Gnathopogon caerulescens, an endemic fish of Lake Biwa, Japan, is one of the most important fishes for lake fisheries. It has been listed as an endangered species following its population decline in the 1990s and thus needs conservation. An increasing number of studies have recently stressed on the importance of incorporating genetic diversity for conservation purposes. In this study, we aimed to evaluate the mitochondrial DNA haplotype composition of G. caerulescens using environmental DNA (eDNA) to determine the genetic diversity of this species. We developed an eDNA-based haplotyping approach using high-throughput sequencing (HTS), which can accurately detect and eliminate erroneous HTS reads, even when the target sequences are differentiated by a single or a few mismatches. Field surveys using the developed method successfully revealed the genetic structure of local G. caerulescens populations. We found star-shaped haplotype networks with two major haplotypes accompanied by several rare haplotypes for all local populations except one, which suggested that the G. caerulescens population had undergone a historical expansion, and its population size did not shrink to the extent that rare haplotypes were eliminated during the population decline in the 1990s. We also observed that the genetic diversity was lower in the northern populations in terms of haplotype number and haplotype diversity (h), suggesting that the decline in population was more severe in the north than in the south. We demonstrated the practical use of eDNA analysis in population genetics of endangered species. Continuous monitoring using this approach would facilitate the conservation and management of fish resources.

Habitat fragmentation is a pervasive threat to biodiversity. Linearly arranged habitats such as stream networks are particularly vulnerable to fragmentation. As the landscape becomes increasingly human dominated, conservation values of fragmented habitat patches cannot be overlooked. It is critical to understand the demographic mechanisms of population persistence or extirpation in fragmented patches. We studied dynamics of spatially structured populations of two Japanese landlocked salmonids persisting for >30 years in a headwater stream network that is highly fragmented due to low-head dams in the mainstem. We parameterised and analysed spatial matrix population models using 9-year mark-recapture data. Tributaries supported higher survival rates in some life stages, and movement was asymmetrical from the tributaries to the mainstem. Accordingly, population growth rates were higher in the tributary patches than the mainstem in both species despite the tributaries occupying only 12% or 18% of the study stream network by surface area. The tributaries harboured more physically and hydraulically complex instream habitats (i.e. higher wood density and flow refugia), indicating that habitat patch quality was more important than habitat patch size in determining the dynamics of these spatially structured populations. Tributary locations in the stream network were important in the trajectory of these populations. The upstream-dwelling charr persisted in the highly fragmented mainstem patch (i.e. six impassable infrastructures in a <500 m patch) due to immigration of fish from upstream including the tributary. However, the downstream-dwelling salmon has been gradually extirpated from the uppermost section of the fragmented mainstem patch because they could not maintain a positive population growth rate after loss of emigrants was accounted for and immigration was prevented due to fragmentation. Synthesis and applications. We conclude that small tributaries have rescued the spatially structured populations from extirpation (charr) or at least slowed down extirpation (salmon). Legal protection of headwaters as aquatic habitats is weak globally. Our results suggest that stream management plans underestimating the demographic value of small tributaries will likely fail to conserve populations of headwater inhabitants and therefore endanger aquatic biodiversity. We discuss conservation implications of this study related to habitat connectivity and fisheries management.

Migration patterns of Japanese dace Pseudaspius hakonensis in and around Lake Biwa, Japan are unknown, although fluvial-lacustrine life history has been known in most tributaries, the many mature individuals migrating upstream into tributaries in early spring. Field surveys and stable isotope analyses were used to clarify the migratory history of Japanese dace in the Ishida River, inflowing to Lake Biwa. In the field surveys, the number of young of the year (YOY) individuals caught per unit effort (CPUE) by hand net and casting net in the tributary and lake-shore area was calculated. The CPUE peaked from May to July, after the spawning season, thereafter decreasing rapidly, suggesting that YOY Japanese dace migrated downstream to Lake Biwa immediately after emerging, possibly due to the abundant food resources there. From September to November, the CPUE of YOY individuals increased in the lower reaches of the stream, although their stable nitrogen isotope values were variable, in some cases being similar to those recorded in the upper reaches and the lake-shore area. This suggests that some YOY individuals migrate upstream from the lake in autumn. In the present study, the juvenile migration pattern of fluvial-lacustrine Japanese dace in Lake Biwa was described using a CPUE survey and delta N-15 and delta C-13 analyses.

