Deep-sea habitats cover about two-thirds of the earth's surface. The understanding of spatial patterns of biodiversity and community structure and the processes that regulate them is the fundamental goal of deep-sea ecology. One of the best documented large-scale diversity patterns is that with water depth. Many studies have reported parabolic pattern of diversity with water depth for many taxa. Most studies, however, have focused only on bathyal and abyssal depths, while studies extending into the hadal zone (trench) are limited, due to their remoteness and difficulties to access. In addition, the biodiversity and community structure of meiofauna around trenches were still unknown although meiofauna is one of important components of deep-sea ecosystem. In this thesis, I analyzed harpacticoid diversity and community structure of harpacticoids around two trench regions around Japan: the Kuril Trench, which is located below a highly productive boreal region, and the Ryukyu Trench, which underlies an oligotrophic subtropical ocean. Furthermore, I investigated the relationship between harpacticoid assemblages and abiotic environments. On 11th May 2011, the 2011 off the Pacific coast of Tohoku Earthquake occurred. I also evaluated the effect of catastrophic disturbance caused by the mega earthquake on the deep-sea meiofaunal assemblage on the landward slope of the Japan Trench.

In the Chapter 2, I provided the harpacticoid diversities around the Kuril and Ryukyu Trenches at the genus level. In the Kuril region, harpacticoid diversity indicated unimodal patterns with water depth, with peaks appearing at intermediate depths, while it monotonously decreased along the water depth in the Ryukyu region. These results would reflect the difference in food availability between two regions, that is, in the Kuril region, where productivity is high, the entirety of the general unimodal pattern of diversity along the water depth appeared, and in the Ryukyu region, where productivity is low, only the deeper side of the general pattern appeared. These findings also suggest that the general relationship between water depth and diversity, which has been described for macrofauna and megafauna, can be extended to meiofauna at all depth ranges. In addition, food availability would affect the harpacticoid diversity in the Ryukyu region, while a multiplicity of factors appeared to regulate it in the Kuril region.

In the Chapter 3, I investigated the community structures of harpacticoids around the Kuril and Ryukyu Trenches at the genus level. Sampling strategy in this study, with stations ranging from bathyal to hadal depths, enabled me to analyze bathymetric trends among harpacticoid assemblages in detail down to hadal depths. In the both regions, gradual changes of the harpacticoid community compositions from batyal to hadal depths were detected for the first time. This finding suggests that the hadal community structure was continuous from the adjacent trench slopes in both regions. Water depth, or certain factors associated with water depth, would affect harpacticoid community structure in the both regions. A large difference was found between the trench floor and the adjacent environments in the Kuril region, while not in the Ryukyu region. This discrepancy would be attributed to the difference in the geological history of each trench, a certain barrier and topographical features in the Kuril region. When the harapcticoid community structures on the same topographic settings were compared between the Kuril and Ryukyu regions, the dissimilarities were increased with water depth. This result may be attributed to the Izu-Ogasawara arc and trench, which are located between the two regions and may prevent the expansion of harpacticoid distribution ranges from both trenches.

In the Chapter 4, I examined whether the analyses at the family level could reveal the spatial patterns in diversity and community structure of harpacticoid copepods around the Kuril and Ryukyu Trenches with a same accuracy as those at the genus level. Virtually the same results of the bathymetric diversity pattern could be obtained by analyses at the genus and family levels, although some differences were found. The results of this study suggest that the family-level analyses are sufficient to illustrate the bathymetric pattern in harpacticoid diversity from bathyal to hadal depths. However, the different patterns were found in comparing community structure within the same regions and between the different regions, suggesting that the analyses at the family level are not sufficient to examine the spatial pattern of community structure within and between regions. In addition, the genus composition shifts were detected within some families.

In the Chapter 5, I examined the meiofaunal assemblages on the landward slope of the Japan Trench (120-5,600 m water depth) 4.5 months after the 2011 off the Pacific coast of Tohoku Earthquake and compared with those before the mega earthquake. Unusual vertical distribution of meiofauna in the sediments was found only after the earthquake, while differences were not detected in meiofaunal density and its bathymetric pattern before and after the mega earthquake. This unusual vertical distribution seemed not to be attributable to the effect of turbidite and the change of organic matter supply.

The static and dynamic states of the deep-sea ecosystem revealed through the present study would potentially lead us to understand the generation and maintenance mechanism of the deep-sea biodiversity.

Fig. 1. General relationship between water depth and diversity. In shallower regions, where a high organic matter flux leads to high population density, strong biological interactions depress diversity. In contrast, in deeper regions, where organic matter is scarce, animal density is so low that the probability of extinction is high, again leading to low diversity.

Fig. 2. Bathymetric patterns of harpacticoid diversity in the Kuril region: (a) the number of genera (GR); (b) the expected number of genera (E(G19)); (c) the Shannon-Wiener index (H'loge); and (d) evenness (J').

