Molecular phylogeny and evolution of the pelagic copepod genus Neocalanus (Crustacea: Copepoda)

Portions of the mitochondrial genome (ca. 4 kb), encoding three protein-coding (COI, ND4L, ND6) and two ribosomal RNA (srRNA, lrRNA) genes, were sequenced for all six currently recognized species, plus one form, of the pelagic calanoid copepod genus Neocalanus. In Neocalanus gracilis, the ND6 gene was not found in the sequenced portion of the mitochondrial genome. Unambiguously aligned sequences were subjected to Bayesian, maximum-likelihood, maximum-parsimony, and neighbor-joining analyses using Eucalanus bungii as an outgroup. The resultant tree topologies from these four methods were congruent, robust, and all nodes were supported by high bootstrap values and posterior probabilities of 92–100%. Two tropical and subtropical species (N. gracilis and N. robustior) occupied the most basal position, and a subantarctic (N. tonsus) and three subarctic Pacific species (N. cristatus, N. plumchrus, and N. flemingeri) diverged subsequently. Transequatorial dispersal of the ancestral population during glaciations is suggested for this pattern of speciation, in which sister clades exhibited antitropical distributions. Although the area of ocean is much broader in the subantarctic than the subarctic Pacific, a higher number of species occur in the subarctic Pacific (three) than the subantarctic (one). The possibility that marginal seas, such as Japan Sea and Okhotsk Sea, function as natal areas for the divergence of species is discussed.     

Vertical distribution,population structure and lifecycle of<Emphasis Type="Italic"> Eucalanus bungii</Emphasis> (Copepoda: Calanoida) in the Oyashio region,with notes on its regional variations

Vertical distribution and population structure of Eucalanus bungii were investigated at site H in the Oyashio region (western subarctic Pacific) from September 1996 through October 1997 to evaluate the species lifecycle pattern and associated ontogenetic vertical migration. 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 lifecycle features of this species. At site H, a marked phytoplankton bloom occurred from mid-March to June, and E. bungii spawned in April/May in the surface layer. Resulting nauplii and copepodite stage 1 (C1) formed a prominent abundance peak in early June. The C1 developed and reached C5 by August. The development of nauplii through C4 occurred in the surface layer. From August onwards, C5 and a small fraction of C3–C4 sank gradually deeper, and entered diapause to overwinter at >500 m depth. The C5 molted to C6 males and females in February and March, respectively. The C6 males and females mated at 250–500 m depth, and only mated C6 females ascended to the surface layer in April for spawning. Judging from the size of lipid droplets in the body, the C3–C5 specimens deposited lipids in the body through the phytoplankton bloom period, and the lipids were consumed gradually during overwintering. Taking account of sampling season, temporal changes in population structure, and vertical distribution, the data collected from the western subarctic Pacific and Okhotsk Sea are consistent with a 1-year lifecycle for the site H population, while the data from the central and eastern subarctic Pacific were consistent with a 2-year lifecycle. The populations from the southern and southeastern Japan Sea did not fit the features of either lifecycle scenario, and because of their very small population size it is suggested that they originated from the northern Japan Sea. Regional comparison of the prosome length of C6 females, including those in the Bering Sea, indicated significantly larger specimens from the Japan Sea and Okhotsk Sea, and smaller specimens in the eastern subarctic Pacific, as compared with those in the western subarctic Pacific (including site H) and Bering Sea. A possible overwintering mechanism of E. bungii is discussed.Communicated by O. Kinne, Oldendorf/Luhe     

Weight and chemical composition of some important oceanic zooplankton in the North Pacific Ocean

