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We discussed the branching and joining of the Tsushima Current around the Oki Islands, based on ADCP and CTD measurements carried out in June 1990 by the quadrireciprocal method (Katoh, 1988). The volume transport of the northeastward current northwest of the Izumo Coast was about 2 Sv. The triple-branch structure of the Tsushima Current was obscure there. This northeastward current divided into the eastward and northward currents, with volume transports of 0.5 Sv and 1.5 Sv, respectively, at the west entrance of the Oki Strait. Most of the first branch of the Tsushima Current seemed to be separated again from the other confluent branches and to pass through the Oki Strait as this eastward current. The northward current was composed of the second and the third branches of the Tsushima Current. It detoured the Oki Islands, and almost all of it returned south to the Tajima Coast. In the vicinity of the Tajima Coast, the eastward current was abruptly strengthened through the confluence of the southward one which was originated from the northward current west of the Oki Islands. This showed that the first branch finally joined the compound of the second and the third branches detouring the Oki Islands. Between the Oki Strait and the Tajima Coast, the two-layer structure of currents was clearly found.

In order to clarify detailed current structures west of Kyushu, ADCP measurements were carried out in July and September 1990 by the quadrireciprocal method (Katoh, 1988) for removing diurnal and semidiurnal tidal flows from observed flows. On the basis of these results, together with data of routine oceanographic observations, we study the downstream transition of the Tsushima Current west of Kyushu in summer. In the southwest of the Goto Islands, a northward current identified as the Tsushima Current was clearly found. In the south of Cheju, a westward current bifurcated from the Tsushima Current. In the northwest of the Goto Islands, the Tsushima Current narrowed and its velocity became strengthened. Salinity of the Tsushima Current water was much diluted by a current from the Cheju Strait. Near the west coast of the Goto Islands, a countercurrent bifurcating from the Tsushima Current often occurred. The volume transport of the Tsushima Current was 2.3 Sv (1 Sv=106 m3s−1) on the northern side of latitude 31°N. The substantial bifurcation of the Tsushima Current toward the Eastern and Western Channels of the Tsushima Strait occurred in the vicinity of Tsushima. The volume transport through the Western Channel was two to three times larger than that through the Eastern Channel. The baroclinic component in volume transport of the Tsushima Current west of Kyushu was much smaller than that in the Japan Sea.

Splenic white pulp of the Aleutian skate (Bathyraja aleutica), an elasmobranch, was investigated using light and transmission electron microscopy. The major cellular constituent was plasma cells, of the typical Marshalko type, characterized by well developed rough-endoplasmic reticulum and a Golgi complex. The morphology of the rough-endoplasmic reticulum was variable, being lamellar in some cells and spherical in others. Plasma cells with distended cisternae of rough-endoplasmic reticulum and cells with Russel bodies were often observed. Only a small number of lymphocytes were encountered. These findings indicate that the splenic white pulp is the major site for immunoglobulin production in this fish, thus confirming our previous immunocytochemical observation. Globules presumably containing immunoglobulin were found consistently associated with the Golgi complex. The secretion mechanism of immunoglobulin by plasma cells is discussed in connection with the globules.

Maturity condition of pelagic walleye pollock Theragra chalcogramma in the winter Aleutian Basin was examined from GSI (gonadosomatic index) changing trend and observations of gonads collected through midwater trawl samplings conducted during December, 1988 to February, 1989. Throughout the whole survey period female GSI values increased gradually and rapidly reached the highest level around late February while those of males remained unchanged. Spawning female and spermatozoa-releasing male pollock occurred during late February in the southeastern waters of the Aleutian Basin. Assuming that pollock migrate eastward in the Aleutian Basin during winter and spawn in the southeastern Aleutian Basin, spawning was suggested to begin late in February in 1989.

