Occurrence of large numbers of carcasses of the large,grazing copepod Calanus cristatus from the Japan Sea

Carcasses of Calanus cristatus were discovered in plankton samples collected from the Japan Sea throughout the year from 1970 to 1985. Many carcasses of copepodite Stages IV and V occurred in the layer between 15 and 300 m below a distinct thermocline. The number of copepodite Stage V carcasses also peaked in the layer between 1 500 and 2 000 m. The highest density of copepodite Stages IV and V carcasses was 169 individuals per 1 000 m³ and 1 573 individuals per 1 000 m³, respectively. Carcasses of adults occurred at depths below 500 m and numbers of males and females per 1 000 m³ were 1 to 16 and 1 to 42, respectively. Living males were larger in catch number than living females, but the relationship for carcasses was the opposite. Weight of carcasses was 15 to 25% of living C. cristatus. Carcasses contained about 51% carbon and 8% nitrogen by weight. Carcasses may have been drifting for more than one year in the epipelagic layer under the thermocline because of their slow decomposition rate.     

Spatio-temporal changes in diversity and community structure of planktonic copepods in Sagami Bay, Japan

Seasonal changes in diversity and community structure of planktonic copepods at a shelf site in Sagami Bay, Japan was studied in relation to cross-shelf interaction of species components. Seasonal mesozooplankton samples were collected from the shelf station (St. M) of the north-west part of Sagami Bay from 1995 to 1997. Vertical multi-layered samples were collected near the center of Sagami Bay (St. P) in June 1996. A total 185 copepod species were identified from the two stations. We observed a clear seasonal succession in calanoid diversity and community structure at St. M from a simple shelf water community (>11 species) during spring blooming periods to highly diverse and mixed communities (ca 20–30 species) of shelf water species coupled with various Kuroshio Current species during late summer to autumn. Cluster and non-metric multidimensional scaling ordination analyses showed two distinct calanoid community groups. One group, which included samples of St. M and the surface layer of St. P, consisted of shelf water species, such as Calanus sinicus, Ctenocalanus vanus, Paracalanus spp., and Kuroshio species, such as, Canthocalanus pauper, Scolecithrix danae, etc. The other cluster was restricted to the samples collected from mid and deep layers at St. P, which consisted of meso- and bathypelagic species and Oyashio species (cold-current species, such as Neocalanus cristatus, Pseudocalanus spp., Eucalanus bungii and Metridia pacifica). In the mid and deep layers at St. P, the population of dormant copepodid stage V (CV) of Eucalanus californicus and C. sinicus were dominant. The deep CV population of C. sinicus might be ecologically discriminated from the surface and shelf water population due to their larger body length and dormant life cycle. E. californicus was also collected at the shelf site during each spring bloom period, whereas the population might descend into the mid- and deep-layers of the central bay before summer. Our results suggest that the seasonal fluctuation of community structure in the shelf water was controlled by both physical (Kuroshio Current) and biological factors, i.e., spring bloom and ontogenetic vertical migration of E. californicus. In particular, transport and diffusion processes of Kuroshio Current in Sagami Bay played a key role in controlling the shelf water calanoid community.     

Annual biomass and production of the oceanic copepod community off Discovery Bay,Jamaica