Wet and dry weight, total carbon, nitrogen, hydrogen, and ash contents were determined on 33 species of zooplankton distributed predominantly in the open sea region of the North Pacific. Sampling covered the waters from 44°N to the equator. Average percentage of dry weight to wet weight was about 19% of all samples from the whole area. Percentage dry weight of carbon in copepods was on an average 51.5%. The highest value, 66.6%, was obtained in eggs of the copepod Pareuchaeta sarsi. Mixed zooplankton was assumed to contain carbon comprising about 35 to 45% of the dry weight. Carbon contained in the zooplankton biomass existing in the upper 200 m in the western parts of the northern North Pacific and Bering Sea during spring and summer was estimated to range from 20 to 85 mg C/m³. Nitrogen content varied considerably with localities. Average ratio of carbon to nitrogen was 8.5 in subarctic copepods, and 4.1 in subtropic-tropic copepods. This ratio also varied with season. In the copepod Calanus cristatus the ratio was highest (10.0) in May, immediately after the spring bloom of phytoplankton, when the animals contained much fat. The ratio fell to 5.1 in December. There seemed to be a large seasonal variation in boreal zooplankton due to great fluctuations of environmental conditions, especially the amount of food available; in tropical species the range was small because of environmental uniformity. Average hydrogen content was about 6 to 10%. The percentage of ash to dry weight amounted to 39.3% in pteropods and 3.4% in copepods.     

Diel changes in gut-cell morphology and digestive activity of the marine copepod Acartia tonsa

Morphometric measurements of the gut cells of the marine calanoid copepod Acartia tonsa were made over a 24 h diel cycle to test the hypothesis that feeding rhythms in this copepod are limited by the cycles of blishter-like cells (B-cells) in the gut. Copepods were collected from Long Island Sound, New York, USA, in June 1991. Experimental treatments included low (1 g chlorophyll al^-1) and high (10 g chlorophyll al^-1) food concentrations. Copepods were fixed and embedded in Epon at six time intervals over the 24 h period, and semi-thin sections through the length of Midgut Zone II were analyzed for gut-cell area and vacuole area, with area measurements integrated to yield estimates of gut-cell volume and vacuole volume. Concurrent measurements of gut fluorescence, a measure of gut fullness, and digestive enzyme activities also were made. A diel cycle in gut fluorescence was observed, most notably in the low-food treatment, which exhibited a 3-fold increase in gut pigments. There was little consistent change in digestive enzyme activities over this time span. Gaps in the gut wall indicative of spent B-cells were observed in 17 of 39 copepods, with no trend over time or food treatment. Generally, only a single gap was seen in any one copepod. B-cells were vacuolated throughout the 24 h period. Morphometric analyses revealed a correlation between gut fluorescence and both maximum vacuole area and vacuole volume, as well as percent vacuolated cell volume, in the low-food treatment. The high-food treatment, which had a relatively smaller increase in gut fluorescence (1.5-fold) during the night cycle, showed no significant increase in vacuole size. B-cells appear to have a life-cycle greater than the diel feeding period, and while B-cell vacuoles respond to the ingested food during the diel cycle, production of B-cells does not appear to be a limiting factor in A. tonsa's feeding cycle.     

Phytoplankton pigments in the gut concents of planktonic copepods from coastal waters off southern California

Phytoplankton xanthophylls in the gut contents of the copepods Calanus pacificus, Corycaeus anglicus, and Paracalanus parvus, collected from 5 stations off San Onofre, California, in June 1982, were measured by reverse phase, high-performance liquid chromatography (HPLC). The dinoflagellate pigment, peridinin, was usually the most abundant xanthophyll in the guts of all three species of copepods. Evidently, feeding was principally on dinoflagellates (which dominated the phytoplankton biomass). The level of feeding activity, rather than the class of phytoplankton ingested, seemed to differentiate the behaviors of the copepods. Xanthophyll content per unit copepod wet weight was higher in Corycaeus anglicus and Paracalanus parvus than in Calanus pacificus. Chlorophyll a fluorescence of the copepod gut contents was measured in conjunction with the analysis of gut xanthophylls. The xanthophyll content of the gut varied directly with the concentration of chlorophyll a in the gut. Xanthophyll content was not related to the concentration of pheopigments in the gut. Apparently, the xanthophylls that were detected were due to the presence of recently ingested phytoplankton biomass.     

