Horizontal distribution and seasonal variability of the epipelagic chaetognath Sagitta elegans in relation to hydrography in the western subarctic Pacific Ocean

The spatial distribution and seasonal variability of epipelagic chaetognaths along the Kurile Islands and off south-east Hokkaido, in the western subarctic Pacific Ocean. were investigated during the period from May 1990 to October 1992. Sagitta elegans was the dominant species among the epipelagic chaetognaths in the study area. Juvenile S. elegans were distributed mainly along the path of the mixed water which was determined by the acceleration potential anomaly (APA) at the iso-pycnal density surface of 26.6 σθ. The location of the path of The mixed water meandered around cyclonic and anticyclonic eddies. Catches of S. elegans were made at temperatures ranging from 4.5 to 22.2C. The juvenile distribution, however, was mostly restricted to between 1 and 4C of the potential temperature determined at the isopycnal density surface of 26.6 σθ. Variability in the juvenile abundance within the path of the mixed water could be caused by predation rather than by food limitation because microzooplankton, which are known to he prey for juveniles, were abundant enough to meet the food requirements of juveniles in the study area. Adults occurred in spring (April-May) and young individuals (juvenile and stage 1) were abundant in summer (June-July), when a strong thermocline developed. The main spawning period appears to be during April-May, with a possible second spawning period in the autumn.     

Variability in timing and magnitude of spring bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan

The spring bloom of phytoplankton is a well-established, regular, seasonal event in the western subarctic Pacific and is considered one of the most important conditions of massive production of pelagic fishes. A series of 12 cruises was conducted from 1990 to 1992 to examine the timing and magnitude of the spring phytoplankton bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan. An interannual variability in the bloom events was also analysed. On the basis of hydrographical characteristics, the study area was divided into three water masses: the Oyashio Water Mass, the Mixed Water Mass, and the Coastal Water Mass. Spring blooms were observed first in April in the Oyashio and the Coastal Water Masses, and continued to May in 1991 and 1992. However, no bloom was recorded in the Mixed Water Mass. High nutrient supply into the surface mixed layer during winter is likely to be one of the factors supporting an intense spring bloom in the Oyashio Water Mass. A significant positive relationship between log-transformed surface chlorophyll a concentration and maximum density gradient (MDG) within the euphotic layer was obtained in April, indicating the importance of vertical stability of the water column in the initiation of spring blooms in the Oyashio and the Coastal Water Masses. The spring blooms in 1991 were much more extensive and lasted longer than in 1990. It is suggested that meteorological conditions and abundance of grazers were responsible for this interannual difference.     

Expression of pancreatic enzyme genes during the early larval stage of Japanese eel Anguilla japonica

ABSTRACT: To reveal the ontogeny of pancreatic exocrine function in the early larval stage of eel, cDNAs encoding major pancreatic enzymes, trypsinogen, amylase and lipase were identified from the Japanese eel Anguilla japonica and their expression pattern in larvae was analyzed. The cloned eel trypsinogen precursor consisted of 224 amino acids and showed 82.2% identity to trypsinogen-2 of winter flounder Pleuronectes americanus . The eel amylase precursor consisted of 512 amino acids and showed 77% identity to winter flounder amylase. Eel pancreatic lipase was composed of 470 amino acids and had 58.3% of identity to human pancreatic lipase. In the eel larvae, mRNA expression of trypsinogen and amylase was first detected at 6 days post-hatching (d.p.h.), and the expression level increased between 7 and 8 d.p.h. In contrast, mRNA expression of lipase was first detected at 8 d.p.h. Eel larvae start to feed actively at 8 d.p.h. Thus, it was indicated that eel pancreas starts to synthesize digestive enzymes at 6 d.p.h. and acquires full function by the onset of exogenous feeding at 8 d.p.h.     

Role of prostaglandin in the control of ovulation in the Japaneseeel Anguilla japonica

ABSTRACT:   The in vitro effects of 17,20β-dihydroxy-4-pregnen-3-one(DHP) and prostaglandins (PGE₁, PGE₂, PGF_1α,PGF_2α) on ovulation in the Japanese eel Anguillajaponica were examined. Oocytes with follicle layers at themigratory nucleus stage (approximately 850–900 µmdiameter) were removed using a polyethylene cannula from artificiallymatured fish. At concentrations of 10 and 100 ng/mL,DHP was found to induce both germinal vesicle breakdown and ovulation.The prostaglandins, except for PGE₁, effectively inducedovulation of previously matured oocytes by DHP treatment in vitro .Prostaglandin F_2α was the most effective. Asignificant increase in ovulation rate was observed even at a concentrationof 0.01 µg/mL PGF_2α.Indomethacin blocked the in vitro ovulation induced by DHPand addition of PGF_2α reversed indomethacin-blockedovulation. Actinomycin D and cycloheximide blocked DHP-induced ovulationand PGF_2α reversed the effects of both inhibitors. Theseresults indicate that DHP induces ovulation through endogenous prostaglandinsynthesis in the follicle layers of the Japanese eel.     

Changes in fertility of rainbow trout eggs retained in coelom

ABSTRACT:    Effects of prolonged retention time of ovulated eggs in the parental coelom on fertilization success were studied in rainbow trout Oncorhynchus mykiss using cryopreserved sperm with a uniform fertilizing ability. Proportions of successful fertilization, eyed eggs and hatched alevins were examined at different time periods up to a retention time of 14 days beyond the ordinary stripping time, and were compared with eggs incubated in artificial coelomic fluids (ACF). Eggs that were retained longer in the coelom showed gradual decreases in all the proportions of successful fertilization, eyed eggs and hatched alevins. The progress of cleavage after fertilization slowed with prolonged retention times. Eggs incubated in ACF lost their fertilizing ability much sooner than those retained in the coelom. The hatching rate of eggs retained for 2 weeks in coelom was 36%, while it was 1% in those eggs incubated for 4 days in ACF. Thus, eggs retained in the coelom showed higher fertilization success than those incubated in ACF.     

Hydrography and spatial variability in the size distribution of phytoplankton along the Kurile Islands in the western subarctic Pacific Ocean

Spatial variability in the size distribution of the phytoplankton community along the Kurile Islands, in the western subarctic Pacific Ocean, was investigated in terms of chlorophyll a concentrations in August and September, 1990. Analysis of dynamic height relative to 1,500 db and temperature–salinity diagrams at 50 m depth revealed five major oceanographic regions in this study area. The characteristic hydrography was clearly related to the surface distribution of chlorophyll a concentrations. As a warm core ring originating from the Kuroshio water was cooled by the surrounding water and meteorological conditions, its physical, chemical, and biological characteristics changed. Dominant picoplankton were replaced by a 10-2 μm size fraction in the warm core ring. In the Oyashio region, physical structures were less variable but concentrations of chlorophyll a were most variable and cells larger than 10 μn dominated. An anticlockwise eddy found in the Oyashio water showed the highest concentration of chlorophyll a. In the Okhotsk region, the chlorophyll standing stock was small with a low percentage of cells larger than 10 μm in spite of high nitrate concentrations. Along the Kurile Islands and Hokkaido Island, the surface mixed layer was not well defined; surface nitrate concentrations were relatively high, and the chlorophyll standing crop was also high with low percentages of picoplankton. The spatial heterogeneity in the dominance of cells larger than 10 μm in phytoplankton assemblages seems to be clearly associated with the effect of nitrate availability determined by water structure.