Monthly samples were collected in oceanic waters off Discovery Bay, Jamaica, in 60- and 200-m vertical hauls, using 200- and 64-m mesh plankton nets, from June 1989 to July 1991. Length-weight regressions were derived for twelve genera of copepods (R²=0.79 to 0.97). For eight occasions spanning the study period, biomass estimates generated from these length-weight regressions differed by only 3% from direct weight determinations. The mean ash content of copepods was 7.1%, and the energy density was 20.8 kJ g^-1 ash-free dry weight (AFDW). Mean annual biomass of the total copepod community in the upper 60 m was 1.83 mg AFDW m^-3 (range 1.14 to 2.89 mg AFDW m^-3), and for the 200-m water column was 0.96 mg AFDW m^-3 (range 0.12 to 1.99 mg AFDW m^-3). Estimates of generation times for five common taxa ranged from 16.1 to 33.4 d. None of the taxa investigated displayed isochronal development; in general, stage duration increased in later copepodite stages. Weight increments showed a significant decrease in later copepodite stages, but with strong reversal of the trend from stage 5 to adult female in most species. Daily specific growth rates also declined in later copepodite stages, and ranged from 1.49 d^-1 in stage 1–2 Paracalanus/Clausocalanus spp. to 0.04 in stage 5-female of Oithona plumifera. Progressive food limitation of somatic copepodite growth and egg production is postulated. Naupliar production was 50.4 to 59.5% of copepodite production, and egg production was 35.1 to 27.7% of copepodite production in the 60-and 200-m water columns, respectively. Total annual copepod production, including copepodites, nauplii, eggs and exuviae, was 160 kJ m^-2 yr^-1 for the upper 60 m and 304 kJ m^-2 yr^-1 for the upper 200 m. Secondary production of the copepod community in oceanic waters off Discovery Bay approaches 50% of the corresponding value in tropical neritic waters.     

Vertical distribution, population structure and life cycle of Neocalanus cristatus (Crustacea: Copepoda) in the Oyashio region, with notes on its regional variations

Vertical distribution and population structure of Neocalanus cristatus were investigated at Site H in the Oyashio region from September 1996 through October 1997 to evaluate their life cycle mode. Additional temporary samplings were also made at several stations covering the entire subarctic Pacific, Okhotsk Sea and Japan Sea, as a basis for regional comparison of life cycles of this species. At Site H, N. cristatus spawned throughout the year below 500 m depth, with a peak from October to December. The resulting eggs and nauplii floated/migrated upward, and formed an abundance peak of Copepodite Stage 1 (C1) in the surface layer in February. In the surface layer, the C1 developed and reached C5 by early June through a phytoplankton bloom which occurred in mid-March to end of June. The C5 migrated to deeper layers in July and August, where they molted to adults. Apparently, the developmental time from C5 to adults was highly variable (>1 month), and some might overwinter. The life cycle of N. cristatus appeared to be annual for the major portion of the population. Taking into account sampling season, temporal changes in vertical distribution and population structure data collected from regions other than Site H, there was a close correlation in the timing of the life cycle over the entire subarctic Pacific, but the reproduction season (April to June) was observed to be different in the Okhotsk and Japan Sea populations. Regional comparison of prosome length of C5 individuals, including those in the Bering Sea, indicated significantly larger sizes of specimens from the Japan Sea and Okhotsk Sea, as compared with those from the entire subarctic Pacific. Possible causes for regional variability in life cycle patterns and body sizes are discussed. Received: 18 December 1998 / Accepted: 19 April 1999     

Vertical distribution, life cycle, and developmental characteristics of the mesopelagic calanoid copepod Gaidius variabilis (Aetideidae) in the Oyashio region, western North Pacific Ocean

Vertical distribution, life cycle, and developmental characteristics of the mesopelagic copepod Gaidius variabilis Brodsky in the Oyashio region were investigated by combining analyses of field copepodite populations with laboratory-rearing data of egg hatching and naupliar development. Field samplings from five discrete depths between the surface and ≤2000 m were made approximately every month for 1 year. Most populations of G. variabilis occurred between 600 and 1000 m depth. A modest degree of reversed diel vertical migration behavior and some stage-specific depth-distribution patterns were noted. All copepodite stages were observed throughout the year, suggesting a year-round spawning of G. variabilis. From a prominent abundance peak of Copepodite Stage 1 (C1) seen in June to August, together with development times of eggs and nauplii obtained in laboratory-rearing experiments, the major spawning season was extrapolated to be April to June, the phytoplankton bloom season. Tracing the peak abundance of each copepodite stage (distinguishing males and females for C4 to C6), the generation times of males and females were deduced as 2 and 1 year, respectively. All between-stage increments in terms of wet-, dry-, and ash-free dry weights were greatest in C3/C4, and least in C5/C6 for both males and females. The increments in C3/C4 and C4/C5 were greater for males than for females, reflecting a longer stage duration of the males. These weights did not increase in C5/C6 males, possibly because feeding ceased in C6 males. These results for G. variabilis are compared with those for some mesopelagic copepods previously reported from other regions. Received: 25 October 1999 / Accepted: 20 March 2000     