Climate oscillations during the Pleistocene had profound effects on the evolutionary history of freshwater fishes now distributed across northern temperate regions. The extent of continental glaciation on the western side of the North Pacific, including areas of East Asia, was more limited as compared with regions of North America, Europe and high-latitude areas of the North Pacific. Therefore, the effects of climate oscillations might have influenced species in dissimilar ways depending on the species' distribution. We used mitochondrial DNA (mtDNA) and microsatellite DNA (msDNA) markers to clarify the evolutionary history of masu salmon Oncorhynchus masou subspecies-complex (family Salmonidae) distributed in historically non-glaciated regions in the western North Pacific. No marked regional or subspecies-specific mtDNA haplotype associations were recognised, except for O. masou subsp., a lacustrine form endemic to Lake Biwa, an ancient lake in central Honshu. The landlocked subspecies O. masou formosanus, with a disjunct distribution on Taiwan Island, exhibited no diagnostic population features differing from the other subspecies, in either mtDNA or msDNA markers. Mismatch distribution and Bayesian skyline plot analyses indicated relatively recent range expansion and rapid population growth for masu salmon during the last glacial period (c. 0.1-0.15 Ma). Contrary to the mtDNA genealogy, Bayesian clustering using msDNA showed two main genetic clusters, mainly northern populations of the subspecies O. m. masou and populations of the subspecies O. m. ishikawae in southern areas of the Japanese Archipelago. Notably, O. m. formosanus on Taiwan Island was included in the O. m. masou group, and O. masou subsp. was included in the O. m. ishikawae group. Our results suggest that the masu salmon subspecies-complex in historically non-glaciated regions of the Temperate Northern Pacific is characterised by weak population structuring and shallow genetic differentiation among the subspecies, except for O. masou subsp. owing to its long isolation in Lake Biwa. Incomplete lineage sorting and historical inter-subspecies hybridisation, possibly due to secondary contact, seem to be plausible explanations for discrepancies in the mitochondrial DNA genealogy and nuclear DNA genetic structure.

Despite a recent study of Honmoroko, Gnathopogon caerulescens, reproduction in lagoons around Lake Biwa having demonstrated the importance of lagoon inlets as spawning habitats for the species, no quantitative studies have examined the conditions determining the environmental suitability of spawning habitats. A survey of Honmoroko egg distribution in the inlets indicated that mature Honmoroko selectively spawn in regions of greater current velocity, utilizing a greater abundance of submerged plants and larger substrate (gravel and pebble) size, not utilizing the emergent plants, habitat characteristics supported by a generalized linear mixed model. The biological information gained is essential for future conservation of Honmoroko spawning habitats.

Otolith growth increments in wild-caught alizarin complex one (ALC)-marked honmoroko Gnathopogon caerulescens were examined to verify the veracity of the age determination method in cyprinids. ALC-marked G. caerulescens recaptured from their natural environment had lapilli increment counts outside the ALC ring mark that had formed on a daily basis during the juvenile stage. This apparently being the first direct evidence of daily periodicity of otolith increment formation in wild-caught cyprinids.

The establishment of fluvial fish populations from anadromous populations by natural or artificial barriers obstructing migration is a good research subject to study life history plasticity. Biwa salmon, Oncorhynchus masou subsp., a salmonid fish endemic to the Lake Biwa water system, exhibit life history variation (e.g., mature stream-resident males) in addition to a typical lacustrine life history type, indicating potential adaptations of life histories in response to emergence of barriers. Currently, fluvial populations that are morphologically similar to both stream-resident Biwa salmon and amago, the fluvial red-spotted masu salmon, Oncorhynchus masou ishikawae, are found upstream of dams which were constructed in the inflowing rivers of Lake Biwa. However, it is unknown whether they are Biwa salmon or amago. To explore that, the genetic characteristics of nine fluvial populations were investigated through AFLP and mtDNA analyses. Bayesian admixture analysis based on the AFLP data revealed that three fluvial populations were admixed populations between Biwa salmon and amago. In addition, a Biwa salmon mtDNA haplotype was detected in some individuals from three populations. However, no genetically pure fluvial populations of Biwa salmon were found, indicating no life history plasticity in this subspecies, and thus hybridization with amago boosted the ability of this subspecies to establish fluvial populations. Nevertheless, other scenarios, such as hybridization after establishment of fluvial populations of Biwa salmon, are also possible. The latter hypothesis could be supported by the fact that amago did not inhabit the river before emergence of barriers. However, a significant genetic population structure was found only in amago, suggesting that this subspecies is native to the Lake Biwa water system. But the possibility that multiple sources of amago have been released into rivers cannot be excluded. Therefore, further studies on the relationships between amago populations in the upper reaches of the Lake Biwa water system and other populations in the surrounding areas of the Lake Biwa water system are needed to clarify the origins of the admixed populations.

