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In the Arctic Ocean, the life cycles of interzonal omnivorous copepods have been studied, whereas little information is available on the life cycles of mesopelagic carnivorous species. Here, the life cycles of two mesopelagic carnivorous copepods (Paraeuchaeta glacialis and Heterorhabdus norvegicus) are described from vertically stratified samples collected at an annual ice-station (SHEBA) in the western Arctic Ocean during 1996-1997. Reproduction estimated to occur between January and March for both species. Vertical distributions and population growth varied between the species. Early copepodite stages and adult males of P. glacialis were distributed in deeper layers, whereas late copepodite stages and adult females were observed in shallower layers. The skewed sex ratio towards females was observed for adults, which may be related to the cease feeding and short longevity for adult males. In contrast, all the H. norvegicus life stages were distributed in the mesopelagic layer. Clear peaks of the young stages and their sequence suggest that H. norvegicus may complete its life cycle within 1 year. A specialized feeding mode (incorporating a venomous injection spine and large beak) could be a key trait facilitating the achievement of rapid growth in H. norvegicus in the food-limited mesopelagic layer.

In the Arctic Ocean, the life cycles of interzonal omnivorous copepods have been studied, whereas little information is available on the life cycles of mesopelagic carnivorous species. Here, the life cycles of two mesopelagic carnivorous copepods (Paraeuchaeta glacialis and Heterorhabdus norvegicus) are described from vertically stratified samples collected at an annual ice-station (SHEBA) in the western Arctic Ocean during 1996-1997. Reproduction estimated to occur between January and March for both species. Vertical distributions and population growth varied between the species. Early copepodite stages and adult males of P. glacialis were distributed in deeper layers, whereas late copepodite stages and adult females were observed in shallower layers. The skewed sex ratio towards females was observed for adults, which may be related to the cease feeding and short longevity for adult males. In contrast, all the H. norvegicus life stages were distributed in the mesopelagic layer. Clear peaks of the young stages and their sequence suggest that H. norvegicus may complete its life cycle within 1 year. A specialized feeding mode (incorporating a venomous injection spine and large beak) could be a key trait facilitating the achievement of rapid growth in H. norvegicus in the food-limited mesopelagic layer.

Pelagic polychaetes are distributed from the sea surface to greater depths of the oceans worldwide, but little information is available regarding their ecology. This study investigated the vertical distribution of abundance, biomass and community structure of pelagic polychaetes at a single station in the western subarctic Pacific based on day-night vertical stratified samples collected from a 0 to 1000 m water column during four seasons covering 1 year. The polychaete abundance and biomass ranged from 0 to 757 ind. 1000 M-3 and 0-6.1 mg WW m(-3), respectively. Ten pelagic polychaete species belonging to nine genera and six families were identified. From cluster analysis based on abundance, the polychaete community was divided into five communities. Each community occurred at different depth layers. Two surface groups seen at 0-200 m were dominated by two carnivorous species: Tomopteris septentrionalis and Typhloscolex muelleri. The deepest group, dominated by the particle feeder Pelagobia longicirrata, was seen at 500-1000 m. Two transition groups occurred in the intermediate depths. These vertical distributions of the pelagic polychaete communities were common throughout the season and day. Water mass, food availability and the oxygen minimum layer are put forth as environmental factors that affect the pelagic polychaete community.

Scaphocalanus magnus is a large copepod species with a worldwide distribution. Information on the ecology of this species is scarce. In this study, S. magnus was collected using vertically stratified net sampling from an ice station in the western Arctic Ocean over a year. The vertical distribution differed between periods of polar night and midnight sun, with core depth distributions of 264–381 m for the polar night and 518–745 m for the midnight sun. The shallower distribution during the polar night may have resulted from the animals moving upwards to find sufficient food during the less productive season. The abundance of early copepodite stages showed clear seasonality. The C2 and C3 stages were abundant June–August and August–September, respectively. After C4, seasonality in abundance was not clear. For C4 and C5, body sizes and masses were greater for males than for females while the opposite was the case for C6. Adult males do not feed and thus have shorter life spans than females, skewing the adult sex ratio towards females. For these mesopelagic particle feeding copepods, a long residence time at C6F suggests that in a varying food environment, it is possible to wait to initiate reproduction until favourable food conditions occur.

Within the calanoid copepods, the bioluminescent species comprise 5-59% of the abundance and 10-15% of the biomass in the world's oceans. Most of the luminous species belong to the superfamily Augaptiloidea. The composition of bioluminescent species within the calanoid copepods shows latitudinal patterns; 5-25% of total calanoid copepods are found in high-latitude oceans, while 34-59% are in low-latitude oceans, reflecting a prey-predator relationship. Bioluminescent species of calanoid copepods are able to produce the light-emitting substrate coelenterazine. It is then transferred to higher predators through the food chain, and might be used for bioluminescence in other luminous organisms. A notable feature of copepod bioluminescence is the secreted-type, and its major function may be as an antipredatory response or a defensive behavior. Identification of more than 20 luciferase genes from calanoid copepods has revealed the highly conserved sequences of those genes. This leads us to the speculation that the genes for luciferase within the group of calanoid copepods have evolved independently of comparable genes outside of this group. We discuss here the ecological and biological functions of copepod bioluminescence, the significant diversity in luminous intensity, which might be evolutionarily relevant to their motility and habitat depth, and the promising future directions of bioluminescence studies.