Feeding ecology of Spinocalanus antarcticus, a mesopelagic copepod with a looped gut

Spinocalanus antarcticus, an abundant mesopelagic copepod in polar seas, has a greatly elongated and looped midgut, contrary to most other copepod species. The total gut length is 1.77, 1.86 and 1.90 times the total body length in adult females, CV and CIV, respectively. Gross morphology of the midgut is similar in all copepodite stages and adults. It is described here from specimens collected in the Arctic Ocean. In stratified samples from the deep Amundsen and Makarov Basins S. antarcticus showed a clear preference for the depth layer between 100 and 500 m. Generally, the guts were packed with material, but most of it was impossible to identify. In most specimens the digestive tract was filled with undefined detritus particles ("detritus balls"). They were almost spherical, heterogeneous organic aggregates of 40-100 µm diameter, with small clay-sized mineral flakes imbedded. Mineral particles in the size range of 1-10 µm were found in large quantities in the guts of many specimens. Cysts of Chrysophycea and dinoflagellates and fragments of dinoflagellates, diatoms, tintinnids and radiolarians, as well as skeletons of silicoflagellates, were rather rare; some animal remnants were also found. A high carbon/nitrogen ratio (8.9) and very high lipid content (54% of dry weight) indicated a very good nutritional state. The adaptive significance and possible feeding strategy of this deep-water copepod is discussed.     

Comparative observations on the biometry and development of Calanus finmarchicus and C. helgolandicus in Copepodite Stage V,with comments on other Calanidae

A similar, sexual dimorphism in Copepodite Stage V was observed in winter generations of Calanus finmarchicus (Gunnerus) and C. helgolandicus (Claus), co-existing in the Firth of Clyde, Scotland. In both species, the frequency distribution of prosome length in the Copepodite V population was bimodal. Among individual Stage V copepodites kept in the laboratory, those of the larger form alone produced adult males. Indications of morphological distinctions between the forms other than in size, however, were absent. Differences in least-squares regressions of individual cephalosome on metasome lengths, previously attributed to a distinction in prosome shape by Grigg et al. (1985), reflected size alone. These two measurements also were greater in the “large” form, but this independently of each other. Variations in their relative contributions to the prosome length distinguished individuals, and were continuous between the forms. Present and previously published observations on members of the family Calanidae have been compared. The presence, or not, of a sexual distinction in prosome length in the Stage V copepodite is variable among calanids, within and between species. A possible association with the extent of seasonal variations in size and life-cycle characteristics is suggested.     

Vertical distribution of larvae of Euphausia nana and E. similis (Crustacea: Euphausiacea) in Sagami Bay and Suruga Bay,Central Japan

Vertical distributions of the larval stages of Euphausia nana Brinton and E. similis G. O. Sars in Sagami Bay and Suruga Bay, Central Japan were studied. Most of the metanauplius larvae of E. nana occurred between 25 and 80 m depth, and they were found at greater depths than the eggs and calyptopis larvae. The nauplii and metanauplii of e. similis were mainly found between 50 and 100 m depth, and they also occurred deeper than the eggs and calyptopes. The larvae of the two species from calyptopis I demonstrate diurnal vertical migration. However, this phenomenon was not clear in the season (March) when a seasonal thermocline was absent. Calyptopes and fruciliae of E. similis occurred deeper and migrated over greater vertical distances than those of E. nana. The distance of migration of furcilia I larvae of E. similis was estimated to be about 200 m.     

