日本沿岸内湾性介形虫類の特性 : 古環境解析の指標として  [in Japanese] Characteristics of the inner bay ostracodes around the Japanese islands : The use of ostracodes to reconstruct paleoenvironments  [in Japanese]

日本列島の沿岸, 特に内湾域における介形虫群集の実体については, 最近かなり詳しく把握できるようになってきた。その中で次の3種, Bicornucythere bisanensis (Okubo, 1975), Cytheromorpha acupunctata (Brady, 1880), Spinileberis quadriaculeata (Brady, 1880)は各地の内湾域で特徴種として非常に高い優占度をもって生息している。これら3種のいずれかの種が優占する群集では, その優占種が群集内に占める割合と種の多様度(H'=-ΣPi ln Pi)との間に一定の関係が成り立つことが知られた。すなわち, 多様性が低い群集ほど3種が群集内で優占する割合が高い。逆に3種の個体数の合計が群集内で50%以下の場合は, 多様度との相関が低いことが示された。さらに, これらの3種は各海域によって, また, 同一海域内においても, それぞれの環境条件(水深, 塩分濃度, 底質など)によってその優占順位が異なることも知られた。3種のそれぞれが優占する地域で環境要素との関係を調べた結果, C.a.は水深2-3m, 塩分濃度27-30‰, 粒度2-3φのごく限られた砂底に最もよく適応しているのに対して, 他の2種は水深をやや異にする(S.q.は水深2-7m, B.b.は水深5-10m)他は, 塩分濃度20-30‰, 粒度5-8φの類似した泥底に適応していることが明らかになった。内湾底質表層部の堆積物を詳しく調べると共に, 殻および付属肢の形態と飼育によるVTR観察から3種の底質中での行動様式を次のように明らかにした。1) B.b.は淘汰度の悪いシルト質の軟泥中で殻を大きく開き, 胸肢を横方向に長く伸ばして生息する。2) S.q.は比較的淘汰度の良いシルト質の軟泥中で平らな殻の腹部を堆積物に乗せて, 堆積物上を滑るように移動している。3) C.a.は細粒ないし中粒砂の間隙中を殻を回転させながら, また鋭い爪を使って固い堆積物上を這うようにして生息している。これらの介形虫類の特性は多くの内湾環境をもつ完新世の古環境解析に応用できるであろう。

Recently there has been a gradual increase in our knowledge of ostracode faunas around the Japanese islands. Cytheromorpha acupunctata (Brady, 1880), Spinileberis quadriaculeata (Brady, 1880) and Bicornucythere bisanensis (Okubo, 1975) live in every inner bay environment, and dominate in the fauna. Low diversity faunas are nearly always dominated by these species. When anyone of these three species dominates the fauna, the relative proportion of the fauna made up by this species is inversely correlated with the species diversity (Shannon-Wiever's diversity index); conversely, in assemblages in which the sum proportion of the three species is less than 50%, this proportion is uncorrelated to diversity. The relative dominance of these three species might be due to environmental conditions such as substratum, water depth, salinity and others. The distribution of these three species could be related to salinity, substrate and water depth. C. acupunctata is more abundant on sandy bottoms (2-3 m in depth, 27-30‰ in salinity, 2-3φ in grain size), and S. quadriaculeata and B. bisanensis are more abundant on silty bottoms (S. q.: 2-7 m, 20-30‰, 5-8φ; B.b.: 5-10 m, 20-30‰, 5-8φ). The detailed structures around the sediment-water interface of inner bays were examined in relation to the habitat of the three species. 1) The surface sediments are divided into three parts, the flocculent layer, oxidized layer and reduced layer. 2) The vertical distribution of the three species are limited to the upper 0.5 cm of the surface sediments, which comprises the flocculent layer and the top of the oxidized layer. 3) The shape and surface ornamentation of the carapace and appendages of ostracodes were studied in relation to the sediment types. C. acupunctata is better suited to a sandy substrate due to its short thoracic legs with sharp claws and the rounded shape of its carapace, which enable it to move through the spaces between sand particles. However, S. quadriaculeata and B. bisanensis, which possess long thoracic legs and flexible claws, are better suited to a silty substrate. The former walk on well sorted sandy bottoms, attaching to the substrate with the flat ventral edge of their carapace pressed to the bottom. The latter walk through the flocculent layer above poorly sorted silty bottoms, attaching to the substrate by opening their carapaces widely and extending their thoracic limbs. Thus the dominance of each species in particular substrates is due to morphological adaptations of the leg and carapace and to behavioral adaptations for movement within the various substrates.