The degree of iteroparity in stream-resident forms of masu salmon Oncorhynchus masou was examined using mark-recapture studies in natural streams. In a partially migratory population, at least 10% of resident males survived after maturation and repeatedly matured for up to 5 years. In the landlocked amago salmon subspecies, the post-maturation survival rate was at least 7% and repeat maturation was observed for up to 3 years for both males and females.

Rhinogobius similis a common goby, widely distributed in Far East Asia, is characterized by an amphidromous life history, adult fish inhabiting the lower reaches of rivers, and larvae in the freshwater area being swept downstream to the sea and returning to the river following a period of pelagic marine existence. Individuals were collected from 6 stations in the Ibi, Nagara and Kamo Rivers, central Japan, and their otolith strontium and calcium environmental signatures examined using electron probe microanalyses. The otolith Sr:Ca ratios of R. similis from the Kamo River suggested a typical amphidromous life history, whereas those of fish collected from the Ibi River suggested two types of migratory life history pattern: amphidromous and river resident. In contrast, the otolith Sr:Ca ratios of fish collected at two stations upriver of a dam in the Nagara River estuary suggested an artificially landlocked life history. Emergent plant vegetation in the lower reaches of the Ibi River created slow currents and pools in the river, thereby possibly enhancing the likelihood of a river resident life history. In the Nagara River, however, landlocked specimens had been obstructed in their downstream migration by the dam. It is suggested that R. similis in central Japan has a variable migratory life history pattern, allowing the species to persist in upstream habitats in large rivers.

Honmoroko Gnathopogon caerulescens is a critically endangered species and important for commercial fisheries; thus stock enhancement programs are being conducted to restore resources. We evaluated the genetic population structure of field collected samples including spawned eggs around spawning areas in Lake Biwa and the extent of genetic diversity in wild samples and hatchery stocks using sequences of mitochondrial cytochrome b gene. Pairwise I broken vertical bar st analysis and AMOVA clearly showed minimal population structure and haplotype network did not reveal any clear geographic pattern in Lake Biwa. It is probable that strays spawn in non-natal spawning areas, resulting in significant levels of gene flow among spawning areas. Genetic characteristics of hatchery stock F1, F2, and F3 were similar to those of wild samples in terms of haplotype diversity, nucleotide diversity and pairwise I broken vertical bar st values. These results indicate that the relatively high genetic diversity at its initiation was retained due to a lot of broodstock over two successive generations. Accordingly we propose that the current Honmoroko breeding method is appropriate for conserving the genetic diversity of Honmoroko and that the hatchery stock are genetically compatable for release and stock enhancement.

Egg size affects larval size, growth rate, survival and fecundity with maternal fitness being maximized by a trade-off between egg size and fecundity. Production of a small number of large eggs maximizes female fitness under poor food conditions, as does a large number of small eggs under rich food conditions. Gnathopogon caerulescens (Honmoroko) spawns over a wide range of water temperature from spring to summer. Thus, we determined whether or not egg size varied with water temperature and how egg size influenced hatchling size at different water temperature. Changes in egg size strongly correlated with seasonal changes in water temperature around the lake, regardless of time and area. An experiment using eggs from the lake indicated that hatchling size has a significant positive relationship with egg size in water temperatures of 24 A degrees C. On the other hand, a lower incubation temperature, similar to that likely to be encountered at the beginning of the spawning season, resulted in a smaller hatchling size, eggs requiring a longer time to hatch. At the beginning of the spawning season, therefore, egg size may have a lesser impact on hatching size in the natural environment of Honmoroko because of the relatively lower temperatures. Although fish larvae below a certain threshold of effective body size are generally not expected to have a high early survival rate due to, for example, a lower competitive ability or a high susceptibility to predation, seasonal egg size variation in Honmoroko may be adaptive so as to enhance the likehood of early survival by establishing an effective body size under changing water temperature conditions.

