Takahisa Nemoto

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.

Comparative Morphology of the Mouthparts and Foregut of the Final-Stage Phyllosoma, Puerulus, and Postpuerulus of the Rock Lobster Jasus Edwardsii (Decapoda: Palinuridae)

ABSTRACT The gross morphology and fine structure of the mouthparts and foregut of the rock lobster Jasus edwardsii were studied in its late larval and early juvenile stages. The stage-XI phyllosoma has third maxillipeds with specialized setae, second maxillipeds and first maxillae with strong setae, mandibles with well-developed molar and canine-like processes and teeth, and a foregut with well-developed grooves and brushes but no gastric teeth. The first-molt juvenile has third and second maxillipeds and first maxillae with rows or tufts of numerous strong setae, wellcalcified mandibles with strong molar and incisor processes, and a foregut with well-developed ossicles, grooves, brushes, and strong gastric teeth. In contrast, the puerulus has maxillipeds and maxillae with only small numbers of setae, uncalcified mandibles with a rudimentary incisor, and a foregut with a small number of ossicles and setae, and rudimentary lateral teeth. These observations suggest that the late-stage phyllosoma of J. edwardsii feeds primarily on large, soft materials and the early postpuerulus on hard, as well as soft, materials. In contrast, the puerulus probably does not feed during some or all of its existence, and any feeding is on small, soft materials. Use of nutrients stored in the digestive gland is one possible way that the puerulus could meet its energy requirements.

Poison arrowworms: a tetrodotoxin venom in the marine phylum Chaetognatha

Abstract We have conclusively detected a neurotoxin which blocks the sodium channel of cell membranes in extracts of the heads of six diverse chaetognath species through the use of a new bioassay method utilizing mouse neuroblastoma tissue culture. Analysis by gas chromatography—mass spectrometry has revealed that the major bioactive component in chaetognath venom is tetrodotoxin or a tetrodotoxin analogue. The existence of a venom which contains tetrodotoxin is previously known from only one other animal and is produced by symbiotic bacteria. Although the source of tetrodotoxin in chaetognaths remains unknown, similarities in the vestibular morphology between different chaetognath species further suggest that the majority of the phylum Chaetognatha may be capable of subduing prey with a tetrodotoxin venom.

Distribution and growth of salps in a Kuroshio warm-core ring during summer 1987

Abstract A salp bloom, accounting for 47% of the macrozooplankton wet weight in the upper 200 m, was observed in a Kuroshio warm-core ring and adjacent areas during September 1987. Although salps had wide distribution and high biomass in the ring and adjacent southern areas, they did not occur north of the northern ring front. Thalia democratica dominated in these areas and Salpa fusiformis was abundant at some stations. Salps were distributed only in the upper 200 m of the water column. The maximum abundance of T. democratica was in the surface mixed layer, 0–20 m. S. fusiformis was most abundant from 50 to 75 m. Diel vertical migration was observed only for solitary zooids of S. fusiformis. All other salps appeared only on the surface. The growth of aggregate zooids of T. democratica was investigated with the time-series sampling during a 28-h sampling period following a drifter. Several cohorts were identified in the length-frequency distributions. The average relative growth rate in length was 8.0% per hour. Carbon consumption by the T. democratica population, calculated from the derived growth rate, suggested that T. democratica was a major consumer of the primary production in the ring.

Summer phytoplankton community structure and growth in a regional upwelling area off Hachijo Island, Japan

Abstract Phytoplankton community structure both in terms of taxonomic composition and size distribution, and growth were examined in a regional upwelling which occurred in a Kuroshio region in summer. Organic carbon abundance of diatoms was markedly enhanced by the upwelling and they accounted for 87% of the total phytoplankton carbon. Nitzschia pungens Grunow was numerically most prominent accompanied by Chaetoceros and Rhizosolenia species. On the other hand, only a small increase was observed in phytoplankton other than diatoms, which consisted of unicellular cyanobacteria, coccolithophorids, gymnodinoids, and so called monads and flagellates. Division rate of diatoms from the upwelled water was twice that of the other forms. This difference in growth response among phytoplankton groups resulted in a shift of community structure to dominance of diatoms from that of the other forms. The time needed for this shift was estimated to be at least ≈ 3 days.