Distribution,growth and life cycle of the mesopelagic amphipod Primno abyssalis (Hyperiidea: Phrosinidae) in the southern Japan sea

The vertical distribution, growth, maturation, brood size and life cycle of the hyperiid amphipod Primno abyssalis (formerly P. macropa) were investigated using seasonal samples collected from Toyama Bay, southern Japan Sea, during the period June 1986 to September 1992. Over four different seasons of the year, P. abyssalis was most abundant in the 200 to 350 m strata at night and the 350 to 400 m strata during the day, indicating 100 to 150 m as the general distance of diel vertical movement. Some differences in vertical migrating behavior were noted among juveniles, adult males and females. Population-structure analysis revealed the occurrence of three cohorts aged 0+, 1+ and 2+ yr. Growth as body length in this species is linear with time. Estimated time to complete one life cycle is 1.8 to 2.5 yr for females, but only 0.8 yr for males. Maximum longevity is 2.8 yr. Instar analysis based on the segment number of pleopod rami indicated that newly hatched juveniles molt ten times to reach adult male, and four more times to reach adult female. Adult instar number was found to be only 1 for males and 5 for females. Ovigerous females occurred throughout the year, but the annual peak of release of juveniles from the female's marsupium is estimated to be arly March. Brood size was not correlated with female size, a maximum brood size of 214 eggs was recorded. The dry and ash-free dry weights of instars suggested that juveniles in the female marsupium, adult males, and older adult females are less active feeding or non-feeding stages. Except for the reduced growth rate and the occurrence of small, short-lived males, most characteristics of P. abyssalis are consistent with the present view of the life modes of mesopelagic animals, including linear growth in length, aseasonal reproduction, and smaller brood size coupled with larger eggs.     

Abundances,growth rates,and production of tropical neritic copepods off Kingston,Jamaica

Weekly samples were collected near Kingston, Jamaica in 27 m vertical hauls, using 200 and 64µm mesh plankton nets, from July 1985 to January 1987. Thirtytwo copepod species were identified; nauplii and all copepodite stages were enumerated. Total copepod abundance ranged from 2.56 to 87.3 × 10⁴ m^–2. The annual abundance cycle was bimodal with peaks in October–November and May–June corresponding to the rainy seasons. Mean annual copepodite biomass was 0.15 g AFDW m^–2 ranging from 0.03 to 0.41 g AFDW m^–2. Mean generation time (from egg to adult) at 28°C was 19.5 d for the common speciesCentropages velificatus, Paracalanus aculeatus, andTemora turbinata. Isochronal development was demonstrated for copepodites ofP. aculeatus andT. turbinata, but not forC. velificatus. Mean daily specific growth rates (G) were 0.63, 0.63, and 0.48 d^–1 forC. velificatus, P. aculeatus, andT. turbinata, respectively. In general, daily specific growth rates decreased in the later copepodite stages. Thus, it is postulated that growth of later stages and egg production may be food limited. Annual copepodite production was estimated as 419 kJ m^–2 yr^–1, while annual exuvial production and naupliar production were 35 and 50 kJ m^–2 yr^–1, respectively. Egg production was estimated as 44% (184 kJ m^–2 yr^–1) of the total copepodite production. Thus, mean total annual copepod production was 688 kJ m^–2 yr^–1. This estimate is within the range of copepod production estimates in coastal temperate regions.     

Larvae of deep-sea fishes (Stomiatoidea) from Biscayne Bay,Florida, USA,and their ecological significance

Twenty stomiatoid fish larvae (3.1 to 21,9 mm) belonging to the genera Bathophilus, Cyclothone, and Stomias were collected in plankton nets from surface waters over depths of 3 to 4 m from Biscayne Bay, Florida, USA. The adults are mesopelagic in the open water of the Florida Current and, therefore, these larvae were presumably advected in parcels of high salinity water into the shallows of Biscayne Bay by winds, tides, or currents. The presence of these larvae suggest that sewage discharged into the Florida Current may be recycling into coastal waters as well as into Biscayne Bay.Scientific Contribution No. 1403, University of Miami, Rosenstiel School of Marine and Atmospheric Science. The support of Environmental Protection Agency Contract FW QA 18050 DIU, Water Quality Office, Division of Water Quality Research is gratefully acknowledged.     

