Chaetognath ecology in relation to hydrographic conditions in the Australian sector of the Antarctic Ocean

Chaetognath ecology in relation to hydrographic conditions was investigated during austral summer in the Antarctic Ocean with two cruises conducted seven years apart. Time series samples were collected at a total of five stations along 140_E in the region of the Antarctic Divergence (AD). The numerically dominant species was Eukrohnia hamata, averaging over 80% of the collected specimens. This species was most abundant north of the AD where bi-modal distribution was common. No species displayed regular diel vertical migration nor were their vertical distribution restricted by hydrographic clines. Breeding of E. hamata appeared unaffected by water conditions-the same maturity stages were present in similar percentages across the water masses. Vertical segregation of size classes among E. hamata was clearest north of the AD in a more stratified water column. In other locations, the size classes overlapped considerably; this is likely due to physical forcing from mixing water. Feeding activity of E. hamata was lowest within the AD and increased both north and south on the edges of the front. Feeding activity appeared continuous with no recognizable diel pattern. E. hamata containing prey items were smaller north of the AD compared to the south, suggesting two separate populations. The percentage of predators with food in their gut was much larger in this study than previously reported work. Often times nearly 50% of the E. hamata collected had highly digested contents in the posterior of the gut

Life history strategy of the chaetognath Sagitta elegans in the World Oceans

Sagitta elegans is the best-studied chaetognatha in the world ocean. This species typically inhabits the upper 100 to 150m in the arctic and subarctic area of both the Atlantic and Pacific. S. elegans was also reported from the North Pacific Intermediate Water and the meso- and bathypleagic layer in the Sea of Japan. Diurnal vertical migration is recognized in the various waters. The size and number of grasping spines and teeth of S. elegans in the Sea of Japan are greater than those from the Pacific. They also have developed intestinal tissue containing oil droplets similar to meso- and bathypelagic species. These features are adaptations to the deep-water environment. Number of generations per year, life span and growth rate depend on the waters inhabited. In general, S. elegans mature at large sizes in lower temperatures and life span is short in warm regions. Food is also an important factor controlling their growth. Feeding activity of S. elegans in the various waters of the Pacific is high during the night time. The major food organisms are copepods. S. elegans inhabiting the open sea consume larger copepods compared to those in coastal waters. The percentage of secondary production consumed daily by S. elegans, was 36% in Bedford Basin, Nova Scotia, 10% in the Bering Sea and 4% in the western North Pacific, respectively

Community structure and vertical distribution of chaetognaths in the Celebes and Sulu Seas

Chaetognath community structure, vertical distribution and migration patterns were investigated in two marginal basins-the Celebes and Sulu Seas. The two seas are part of an island arc located in the southeastern Asia region of the western equatorial Pacific Ocean. The Sulu Sea had higher species diversity (Shannon Index, H\u27) and higher overall chaetognath abundance than the Celebes Sea. Twenty-two species from 4 genera were collected from the Sulu Sea and 19 species from 4 genera were collected from the Celebes Sea. Three species were collected exclusively in the Sulu Sea: Sagitta bipunctata, Sagitta nagae and an unidentified Sagitta species. Sagitta enflata was the numerically dominant species at both locations. Species diversity in the Celebes Sea was fairly even between the surface and 1000m. However, in the Sulu Sea substantial drops in diversity occurred at 175-200m-below the oxycline, thermocline and halocline-as well as at 900-1000m. The drop at 175-200m and 900-1000m was largely due to the numerical dominance of the species Sagitta decipiens and Sagitta macrocephala, respectively. In the Celebes Sea only, a large portion of the overall chaetognath population migrated downwards during the day into the 125-200m layer. Cluster analysis using Ward\u27s method, along with weighted mean depth values, revealed that this migration was largely attributable to the species Sagitta pacifica, Sagitta ferox-robusta, Sagitta neglecta and Pterosagitta draco. A similar migration of the overall population was blocked in the Sulu Sea-apparently by the oxycline. It seems the group of migrating chaetognaths in the Celebes Sea prevented the numerical dominance of S. decipiens at the lower edge of the epipelagic region, possibly through competition, and consequently prevented the same drop in diversity (H\u27) at 175-200m as was recorded in the Sulu Sea