The food of some Aantarctic fish in the western Ross Sea in summer 1979

SummaryThe food of fish in the western area of the Ross Sea was studied. Four locations were sampled using a bottom trawl. Twenty-four species of fish belonging to six families were caught. Pleuragramma antarcticum was the most important species among them and Chionodraco myersi was also abundant. The food chain, starting from small crustaceans such as Parathemisto gaudichaudii, copepods and Euphausia crystallorophias and extending through P. antarcticum to channichthyids is considered the most important chain in the food web of the fish community of the western Ross Sea in summer. The significance of the ecological role of P. antarcticum in the food web of the fish community of this area where the krill, Euphausia superba, is scarce is discussed.

Aspects of the Roles of Squid in Food Chains of Marine Antarctic Ecosystems

Squid (Cephalopoda) are important organisms in marine food chains. However, there are very few studies of their ecological roles in marine ecosystems of the Antarctic. Seventy-six specimens of squid, both large (ca 355 mm in mantle length) and small (ca 40 mm), have been collected by large mid-water trawl operations of the Japanese pilot fishery for Antarctic Krill, Euphausia superba, in the Antarctic during 1980–81 and 1981–82. The squid collected are all considered to be young juveniles distributed at shallower depths than adults. Two species, Kondakovia longimana and Moroteuthis knipovitchi, are dominant. Alluroteuthis antarcticus, Pholidoteuthis boschmai, Brachioteu this picta and Galiteuthis glacialis also occurred in the collections. These squid consume various kinds of food, but they feed mainly on macro-zooplankton and micro-nekton. Krill is the main food item of squid in the Antarctic. An amphipod, Themisto gaudichaudii, is also a major component of the food of Kondakovia longimana. Small euphausiids, Thysanoessa macrura, and large chaetognaths, Sagitta gazellae, are also found in the stomachs of K. longimana, which indicates that the species is a plankton feeder. Fish are considered to be a major food item for Moroteuthis knipovitchi, together with E. superba. Cannibalism appears to be rather common in these squid. Predation by deep-diving Sperm Whales, Physeter catodon, on squid larger than 400 mm in mantle length was observed, and this, associated with the fact that such large squid were not caught in mid-water trawls, suggests that squid are segregated into different water depths according to size and maturity in the Antarctic.

The diet of mesopelagic fish from the Pacific coast of Hokkaido, Japan

The diet of at least 28 species of mesopelagic fish from the Pacific coast of Hokkaido was examined. The dominant family was the Gonostomatidae (42%) which was represented by five species. The most abundant species wasCyclothone atraria which together with the other species of this genus preyed predominantly on copepods. Euphausiids and copepods were dominant in the diet ofGonostoma gracile. The next most abundant family was the Myctophidae (32%) which was represented by seven species. The dominant species,Stenobrachius nannochir, preyed mainly on copepods. Copepods were also the dominant food item of the other myctophids except forLampanyctus jordani which fed mainly on euphausiids. The other important family was the Bathylagidae (21%).Leuroglossus schmidti was the dominant species and its diet was more diverse with ostracods, copepods, molluscs and larvaceans being the most important food items.Bathylagus ochotensis had a similar diet. Copepods were the most important food items for all but a few species and their occurrence in the fish stomachs was related to the known vertical distribution of both predators and prey. Ostracods and euphausiids were also important prey items, the latter especially in large fish species. Molluscs and larvaceans were restricted to the two species of the family Bathylagidae.

Scanning electron microscopy investigation of bacterial colonization of the marine copepod Acartia clausi

Association between copepods and bacteria was observed in many scanning electron micrographs. Particular sites on the copepods were selectively colonized by bacteria; the joints of segments and legs, swimming legs and depressed parts of the body surface were found to be densely covered with bacteria. In comparison, bacterial attachment to copepod skeletons in fecal pellets excreted by chaetognaths was not selective; bacteria were sparsely found all over the copepod. Between 9 to 30% of copepods in Tokyo Bay waters had attached bacteria in January and April 1983.

Phytoplankton in the subtropical convergence during the austral summer: community structure and growth activity

Two kinds of phytoplankton communities were identified at the Subtropical Convergence (STC) at 150°E during the austral summer of 1983–1984: a sub-Antartic diatom-dominated population (>90% in terms of carbon abundance) and a northern population with less diatom dominance (about 50%). Spatial distribution of these populations were closely associated with the hydrography. The mixing of the two occured at the surface in a horizontal scale of <16 km. The northern warm and saline water mass intruded southward into the subsurface layer (30–70 m), and the colder and less saline sub-Antarctic water overlay it, producing a temperature inversion and an associated shallower pycnocline at the central part of the STC. The inversion confined the sub-Antarctic population to the shallower mixed layer which was rich in nitrate and phosphate, resulting in increased chlorophyll a and primary production..