Seasonal observations on the biometry and development in Copepodite Stage V of Calanus finmarchicus from the Firth of Clyde

Relationships between linear dimensions of casts produced at the final moult in the laboratory and definitive sex confirm that Calanus finmarchicus (Gunn.) is sexually dimorphic in Copepodite Stage V. Two forms of Stage V copepodite were present at all times of the year in the Firth of Clyde, Scotland. Except in early spring, when size was similar, these were consistently distinguished by the prosome length. The small form moulted to adult females and the large form to adults of either sex. Relative to the metasome, the length of the cephalosome was greater in the large than small form of Stage V copepodite. This, to an increased extent, distinguished males from all females in the adult. Differences between females of the large and those of the small form were similar in the Stage V copepodite and adult. Feminization in the large form entailed an arrest in the differentiation of secondary male sex-characters. Most morphological females moulting from both forms of Stage V copepodite possessed an apparently normal ovary, with eggs extending into oviducal diverticula. In the case of the large form, however, eggs were occasionally absent from diverticula. The gonad in two specimens resembled an immature testis. In males, a normal testis was always present. Differentiation into the adult male entailed the internal suppression of feminizing factors. On average, within collections, females of the large form moulted later than the small form and definitive males. A similarly consistent difference between females of the small form and males was absent. Moulting times in the large form of Stage V copepodite depended on the presence or not of feminization. Differentiation into the adult male required the environmental stimulation of development. Comparisons with moulting patterns in the sea suggest a mechanism that regulates the recruitment of males to the adult.     

Ecological and biological features of a mesopelagic ostracod,Conchoecia pseudodiscophora,in the Japan Sea

The abundance and vertical distribution pattern of a halocyprid ostracod,Conchoecia pseudodiscophora, were investigated in the Japan Sea in 1985, 1987 and 1989. Vertical sampling from 500 m depth to the surface in the water around Yamato Rise revealed that this ostracod was second in dominance by number and third to fourth by biomass of the total zooplankton collected with a 0.35 mm mesh Norpac net. Horizontal net tows in Toyama Bay indicated that the major population ofC. pseudodiscophora was distributed below 250 to 300 m depth. No diel migration pattern was evident. Its contribution to total zooplankton there was 5 to 10% or more in terms of biomass. A total of five subadult instars (II to VI) and adult males and females were identified from instar analysis based on sizes and morphological characteristics of specimens collected with 0.10 mm mesh Norpac nets. Data on body length, wet weight and dry weight of each instar are presented. Carbon content of 35 to 48% of dry weight, and nitrogen content of 5.3 to 7.3% of dry weight, were recorded on fresh, freeze-dried specimens of selected instars (subadult Instars IV to VI, adult females). Water and ash contents of mixed specimens of these four instars were 76% of wet weight and 25% of dry weight, respectively. Feasibility of laboratory maintenance ofC. pseudodiscophora was tested, and it produced characteristic J shaped faecal pellets. Oxygen consumption rates of subadult instars V and VI, and adult female ranged 0.011 to 0.021µl O₂ ind.^–1 h^–1 at 1 °C, or 2.9 to 6.1µl O₂ (mg body N)^–0.85 h^–1 in terms of Adjusted Metabolic Rate (AMR_{o 2}). There was no appreciable metabolic reduction inC. pseudodiscophora compared to other ostracods, despite their mesopelagic life mode. Subdominance in total zooplankton and nonreduced metabolic activity ofC. pseudodiscophora suggest that this species may be an important link in mesopelagic energy-flow and matter cycling in the Japan Sea.     

