics allow this species to annually recolonize the upper areas of Tokyo Bay.

Phylogenetic Relationships between the Tideland Snails Batillaria flectosiphonata in the Ryukyu Islands and B. multiformis in the Japanese Islands

Abstract 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.

Life cycle studies on the introduced spider crab Pyromaia tuberculata (Lockington) (Brachyura: Majidae). II. Crab stage and reproduction.

ABSTRACT Introduced populations of the spider crab Pyromaia tuberculata have become established in organically polluted bays in Japanese coastal waters. Pyromaia tuberculata is a predominant species in the epibenthic animal community in upper Tokyo Bay where summer hypoxia of bottom water causes a seasonal defaunation of the benthic community. Crabs recolonized the upper bay area after bottom oxygen recovered, settling in midfall, maturing in midwinter, and producing larvae from midspring until the return of hypoxia in summer. While all crabs in the upper part of Tokyo Bay were killed in late summer by the hypoxia, crabs, including ovigerous females, were found throughout the year in the lower part of the bay, where the hypoxia was not severe. Crabs in the lower part of the bay may be parents of recolonized crabs in the lower part of the bay. The purpose was to clarify the life cycle of P. tuberculata in a hypoxic environment. Crab growth and reproduction of P. tuberculata were studied by field surveys and laboratory experiments. Females reached maturity within 6 months after settlement, and the size of adult females tended to decrease when matured in higher temperatures. Developed eggs in which eye spots or eyes had appeared were found throughout the year. Puberty molt of the female could occur at ecdysis to the seventh crab instar. The final instar stage at which males reach maturity may vary individually from the sixth to eighth instars. This research, combined with earlier studies, demonstrates that P. tuberculata in Tokyo Bay reproduces throughout the year, and completes at least two generational cycles annually. These characteristics allow for the opportunistic exchange of larvae between temporary hypoxic and permanently healthy areas, and thus may be responsible for the ability of P. tuberculata to maintain its population in seasonally hypoxic waters.

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.

Phylogeography of the endangered tideland snail Batillaria zonalis in the Japanese and Ryukyu Islands

The phylogeography of the endangered tideland snail Batillaria zonalis in the Japanese and Ryukyu Islands was analyzed on the basis of nucleotide sequences of a mitochondrial gene for cytochrome oxidase c subunit I (COI). Extremely low genetic diversity was found in populations at both the northern and southern boundaries of the geographic distribution of this species in Japan, i.e., Sendai Bay and Iriomotejima Island, respectively, which might be attributed to the population bottleneck due to historical environmental variations and/or the recent foundation of populations in the marginal part of the inhabitable range. Most populations contained unique rare haplotypes, and significant genetic differentiation on the whole was shown, while no clear geographic genetic structure was detected between the Japanese and Ryukyu Islands or over the distribution area of B. zonalis in Japan, with the exception of significant genetic divergences in Ago Bay in the central part of Honshu and the southern part of Okinawajima Island.

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.

CLARIFICATION OF THE PROPERTIES OF FLUCTUATION OF BOTTOM SEDIMENT BASED ON LONG-TERM MONITORING AROUND THE MOUTH OF THE TAMA RIVER IN TOKYO BAY

With the focus on the characteristics of temporal and spatial fluctuations of the bottomsediment properties and their vertical distributions around the mouth of the Tama River in Tokyo Bay, the properties of surface sediments, such as grain-size distributions and water contents, had been monitored by taking bottom-sediment core samples from several sampling stations around the river mouth with the intervals of 3 month for a period of five years from 2006 through 2010. Based on the monitored data, the impact of flood events which occurred during the survey period on bottom sediment properties and the recovery processes after the flood event were evaluated. This study clarified that seasonal fluctuations of surface bottom sediment qualities clearly appear on river mouth shoals and tidal wetlands and the impact of the river flood was relatively slight and the damages were restored over relatively short period of time after the flood event. On the other hand, the flood event remarkably caused rapid decline of the water content of bottom sediments at offshore of the river mouth, and it took about three years for their bottom sediments to return to its normal as before the flood event. This paper also discusses the recovery process of the sediment properties at the offshore region and shows that the long-term evolution of the sediment properties was caused by the constant settling of new deposition of suspended particles from the water column and accumulation rate was estimated at 2.5 cm per year.