ホンモロコは一妻多夫の産卵行動を行う。これまで群れでの産卵行動に関する知見しかなかった。このためホンモロコの再生産形質を詳細に調査する方法として，水槽を用いて雌雄1個体の4組を飼育し，1産卵期を通して産卵回数や孵化率などを調査してペアー産卵による調査法の妥当性を検討した。産卵巣として方形枠に張った寒冷紗の膜を使用し，これを水槽の表層に水平に設置した。産卵は4月から7月にかけて産卵巣上に6-9回行われ，これまで報告されている回数よりもかなり多かった。本研究において，4ペアーの孵化率，奇形率の平均値は82.7%（44.4-100%），1.5%（0-8.6%）であり，過去に群れで求められた値と遜色がないことが明らかになった。以上の結果は，今回の雌雄1個体のペアーによる産卵が，ホンモロコの再生産形質を個体別に調査する方法として応用可能であることを示していると考えられる。

琵琶湖固有種セタシジミの肥満度と採苗量の関係を調査した。琵琶湖ののべ 19 漁場で採捕したセタシジミの体重に占める軟体部の乾燥重量の割合を測定し肥満度を求めた。また，それらの漁場で採捕した個体を用いて人工採卵し，肥満度と得られた D 型仔貝の数との関係を解析した。その結果，各漁場の肥満度と採苗量に有意な回帰直線が得られ，正の相関が認められた。このことから，肥満度が採苗量の指標として有効であると考えられた。

2012 年と 2013 年の 3 月から 5 月にかけて琵琶湖の内湖の一つである伊庭内湖周辺においてホンモロコ釣りの遊漁による釣獲尾数の推定を試みた。調査日を 1 次抽出単位，遊漁者を 2 次抽出単位とする 2 段抽出法により釣獲尾数を推定した。ホンモロコ釣りの遊漁による釣獲尾数は 2012 年は，272,275（変動係数=14%）尾，2013 年においては 85,489（変動係数=13%）尾と推定された。琵琶湖のホンモロコ資源を評価し管理する上で，ホンモロコ釣りの遊漁による釣獲量の把握は重要であることが明らかになった。

The sex ratios of the nigorobuna Carassius auratus grandoculis reared in paddy fields during the sex differentiation periods of larval and juvenile stages were examined, since the phenotypic expression of sex in the fish is thermolabile, so that sex determination is under the control of genetic factors and temperature. After 2 days from hatching, the larvae were introduced into five paddy fields at five different densities in late May. After 37 or 41 days from hatching the juveniles were collected when the water was drained from the paddy fields, and reared in laboratory tanks supplied with running freshwater pumped from the lake shore until the age of 144-164 days. The mean water temperatures in the paddy fields were 22.4-24.1 A(0)C, and the maximum water temperature during the day frequently exceeded 30 A(0)C during the supposed critical period of sex determination of the fish. The sex ratios in the five paddy fields did not show a male-bias. Instead, all fields produced either balanced or slightly female-bias sex ratios. These results suggest that temporary high and fluctuating temperatures in paddy fields with low mean temperatures have hardly any masculinization effects on genotypic females of C. auratus grandoculis during sex differentiation stages.

ホンモロコGnathopogon caerulescensはコイ科タモロコ属の琵琶湖水系固有種で(細谷，1995)，秋から冬にかけては琵琶湖の沖合の水深40-90mの深層域に生息しているが，2月下旬から6月にかけて琵琶湖の沿岸や内湖に来遊して，ヨシPhragmites australisやヤナギ類Salix spp.の根などに産卵する(牧，1966; 中村，1969)。本種は琵琶湖漁業の重要な漁獲対象種であり，1995年までは毎年150-350tの漁獲量が記録されていたが，それ以降は急速に減少し，2008年には10tにまで落ち込んだ(藤岡，2013)。そのため，本種は滋賀県レッドデータブックでは絶滅危惧増大種として(藤岡，2011)，環境省レッドリストでは絶滅危惧IA類として位置づけられている(環境省，2013)。資源量が減少した要因として，産卵場所であるヨシ帯の減少，オオクチバスMicropterus salmoidesやブルーギルLepomis macrochirusによる食害の影響が大きいと言われている(藤岡，2011，2013)。またホンモロコは，水際のヤナギの根などに産卵することから，琵琶湖水位のわずかな低下が産着卵の干出死亡につながることが指摘されている(臼杵，2007; 亀甲，2011; 藤岡，2013)。