Population structure and life cycle of the mesopelagic ostracod Conchoecia pseudodiscophora in Toyama Bay,southern Japan Sea

The population structure and life cycle of the mesopelagic ostracod Conchoecia pseudodiscophora Rudjakov in Toyama Bay, southern Japan Sea, were investigated using a time-series of samples collected during 0 to 500 m vertical hausls with twin-type Norpac nets (0.35 and 0.10 mm mesh) over one full year (1 February 1990 to 30 January 1991). Additional samples were also collected with a single-type closing Norpac net (0.06 mm mesh) to examine the vertical distribution patterns of eggs and all instars of this species. The proportion of gravid females present indicated that reproduction of C. pseudodiscophora continues throughout the year, but peaks in April–July. Eggs and Instars I and II were distributed below 500 m, while the more advanced instars were most abundant in the 350 to 500 m stratum by both day and night. Based on the abundance peaks of each instar in the time-series samples, development times were estimated to be 2.5, 4, 3, 7 and 11 mo for Instars III, IV, V, VI, and VII, respectively. Thus, a total of 30 mo is required for newly spawned eggs to hatch and reach adulthood. Stomach-fullness indexes revealed no seasonality in the feeding activity of any instar stage, but that feeding activity was low in older instars, particularly in adult males. The present results are compared with those for a few other ostracod species, in an attempt to characterize the life cycle of C. pseudodiscophora inhabiting waters of subzero temperature in the mesopelagic zone of the Japan Sea.     

Size-weight relationships and biomass of tropical neritic copepods off Kingston,Jamaica

Nine genera of neritic tropical copepods were collected near Lime Cay, Jamaica, between July 1985 and January 1987. Length-weight regressions were derived for each genus (R ²=0.74 to 0.98), for all calanoids combined (R ²=0.88), and all cyclopoids combined (R ²=0.85). Width-weight regressions were also derived for the same genera but coefficients of determination were generally lower (R ²=0.52 to 0.98) and were much lower forOithona spp. (R ²=0.21). Over a 12 mo period, biomass estimates generated from these length-weight regressions differed by only 3% from direct weight determinations. There were no significant annual variations in prosome lengths for copepodite Stages 1 to 5 inCentropages velificatus, Paracalanus aculeatus orTemora turbinata; prosome lengths in femaleT. turbinata did significantly vary seasonally. No significant seasonal differences in length-weight relationships were observed forC. velificatus. The mean ash content of mixed copepod samples was 6.4%, and the energy density was 25.0 kJ g^–1 AFDW. No significant loss of weight was observed 10 mo after preservation in 10% formalin.     

Ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus in the Inland Sea of Japan

The ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus was investigated in the Inland Sea of Japan in November 1988 and March 1989, when the water temperature was weakly stratified in a reversed manner. In both investigations a pronounced ontogenetic difference in vertical distribution was found. Spawning always occurred during nighttime, being confined to the upper 40 m water column in November but to the layer below 35 m in March. The distribution of pre-feeding nauplius stages, NI and NII, was more or less similar to that of the eggs. The first-feeding NIII performed a marked upward migration, and late nauplius stages (NIV to NVI) and early copepodite stages (CI and CII) continuously aggregated in the upper water column where phytoplankton was abundant. CIII to CVI (adult female and male) tended to disperse in the whole water column. In November, however, they avoided the upper 10 m strate during daytime and some individuals migrated upward to the surface during nighttime. In March, CV and CVI aggregated in the layer between 5 and 15 m deep in the daytime and migrated both upward and downward at dusk, resulting in homogeneous distributions during the nighttime.     

Life cycle of Pseudocalanus acuspes Giesbrecht (Copepoda, Calanoida) in the Central Baltic Sea: I. Seasonal and spatial distribution

