Makoto Terazaki

15N and 13C abundances in the Antartic Ocean with emphasis on the biogeochemical structure of the food web

Abstract Distributions of δ15N and δ13C for biogenic substances in the Antarctic Ocean were investigated to construct a biogeochemical framework for assessing the Antarctic ecosystem. Phytoplankton exhibited particularly low δ15N (0.5%) and 13C (-26.9%) values in pelagic plankton samples. High nitrate concentrations, and high PCO2 in the surface waters on the southern side of the polar front and the resulting slow growth rate of phytoplankton under low light intensity are suggested as possible factors in causing the low isotopic compositions. Mean fractionation factors of 1.029 and 1.006 were estimated for photosynthetic carbon fixation and for the assimilation of inorganic nitrogeneous compounds (ammonium plus nitrate) during algal growth, respectively. Enrichment of 15N with increasing trophic level was confirmed for Antarctic ecosystems: δ15Nanimal% = 3.3 (trophic level −1) + δ5Nalgae, whereas 13C content did not increase in the same manner. Differences in lipid content among animals may be the main factor in causing this δ13C anomaly. 15N and 13C abundance of sedimentary organic nitrogen differed from phytoplankton and settling particles. An exact mechanism for explaining the high δ15N (around 5%) is not known. The very high δ13C value of −20.5% at Sta. 3B may originate in ice algae that had grown under CO2-limited conditions. Particles collected by sediment traps gave characteristically low δ15N values (−3.0 to 0.9%), strongly suggesting a phytoplankton origin. The δ15N and δ13C values of settling material showed similar vertical profiles with depth which might arise from temporal variation of algal growth.

On the Polyps of the Common Jellyfish Aurelia aurita in Kagoshima Bay

There is the natural habitat of polyps of the common jellyfish Aurelia aurita in the Taniyama area, Kagoshima Bay. We examined the attachment substrata, density, colony structure and strobilation of the polyps. The polyps were observed only on the horizontal undersurface of floating piers. They attached specifically to Mytilus shells, solitary ascidians, calcareous polychaete tubes, muddy amphipod tubes and the gap space that fouling animals peeled off the substrata. The polyp colonies were distributed in patches. Spatial distribution patterns of the polyps within their colonies were uniform. Strobilation occurred during late December to March, when water temperatures were 16–17°C, and a large number of ephyrae were released. An increase in man-made structures such as floating piers in coastal areas may lead to bloomings of Aurelia aurita medusae.

Distribution of salps near the South Shetland Islands during austral summer, 1990–1991 with special reference to krill distribution

Distribution and biomass of salps and Antarctic krill (Euphausia superba) were investigated near the South Shetland Islands during austral summer 1990–1991. Salp biomass ranged between 0 and 556 mgC·m−3 and was greatest at a station in the Bransfield Strait in late December 1990. Salp biomass was lower than that of E. superba. Two species of salps; Salpa thompsoni and Ihlea racovitzai were found, and the former was dominant numerically. Spatial distribution and generation composition of these two species was different. Spatial distributions of salps and E. superba did not overlap particularly so the January–February period. While E. superba was found mainly in the coastal area which showed high-chlorophyll a values, salps exhibited high biomass in the oceanic area with low chlorophyll a concentrations. Predation by salps on small krill and the competitive removal of food by them, are discussed as potential reasons for the relatively low abundance of E. superba at the stations where salps were present in great numbers.

Life history and vertical distribution of pelagic chaetognaths at Ocean Station P in the subarctic Pacific

Seven species of two genera of pelagic chaetognaths (Eukrohnia hamata, E. bathypelagica, E. fowleri, Sagitta elegans, S. macrocephala, S. scrippsae. and S. zetsios) were collected at Ocean Station P (50°N, 145°W) from February 1980 to January 1981. The most abundant species, E. hamata, E. bathypelagica, E. fowleri, and S. elegans comprised 71.0, 18.0, 7.2, and 3.8%, respectively, of the chaetognaths in the upper 2000 m. S. elegans inhabits the epipelagic layer (0 to 200 m), E. hamata the epipelagic and upper mesopelagic layers, E. bathypelagica the mesopelagic layer (200 to 1000 m), S. macrocephala the lower mesopelagic and bathypelagic layers (500 to 2000 m), and E. fowleri the layer below 1000 m. E. hamata and S. elegans both had three spawning periods during the year, separated by distinct gaps in both presence of mature individuals and appearance of new juveniles. Generation lengths were 8 to 10 months for E. hamata and 6 to 10 months for S. elegans. Factors that produce the overlapping, two-thirds year generations remain to be demonstrated. Mean growth rates determined by following their cohorts were 2.5 to 3.0 mm per month for E. hamata and 5 to 6 mm per month for S. elegans. Breeding and development were continuous through the year for E. bathypelagica and E. fowleri.