琵琶湖でのビワマス引縄釣遊漁の現状を把握するために，2008 年 12 月から遊漁者に対して琵琶湖海区漁業調整委員会指示により届出と採捕報告書の提出を義務付けた。2011 年 9 月までの結果を集計したところ，届出者数は年々増加していた。採捕報告書の提出率は約 90% であった。釣行日数と総採捕尾数は冬期に少なく，夏期に多い傾向を示した。毎年約 1 万尾が採捕され，そのうち 41.4～67.3% は再放流されていた。遊漁者による採捕量は 6.6～8.6 t と推定され，漁業者による漁獲量は 23.2～45.8 t であった。

水田を利用したホンモロコ Gnathopogon caerulescens の初期育成の可能性を検討するため，2009年と2011年にホンモロコ孵化仔魚を滋賀県内の水田（合計6筆）に 1 m² あたり20個体から80個体の密度で放流し，中干し時までの成長と生残を調査した。水田へ放流後19～25日間で，15.2～21.9 mm まで成長し，天然下や飼育下での成長よりも早く，生残率は23.1～43.9%であった。以上の結果から，水田はホンモロコの初期育成の場として有効であると考えられた。

水田を利用してゲンゴロウブナ Carassius cuvieri の初期育成の可能性を検討するため，2009年と2011年にゲンゴロウブナ孵化仔魚を滋賀県内の水田（合計7筆）に 1 m² あたり20個体から80個体の密度で放流し，中干し時までの成長と生残を調査した。水田へ放流後23～27日間で，12.2～20.0 mm まで成長し，飼育下での成長よりも早く，生残率は37.9～76.1%であった。以上の結果から，水田はゲンゴロウブナの初期育成の場として有効であると考えられた。

コイ科のホンモロコの繁殖形質とそれに及ぼす親の誕生時期の影響を検討するため，産卵時期の異なる天然卵から育てた 5 組の親を用いて調査を行った。産卵は 4 月から 7 月までの水温 10.5℃ から 25.9℃ の範囲で行われ，この間に 1 尾あたりの平均産卵数は 3,000～4,000 粒，産卵のあった平均日数は 1.5 から 3.5 日であった。5 月上旬までに生まれた 3 組の親は 20℃ 未満で総産卵数の 8 割以上を産卵したが，5 月下旬以降に生まれた 2 組では 7 割以下と少なく，親の誕生時期の影響が示唆された。

An investigation to determine the extent of introgression between Biwa salmon Oncorhynchus masou subsp. and lake-run Amago salmon O. m. ishikawae from Lake Biwa was conducted using mtDNA sequence and amplified fragment length polymorphism (AFLP) analysis. Hatchery-reared Amago salmon from the Samegai Trout Farm, the main source of stock released into the inflowing rivers of Lake Biwa, were also investigated. The extent of mtDNA introgression between these two subspecies was quite low: an anomalous haplotype was detected in just 1 of 55 Biwa salmon and 2 of 63 lake-run Amago salmon. Many of the mtDNA haplotypes found in lake-run Amago salmon were common to those in hatchery-reared Amago salmon. In the AFLP analysis, the nuclear genome of Amago salmon was hardly detected in Biwa salmon, but the Biwa salmon genome comprised 13.4% of the total amplified fragments from lake-run Amago salmon, being detected in different proportions in 28 of the 63 investigated individuals of the latter subspecies. These results suggest that introgression has occurred between Biwa salmon and lake-run Amago salmon descending from hatchery-reared Amago salmon from the Samegai Trout Farm.

Size at maturity of fluvial white-spotted charr, Salvelinus leucomaenis, was studied in small headwater tributaries of nine rivers around the Lake Biwa water system, Japan. Threshold size at maturity in both sexes showed significant positive relationships with water discharge, indicating that smaller threshold sizes at maturity of fluvial white-spotted charr occurred in smaller habitats. These results provide a link between size at maturity and habitat size and have important implications for the management of both habitats and white-spotted charr populations.

The relationships between census population size and tributary length and between haplotype diversity of the mitochondrial DNA and census population size in ten white-spotted charr populations in the Lake Biwa water system and its adjacent basins were investigated. The census population size (number of fish with a parts per thousand yen100 mm in standard length) significantly increased with the tributary length. In the eastern part of the Lake Biwa water system, haplotype diversity increased with the census population size. On other hand, in the western part of the water system and adjacent basins, haplotype diversity was zero irrespective of the census population size. These results suggest that white-spotted charr populations in the eastern and western part of the Lake Biwa water system have undergone different levels of bottlenecks related to the habitat size in the postglacial warming.