We identified the presence in the Aleutian skate, Bathyraja aleutica, of two classes of immunoglobulins (Ig), a high molecular weight Ig analogous to mammalian IgM and a low molecular weight Ig, similarly to the spiny rasp skate, Raja kenojei, (Kobayashi, K. et al., Mol. Immunol. 1984. 21: 397), using an immunological cross-reaction with the specific antisera to the spiny rasp skate Ig components. The antigenic similarity of the heavy chains of the Ig of the Aleutian skate and those of the spiny rasp skate was less than that between their light chains. Two types of Ig-producing cells, one producing the high molecular weight and the other forming the low molecular weight Ig, were present in the spleen of embryos and adults in the Aleutian skate at a ratio of 5-6:4-5. The number of these Ig-producing cells increased with advancing development of the embryos but was 1/20 to 1/50 of those of adults. Cells, each of which were capable of forming both classes of Ig, were found in the spleen of embryos but not in that of adults. These results suggested that the spleen of the Aleutian skate is the primary lymphoid organ for B lymphocyte differentiation and proliferation, possibly equivalent to the bursa of birds.

Organogenesis in embryos (1 and 5 mm-disk widths) of the Aleutian skate,Bathyraja aleutica was described histologicaily in complete serial paraffin sections. Special attention was paid to localization of primordial germ cells (PGC) and development of lymphoid tissues. The embryo of 1 mm-disk width was at the somite stage with no evidence of organogenesis. PGC, gathered in the genital ridge, were seen in the somatic mesodermal layer as well as in the mesenchyme between the endoderm and splanchnic mesoderm of the 1 mm-embryo. Most of the visceral organs were developing in the embryo of 5 mm-disk width. With respect to the development of immune system, two pairs of thymus anlagen, filled with numerous thymic lymphocytes, were recognized in the pharyngeal region. A small focus of numerous immature blood cells appeared to be an anlage of the spleen was found between the stomach and the liver.

The structure and function of epithelial cells of the ductus vitellointestinalis in the Mustelus griseus embryo were studied through light and electron microscopic observations. The ductus vitellointestinalis is contained within the yolk stalk, which connects an embryo with the yolk sac, The internal surface of the ductus vitellointestinalis is a simple epithelium lining, and consists mainly of ciliated squamous cells and columnar cells with microvilli. Structural characteristics of the cells suggest that the ciliated squamous cellsmay aid in the transportation of yolk and that the columnar cells with microvilli may function as absorptive cells. It may be considered, therefore, that the ductus vitellointestinalis in the M. griseus embryo is engaged not only in the transportation of yolk, but also in the absorption of yolk being conveyed into the intestine.

Morphology of intestinal epithelial cells in the freshwater elasmobranch Potamotrygon magdalenae was studied by electron microscopy. Columnar cells lining the internal surface of the intestine as a single layer in P. magdalenae are characterized by microvilli densely arranged on the free surface, intermicrovillous invaginations, numerous vesicles beneath the microvilli and granules of varying sizes. These morphological characteristics suggest that columnar cells with microvilii function as absorptive cells. Interdigitations of the lateral plasma membranes, which are not found in teleosts, occur at junctions of adjacent cells in the P. magdalenae intestine. It is shown that these interdigitations exist not only in marine, but also in freshwater elasmobranchs.

The urinary bladder of the Pacific cod,Gadus macrocephalus was investigated. A pair of ureters was found to unite and form a sac-like protrusion on the posteroventral aspect of the gonad. This protrusion was confirmed as the urinary bladder based on its histology and the presence of ammonia and urea in the fluid within this structure. The urinary bladder consisted of a central lumen and two lateral expansions found on the right and left sections. The luminal epithelial cells of the Pacific cod urinary bladder, unlike those of other animals, were characterized by microvilli on the free surface, and the presence of a number of vesicles and mitochondria across the apical portion on the cells. This suggests that the luminal epithelium of the urinary bladder might be actively engaged in transportation of water or other materials from the urine. The Pacific cod urinary bladder may therefore be closely associated with osmoregulation by reabsorbing water from the urine. Ammonia-N and urea-N levels of 65–213 μg/dl and 1.5–3.5 mg/dl were measured in the fluid which filled the urinary bladder.

In order to estimate the size at maturity of Pacific cod Gadus macrocephalus, daily changes in the gonadosomatic index (GSI) were examined for both male and female cod by size class during the six-month period from July to December. Gonads used for the present study were taken from 1, 899 individuals of cod sampled during Japan-U. S. cooperative groundfish surveys conducted in the Bering Sea in 1981 and 1982. Based on the data in November and December when GSI in both sexes showed a rapid increase, the percentage of maturing population by fork length was obtained. The result indicates that 50 percent maturity was reached at about 60cm for male and 62cm for female cod from the eastern Bering Sea.