Fig. 3. Bathymetric patterns of harpacticoid diversity in the Ryukyu region: (a) the number of genera (GR); (b) the expected number of genera (E(G15)); (c) the Shannon-Wiener index (H'loge); and (d) evenness (J'). Open circles indicated the stations where the number of identified adults was less than 10.

Fig. 4. The nMDS ordination based on the genus composition in the Kuril and Ryukyu regions.

Fig. 5. Dissimilarity values in harpacticoid community structures between the Kuril and Ryukyu regions.

Fig. 6. Vertical profiles of the meiofaunal density on the landward slope of the Japan Trench off Sanriku in the summer of 2011.

本論文は6章からなり、第1章はgeneral introduction、第2章は海溝域における生物多様性の深度変化、第3章は群集構造の深度変化、第4章は分類レベルの違いが多様性研究に及ぼす影響、第5章は大規模撹乱の深海底生生物群集への影響について述べられており、第6章はgeneral discussionである。

深海底は地球上で、最も生物多様性が高い場所のひとつである事が知られているが、これまでの研究は水深6000m以浅の陸側斜面までを対象としており、海溝域(超深海)や海溝軸の海側に広がる斜面や大洋底を含めた生態学的研究は極めて限られていた。また既往研究の多くは比較的大型の動物群を対象としてきた。論文提出者はメイオベントス(体長1mm以下の小型底生生物)の優占的分類群のひとつであるソコミジンコ類に注目し、日本列島に隣接する栄養環境が大きく異なる2つの海溝、海洋表層における生産性が高い千島海溝と生産性の低い琉球海溝の周辺に棲息するソコミジンコ類の生物多様性や群集構造の空間的な変化を詳細に解析した。また2011年の東日本大震災に伴う環境撹乱が深海生態系に及ぼした影響の調査をおこなった。

第2章では、千島海溝と琉球海溝の大陸斜面から海溝底、海側斜面、大洋底に至る計32ヶ所で定量採集したソコミジンコ類を対象に属レベルでの分類をおこない、多様性の深度変化を解析した。その結果、千島海溝では中程度の水深で多様性が最大になるのに対して、琉球海溝では多様性が水深と共に単調に減少する事が示された。またDISTML解析により琉球海溝では栄養供給が多様性に影響しているのに対して、千島海溝では多くの要因が多様性に影響している事が示された。こうした両海域の違いは、栄養条件の違いによるものと結論づけられた。第3章では第2章で得られたデータに基づいてソコミジンコ群集の構造の空間変異を検討した。どちらの海域でも群集構造が連続的に変化している事が示されたが、陸側斜面の群集と海溝底の群集の間の非類似度は千島海溝でより高かった。DISTLM解析により、どちらの海域でも水深または水深と関連する要因が群集構造に影響している事が示された。また2つの海域間の同じ水深帯の群集間の非類似度が大陸斜面に比べて、海溝底や大洋底で高くなる事が示された。その原因として、伊豆小笠原の島弧海溝系がソコミジンコ類の分散障壁として働いている可能性が指摘された。第4章では、前の2章の結果と科レベルの分類に基づく解析結果を比較し、多様性の深度変化については同等の結果が得られたのに対し、群集構造の空間変異に関しては必ずしも同じ結果にはならず、科レベルのデータでは不十分である事が示された。第5章では、2011年夏に三陸沖深海底でメイオベントスの定量採集をおこない、震災前のデータとの比較をおこなった。その結果、震災前後で生物量の深度変化パターンには変化が見られなかったが、堆積物中の垂直分布パターンが変化しているのを発見した。

本博士論文は、海溝域を海側斜面やその先の大洋底を含めたシステムと捉え、栄養環境の大きく異なる2つの海溝を対象に、底生生物の生物多様性や群集構造を定量的に解析した世界で初めての研究を報告したものである。最近、地球規模の気候変動が深海生態系にも強い影響を及ぼしている証拠が見つかっており、地球温暖化等の人間活動に起因する環境変動が深海域の生物多様性を減少させる可能性が懸念されている。本研究はそうした環境変動の影響を予測する上で貴重な基礎的知見となるものである。また東日本大震災の半年後に震源付近の深海底に棲む底生生物の定量調査をおこない、初の定量的なデータを提出しており、今後の同海域における経年的な調査の出発点として重要な貢献となるものと評価できる。

なお本論文第2~4章は川村喜一郎、小島茂明、嶋永元裕、Gritta Veit-Kohler、Roberto Danovaro、John Tietjen、第5章は小島茂明、嶋永元裕、Robert G. Jenkins、野牧秀隆、藤倉克則との共同研究であるが、論文提出者が主体となって分析及び検証を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(環境学)の学位を授与できると認める。