The genetic variations-and the time dependence of such variations-of natural populations of the white-spotted charr ,Salvelinus leucomaenis, in the Lake Biwa water system as well as those of a hatchery-reared population were inferred from AFLP. Upon the application of principal coordinate analysis using 118 polymorphic AFLP fragments based on the Jaccard similarity index, specimens of each of six natural local populations from the inlet rivers of Lake Biwa grouped roughly together, suggesting that each local population was genetically differentiated. The hatchery-reared population was shown to be closely related to the local population in the Seri River, suggesting that the Seri River population originated from hatchery-reared charr due to extensive stocking. Furthermore, specimens of the Yasu River grouped in a somewhat different position from the other natural populations, agreeing well with its geographic distance from the other populations. The nucleotide diversities of six natural populations (Harihata River, Ishida River, two reaches of the Takatoki River, Ane River, and Yasu River) in 2002 or 2003 were relatively low (pi a= 0.067-0.146%) compared with that of the Seri River (0.278%) and the hatchery-reared charr (0.316%). The nucleotide diversity in the five local populations (Ishida River, two reaches of the Takatoki River, Ane River, and Yasu River) remained at a low level from 1994 to 2002/2003, but only the nucleotide diversity in the Harihata River actually decreased. From 1994 to 2002/2003, the nucleotide diversity in the Seri River remained at a higher level among the natural populations from 1994 to 2002/2003; it was enhanced by the artificial release of hatchery-reared charr before 1994. In order to conserve the genetic diversity of the white-spotted charr in the Lake Biwa water system, it is necessary to prevent the stocking of hatchery-reared charr in reaches where hatchery-reared charr have not previously been stocked.

A phylogeographic analysis of mitochondrial DNA sequences was performed in order to elucidate the origin, dispersal process, and genetic structure of white-spotted charr in the Lake Biwa water system. Two haplotypes were most common in the Lake Biwa water system, and were also common in the adjacent inlet rivers of the Sea of Japan. These results suggest that in the glacial periods of the Pleistocene, white-spotted charr dispersed into the northern inlet rivers of Lake Biwa from adjacent inlet rivers of the Sea of Japan by watershed exchanges, colonizing the whole of the Lake Biwa water system. Mitochondrial DNA diversity contrasted sharply between the western and eastern parts of the system, suggesting that the populations in the western part might be more reduced than those in the eastern part in relation to the smaller habitat size. The high overall FST estimate (0.50), together with pairwise comparisons of FST, indicated significant genetic divergence between populations due to isolation and small population size. Hierarchical analysis (AMOVA) also showed that genetic variation was more pronounced among regions (28.39%) and among populations within regions (47.24%) than within populations (24.37%). This suggests that each population in and around the Lake Biwa water system should be treated as a significant unit for conservation and management.

The occurrence and distribution of the threatened nagaremon-charr, a morphotype of Salvelinus leucomaenis japonicus, were investigated in the Ane River system, Lake Biwa basin, Shiga Prefecture, central Japan, in August, 2005. The total numbers of nagaremon-type charr and typical-type charr captured were 20 and 38, respectively. The occurrence of nagaremon-type charr has dropped dramatically compared with the 1970s, suggesting that artificial transplantation might have obscured indigenous gene pools and modified evolutionary distinctions of that morphotype. Nagaremon-type charr were distributed further upstream than the typical-type, a significant only the former being captured above waterfall. These observations suggest that habitat restoration, prohibition of fishing and removal of Salvelinus leucomaenis pluvius are essential for practical conservation of the nagaremon-type charr.

We have cloned a cDNA encoding the matrix metalloproteinase (MMP) from oyster Crassostrea gigas. The clone contains a 1797 bp open reading frame encoding a protein of 599 amino acids. Oyster MMP (Cg-MMP) has a transmembrane domain at its C-terminal and a furin/ prohormone convertase cleavage site at the end of a propeptide domain, which are commonly observed in membrane-type MMPs (MT-MMPs). This suggests that Cg-MMP is an MT-MMP. The deduced amino acid sequence of oyster MMP shares approximately 30% identity with human MT4-MMP and MMPs from fruit fly and hydra. Cg-MMP mRNA was detected in the gill, mantle and adductor muscle, and more intense signals in the northern blot analysis were recognized in the gill and adductor muscle. Similar tissue distribution was observed for tissue inhibitor of metalloproteinase (Cg-TIMP) in oyster. In response to hypoxic stress, the abundance of Cg-MMP mRNA was elevated in the gill, while that of Cg-TIMP mRNA remained almost constant. These findings suggest that promotion of collagen metabolism may be implicated in the hypoxic adaptation in oyster.