First record of a leptocephalus larva ingested by a chaetognath

An approximately 17mm Nemichthys scolopaceus leptocephalus was found in the gut of a 25mm chaetognath, Sagitta (Flacdsagitta) hexaptera, collected in the North Equatorial Current region of the western North Pacific. The chaetognath was photographed before preservation. The leptocephalus was folded in half and was positioned deep within the hindgut. Although the chaetognath was caught during sampling for leptocephali, its location in the hindgut suggests that the leptocephalus could have been ingested prior to capture by the net. This first record of ingestion of this unique type of fish larva suggests that if contact is made with small leptocephali, it is possible for them to be ingested by relatively large sized chaetognaths that may identify them as potential prey

Euphausiids collected from the Australian Sector of the Southern Ocean during the BIOMASS SIBEX cruise (KH-83-4)

Euphausiids were collected in the Australian Sector of the Southern ocean during the BIOMASS SIBEX cruise (KH-83-4), with a 10-foot Isaacs-Kidd Midwater Trawl. The biomass of euphausiids in the epi- and mesopelagic layers of the Southern Ocean was 0.58-16.96g (wet wt.) per 1000m^3 and the total number of individuals was 6.8-103.1/1000m^3,accounting for 1.3-11.2% of the total biomass. Generally, the biomass and the total number of euphausiids in the Subtropical Water were the smallest in the Southern Ocean. Six genera including 18 species occurred, and Thysanoessa macrura, Euphausia superba, E. triacantha, E. crystallorophias and E. frigida were collected from the Antarctic Zone. A total of 270 surface swarms were observed by sighting survey at 62°S and southward in the Antarctic Zone. The surface swarms of E. superba consisted almost exclusively of immature individuals, 22-25mm in mean body length. The density of these swarms was 147-778g/m^3. As a result of a parallel grid survey by using the acoustic system, many scattering layers appeared at the depth between 10 and 100m. The main organisms scattering were adults of E. superba and T. macrura. The evidence of mating of E. superba was recognized in the females larger than 38mm in body length and the occurrence of many females with spermatophores attached suggests that they were in the main breeding season

Activities of the Center for International Cooperation

Abstracts of the UNU and ORI joint international workshop for marine environment in 2000海洋環境国際ワークショップ（大槌, 2000年2月21日～25日）の講演要

Distribution of chaetognaths in the Australian sector of the Southern Ocean during the BIOMASS SIBEX cruise (KH-83-4)

Studies on pelagic chaetognaths were carried out on the plankton samples collected during the R.V. HAKUHO MARU KH-83-4 cruise in the Australian Sector of the Southern Ocean as part of the BIOMASS SIBEXI (1983-1984) investigations. The density of chaetognaths in the epipelagic layer in the Southern Ocean was 2.6-17.3 individuals/m^3 and high values were observed in the northern Antarctic region. Eukrohnia hamata and Sagitta gazellae were distributed widely in the Southern Ocean. S. tasmanica was a dominant species in the Subtropical region. Complicated hydrography in the Subtropical Convergence (STC) and Antarctic Convergence (AC) regions was reflected in the distribution of chaetognaths. The occurrence of S. minima in the subsurface layer of the central station of the STC region suggests that the northern warm and saline water mass intruded southward into the depths of 30-70 m. The plural populations of E. hamata exist in the AC and STC regions. Vertically segregative distribution of chaetognaths was observed in the Antarctic and AC regions: S. gazellae at the depths of 50 to 150m, E. hamata at 100 to 400m, and S. maxima at 200 to 500m