The seasonal and spatial distribution of Pseudocalanus acuspes in the Bornholm Basin (Central Baltic Sea) was studied on 16 cruises between March 2002 and May 2003 from stratified (10 m) multinet samples. The highest abundances were reached in May 2002 and April 2003 (618×10³ and 869×10³ ind. m⁻², respectively). Ontogenetic vertical distribution was stage specific with differences of mean annual weighted mean depth >30 m between nauplii and males; it followed closely the hydrography which was characterized by a permanent halocline and a summer thermocline. The vertical distribution showed a positive correlation with salinity especially in the older developmental stages; the relationship to temperature was negative in the nauplii and copepodite stage I (CI). Most of the stages performed a seasonal migration. The consequences of the vertical distribution patterns in relation to the effects of climate and predation are discussed. A stage shift from nauplii in April/May to CIV and CV as overwintering stages indicated slow seasonal development. However, nauplii were observed all the year round, and the resulting stage structure did not allow to distinguish generations. Changes in the prosome length of females seemed to be related to the advection of water masses with different temperatures rather than to different generations. It could not be clarified whether the strong increase of nauplii and adults after an inflow event of cold, saline North Sea water in the beginning of 2003 was a result of advection or improvement in habitat conditions.     

Seasonal variations in distribution and population structure of Microcalanus pygmaeus and Ctenocalanus citer (Copepoda: Calanoida) in the eastern Weddell Sea,Antarctica

The abundance, vertical distribution and population structure of two important small calanoid copepod species, Microcalanus pygmaeus (G. O. Sars) and Ctenocalanus citer Heron and Bowman, were studied in the eastern Weddell Sea in summer (January/February 1985), in late winter/early spring (October/November 1986) and in autumn (April/May 1992). The population of Microcalanus pygmaeus consisted mainly of copepodite stages CII and CIII in late winter/early spring and were concentrated between 500 and 200 m depth. In summer, stage CIV was the modal stage and the bulk of the population had ascended above 300 m. In autumn the population structure was bimodal with CI and CV dominating. Most of the population was concentrated between 300 and 200 m. In all investigation periods M. pygmaeus had their maximal concentrations in the thermo-pycnocline. The developmental stages CIII to CV of Ctenocalanus citer formed the bulk of the population in late winter/early spring. In October all developmental stages had their main distribution between 500 and 200 m, except females, which were concentrated in the upper 50 m. In November most of the population occurred between 200 and 50 m. The summer population was concentrated in the upper 50 m, and numbers increased dramatically as the new cohort hatched. Copepodite stages CII and CIII dominated the population at the end of January, while CIV dominated 2 wk later. In autumn, CV was the modal stage. The majority of the population was concentrated in the upper 100 m, but there was an increase in abundance below 300 m compared to summer. Age structure changed with depth with a younger surface population and an older one in deeper water layers. The seasonal change in number of M. pygmaeus is much smaller than that of C. citer; the summer:winter:autumn ratio of the former being about one, whereas the winter:summer/autumn of the latter was about nine. Early copepodite stages and adults of M. pygmaeus occurred throughout all investigation periods. The large proportion of early copepodite stages in April and in mid-October suggests autumn and early to midwinter breeding. Apparently, M. pygmaeus may reproduce and grow year-round or perhaps has a 2-yr life-cycle. In contrast, the dramatic increase in abundance of early copepodite stages of C. citer in summer suggests springtime reproduction.     

Preliminary rearing experiments on the larvae of Sergestes lucens (Penaedia,Natantia, Decapoda)

Sergestes lucens Hansen, a mesopelagic shrimp fished commercially in Suruga Bay, Japan, was successfully reared from egg to post-larval stage V under laboratory conditions. Chaetoceros ceratosporum and Artemia nauplii were found to be satisfactory food in the laboratory during rearing. Growth, mortality, food preference, and feeding and swimming activities during the various developmental stages were investigated. Temperature changes greatly affected the speed of development and the mortality of the larvae. The optimum temperature range for larval development was 18° to 25°C. The growth rate (length) of larval stages was as rapid as 0.16mm/ day at 20 °C and 0.21 mm/day at 23 °C. The larvae first started feeding on phytoplankton at elaphocaris stage I, and then gradually became predators in the post-larval stages. It is suggested that the critical period for the species occurs in the elaphocaris stages. Environmental data, vertical distribution of the species, and data obtained from laboratory experiments suggest that the fluctuation in the abundance of S. lucens is greatly influenced by the water temperature at around 50 m from June to August. Feeding mechanisms observed in the post-larval stages are described.