Mesoscale anticyclonic eddies have been observed south of the Aleutian Islands. Eddies farther east, in the Gulf of Alaska, are known to transport coastal water and coastal zooplankton to the offshore open ocean. The impacts of mesoscale anticyclonic eddies formed south of the western Aleutian Islands (Aleutian eddies) on the zooplankton community are not fully understood. In the present study, we describe zooplankton population structures within an Aleutian eddy and outside the eddy during July 2010. Based on the sea-level anomaly, the Aleutian eddy was formed south of Attu Island (172 degrees 54'E) in February 2010, and it moved southeastward in the next 5 months. Large oceanic copepods, Neocalanus cristatus, Eucalanus bungii and Metridia pacifica were more abundant inside the eddy than the outside. Inside the eddy, the life stage distribution of N. cristatus was more advanced than that outside, and Neocalanus spp. had accumulated more lipids. These conditions probably reflect the greater primary production in the eddy, production enhanced by nutrients advected into the eddy. The Aleutian eddy contained mostly oceanic copepods because it was formed in the offshore water and/or eddy-eddy interaction occurred after its formation. The sufficient food condition in the eddy presumably resulted in higher growth and survival rates of these oceanic copepods, resulting in the greater abundance, advanced development stages and greater lipid accumulation.

The subarctic Pacific is known to have east-west gradients in the oceanic environment and phytoplankton community. The western subarctic Pacific is characterized by low temperature and high chlorophyll-a while the eastern region by high temperature and low chlorophyll-a. Although there is little information on the differences in the zooplankton community between the eastern and western subarctic Pacific, the gradients in the oceanographic environment and phytoplankton community may markedly affect the zooplankton community in this region. The aim of this study is to clarify east-west differences in the subarctic Pacific zooplankton community. Zooplankton were sampled at stations along the 165°E line (western subarctic Pacific from 41°30'N to 49°30'N) and 165°W line (eastern subarctic Pacific from 39°N to 53°30'N) using 0.335 and 0.100 mm mesh size Twin NORPAC net during the summers of 2003-2006. East-west differences in the zooplankton community were characterized as: 1) greater total zooplankton abundance in the west-and 2) larger body size of calanoid copepods within same copepodid stage in the west. Differences in east-west zooplankton abundances are attributed to differences in the magnitude of primary production (high in the west) and the size of primary producers (large in the west). Larger body sizes of calanoid copepods in the west are attributed to the lower temperature. Thus, differences in zooplankton abundance and body size are concluded to be due to east-west gradients in the oceanographic environment and phytoplankton community.

Fine-scale vertical distribution patterns of Neocalanus flemingeri, Neocalanus cristatus, Eucalanus bungii and Metridia pacifica were studied by stratified sampling (9 strata between 0 and 1000 m) with a vertical multiple plankton sampler (VMPS) every 3 h for 24 h during 21-22 March 2005 in the Oyashio region. Most C1-C5 stages of N. flemingeri and N. cristatus occurred in the 30-90 m and 70-200 m strata, respectively, both day and night. Irrespective of day and night, C3-C6 E. bungii were distributed broadly in the 200-430 m stratum (C5 inhabiting the shallowest layer). Thus, these three copepods exhibited vertical partitioning of their habitats in the upper layers. The C1-C6 of M. pacifica were distributed broadly from 140 to 400 m during daytime and migrated up to the 100-180 m layer at night (except for non-migrant C1 and C6 males). The within-species zonation of copepodid stages was less marked, but older stages tended to occupy deeper zones (N. flemingeri, N. cristatus and M. pacifica) or shallower zones (E. bungii). Gut content analyses of N. flemingeri and N. cristatus revealed the predominance of diatoms in both species, but the proportion of broken cells was greater in the deep-dwelling N. cristatus than in the shallow-dwelling N. flemingeri, suggesting the importance of sinking phytoplankton aggregates as food source for the former species.

Diel and seasonal vertical distribution, life cycle and body allometry of Pleuromamma scutullata and Heterorhabdus tanneri were investigated in the Oyashio region during September 1996 through October 1997. Monthly samples were collected with 0.1 mm-mesh closing nets towed through five discrete depths between the surface and ≤2000 m. Copepodite stages 1 through 6 of P. scutullata and 3 through 6 of H. tanneri were collected effectively with the nets. Day-night samplings during December 1996, April and October 1997 revealed both species ascend at night. Seasonal differences in the vertical distribution patterns of both copepods were insignificant, and P. scutullata was distributed much shallower (250-500 m) than H. tanneri (600-850 m). By tracing the sequential changes in abundance peaks of each copepodite stage, both species are considered to produce one generation per year. The main reproduction season was estimated to occur in spring for P. scutullata and in winter for H. tanneri. The relationships between prosome length and wet weight, dry weight, and ash-free dry weight indicated that the increase in weight per molt was much greater in H. tanneri than in P. scutullata. These dissimilar features are discussed in relation to the dissimilar food habits of these two species and production cycle of food organisms in the Oyashio region.