Feeding behavior of the poecilostomatoid copepods Oncaea spp. on chaetognaths

Feeding behaviors of the poecilostomatoid copepods Oncaea venusta, O. mediterranea and O. conifera were examined in the coastal area of Cheju Island, south of the Korean Peninsula, from April 1993 to November 1995. Late copepodid stages (CV, adult female and male) showed a high frequency of association with larger zooplankton such as Sagitta spp. (Chaetognatha), Oikopleura spp. (Appendicularia), and Salpa spp. (Thaliacea) in a coastal upwelling zone and around the small Munsum Island. Attack behaviors on Sagitta observed under the microscope and in the field showed that Oncaea approached Sagitta using their swimming legs, and then crawled chiefly to the neck regions and caudal septum (around the tail) of Sagitta spp. using their second antennae. They pierced the body of Sagitta with their sharp maxillipeds, and moved their maxillae and mandibles repeatedly. Midgut contents of Oncaea spp. consisted mainly of unidentified fine particles, and the greater part of the gut was empty. Their mouthpart structure is remarkably different from other pelagic copepods. Such differences of the mouthpart structures and the feeding habits in the genus Oncaea around Cheju Island are discussed in terms of feeding behavior and ecology in this coastal upwelling area.

Eradication of predatory fishes in shrimp farms by utilization of Thai tea seed

Abstract The chief predatory fishes found in shrimp culture ponds in Thailand are Tilapia mossambica, Lates calcarifer, Scatophagus argus, Eluetheronema tetradactylum, Goviupterus chuno, Mystus sp., and other gobies. The eradication of predators, mostly fishes, is necessary for good management of any shrimp farm. Tea seed grown in northern Thailand has been used for this purpose. Quantitative analysis of crude saponin in the tea seed was conducted and some biological tests on the toxicity of crude saponin to fish were performed. The effective dosage of crude saponin for the eradication of predatory fishes was 1.1 ppm, but shrimp, crabs, copepoda, rotifers and brine shrimp (Crustacea) all survived this concentration. The lethal time for fishes increased in proportion to their body weight and the salinity of the pond. The toxicity of the saponin weakened with time.

Horizontal and vertical distribution of chaetognaths in a Kuroshio warm-core ring

Abstract The horizontal and vertical distribution and diel migration of chaetognaths were investigated in the Kuroshio warm-core ring (KWCR) and adjacent waters during two cruises in August and September 1987. Four genera, with a total of 19 species, were identified from the KWCR and adjacent waters. The chaetognaths were most abundant in the marginal areas including fronts, and decreased rapidly from outside toward the center; four water types characterized by different species assemblages, were recognized. The horizontal distribution of epipelagic species was limited by fronts between the KWCR and the Oyashio water or a warm streamer. Sagitta scrippsae , neritic species and warm-water species are carried from the marginal areas to the central core water by the clockwise flow of the surface layer above the seasonal thermocline. Sagitta elegans and Eukrolmia hamata reach the epipelagic layer of the central water by vertical migration from the intermediate water existing under the core water. Warm-water species, S. elegans and E. hamata cannot survive in the central core water because they cannot adapt to the changes in temperature and salinity. S. scrippsae, S. minima and S. nagae are suited for living in the KWCR, but their normal breeding is controlled by food conditions.

Diet of calyptopes of the euphausiidEuphausia pacifica in the Yellow Sea

Foregut contents of calyptopes (the first feeding stage) ofEuphausia pacifica Hansen, collected from the Yellow Sea in June 1989, were examined using scanning electron microscopy with the dry-fracturing method. There was no clear evidence of phytoplankton ingestion by calyptopes. The foregut contents consisted entirely of amorphous organic detritus, crustacean remains and inorganic particles. The inorganic particles adhered to amorphous organic detritus, and bacterial populations were found on peritrophic membranes of fecal pellets which enveloped crustacean remains. These materials seemed to have originated from aggregates of organic and inorganic detritus. It is quite possible that the detrital aggregates are utilized extensively as food by calyptopes ofE. pacifica in the Yellow Sea.

Some aspects of the life history of the subarctic copepodNeocalanus cristatus (Calanoida) in Sagami Bay, central Japan

A typical subarctic copepod,Neocalanus cristatus, occurred in the mesopelagic layer (500 to 1000 m) in Sagami Bay, central Japan, throughout the year. Specimens were collected from 1982 to 1986. A small number of adult females were distributed from 800 to 900 m only, but no adult males were collected. This species appeared to be abundant in April and August, when intermediate Oyashio water flowed strongly into Sagami Bay. Mean prosome lengths of copepodite stages IV and V and adults were 4.33, 6.87 and 6.87 mm, respectively. The condition factor [wet wt/(prosome length)3 × 100] of copepodite stage V did not vary remarkably, and mean values ranged from 4.7 to 5.0. Prosome length, body weight and condition factor ofN. cristatus collected from Sagami Bay were smaller than those of copepods in the northern North Pacific.N. cristatus transported from the north cannot molt to adult stages (except for those originating in mesopelagic waters) due to the adverse environmental conditions in Sagami Bay; instead, they die in the mesopelagic layer and sink to the bathypelagic layer (1 000 to 1 400 m), close to the bottom. Since nauplii and early copepodite stages did not occur in any season,N. cristatus probably do not reproduce in Sagami Bay.

Feeding of Carnivorous Zooplankton, Chaetognaths in the Pacific

The phylum chaetognatha consists of some 100 species arranged in 22 genera (Bieri, 1991). Chaetognaths are found in every marine habitat, from the benthos to all zones of coastal waters and the open oceans. Although small (2–120 mm), they are often abundant and play an important role in the marine food web as the primary predators of copepods. The biomass of chaetognaths has been estimated as 10–30 per cent of that of copepods in the world oceans, so that they are of great significance and Grey (1930) also described a similar event. Parry (1944) provided the first report of feeding in the laboratory other than by chance observation. Though he found Sagitta setosa difficult to keep in the laboratory, he was able to report on the difference between day and night feeding.