Recent Ostracoda from the Northeastern part of Osaka Bay, Southwestern Japan

Abstract

Ostracodes from Osaka Bay, southwestern Japan, were quantitatively studied in order to examine the relationships between environmental factors and species distribution. At least 109 species belonging to 43 genera were identified from 31 surface sediment samples. On the basis of Q-mode cluster analysis, four biotopes (A, B, C and D) are recognized. Characteristic species of each biotope are as follows: A (central bay at 17.2-37.2 m water depth), Trachyleberis scabrocuneata, Cytheropteron miurense, Kobayashiina donghaiensis, Schizocythere kishinouyei and Krithe japonica; B (inner part of the bay at 16.3-21. 1 m water depth), Bicornucythere bisanensis (form A), Bicornucythere bisanensis (form M), Cytheromorpha acupunctata, Loxoconcha viva and Nipponocythere bicarinata; C (innermost part of the bay at 9.2-15.9 m water depth), B. bisanensis (form A), Loxoconcha tosaensis and Spinileberis quadriaculeata; D (inner part of the bay at 12.6 m water depth), C. acupunctata. These results suggest that species distribution is controlled by bathymetry. High abundance and high species diversities are found in the westernmost area. In contrast, low abundance and low species diversities are observed in the eastern area, where water masses with low dissolved oxygen content spread, especially in the summer. It is shown that the tidal current and the dissolved oxygen contents strongly influence abundance and species diversity of ostracodes in Osaka Bay.