Mosaic horizontal distributions of three species of copepods in the subarctic Pacific during spring

 Continuous abundance estimates (510 m resolution) of the copepods Neocalanus cristatus, N. flemingeri and Metridia pacifica were obtained with an electronic particle counter along cruise tracks in the subarctic western North Pacific in spring. For all three species, the number of patches decreased exponentially with increasing patch size. Most patches (63 to 83%) were dominated by one species, and patches of the same species more closely spaced than patches of different species. The patches of M. pacifica tended to coexist with those of N. cristatus. In contrast, patches of N. flemingeri rarely co-occurred with those of other copepods. These patterns were more clearly observed in fine-scale observations with sampling intervals of <31 m. Coherence analysis of copepod species pairs showed no characteristic scale at 2 to 50 km wave lengths. At shorter wave lengths (<2 km), frequent positive correlations were observed between N. cristatus and M. pacifica. Thus, the distribution of copepods appears to be a mosaic assemblage of patches of each copepod species. These results suggest that copepods have a mechanism to form species-specific aggregations, and the aggregation and segregation processes are maintained at a scale of <2 km. Received: 24 February 1999 / Accepted: 25 April 2000     

Characterizing dietary variability and trophic positions of coastal calanoid copepods: insight from stable isotopes and fatty acids

The spring zooplankton community in the Strait of Georgia (British Columbia, Canada) is characterized by the presence of several calanoid copepod species which collectively make up ~90% of the mezozooplankton biomass. Here, we investigate interspecific, interannual, and geographic variability in the diets and trophic positions of these copepods using a combination of fatty acids and stable isotopes. To characterize geographic variability in diet, we compare our findings from the Strait of Georgia with similar data from Ocean Station P in the subarctic northeast Pacific. Both fatty acid and stable isotope signatures indicate the existence of three trophic levels, even within the limited size range of these copepods: Neocalanus plumchrus and Calanus marshallae are primarily omnivorous, while Euchaeta elongata is carnivorous and Eucalanus bungii is herbivorous. Fatty acid markers of trophic position (e.g., DHA/EPA, 18:1n-9/18:1n-7) correlate significantly with δ¹⁵N, while markers indicating the proportion of diatoms to flagellates in the diet (e.g., 16PUFA/18PUFA and DHA/EPA) correlate significantly with δ¹³C, after the effect of lipid concentration on δ¹³C is accounted for. Despite the general correlation between stable isotopes and fatty acids, the former are not sensitive enough to capture the range of interannual variability observed in the latter, and can only capture substantial shifts in the diet over geographic scales. However, regardless of variability in food quality, the relative trophic positions of these copepods do not change significantly either spatially or temporally.     

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     

Quantitative PCR to estimate copepod feeding

Copepods play a central role in marine food webs as grazers of plankton and as key prey for many predators. Therefore, quantifying their specific trophic interactions is critical for understanding the role of copepods in ocean processes. However, because of methodological constraints, it remains difficult to investigate in situ copepod feeding without reliance on laborious intrusive and potentially biased incubation approaches. Recent advances in PCR-based methodologies have demonstrated the feasibility of directly identifying copepod diets based on prey DNA sequences. Yet, obtaining quantitative information from these approaches remains challenging. This study presents results of systematic efforts to develop a quantitative PCR (qPCR) assay targeted to 18S rRNA gene fragments to estimate copepod gut content of specific species of prey algae. These results were first compared to gut content estimates based on fluorescence in the copepod Calanus finmarchicus fed monocultures of two different microalgae species in controlled laboratory studies. In subsequent field studies, we compared feeding rates obtained by microscopy and qPCR for Temora longicornis and Acartia clausi feeding on the haptophyte Phaeocystis globosa in natural blooms. These investigations demonstrate a semi-quantitative relationship between gut content estimates derived from qPCR, gut pigment, and direct microscopy. However, absolute estimates of gut content based on qPCR methodology were consistently lower than expected. This did not appear to be explained by the extraction methods used, or interference by non-target (predator) DNA in the PCR reactions, instead suggesting digestion of prey-specific nucleic acids. Furthermore, the 18S rDNA target gene copy number of the phytoplankton varied with growth phase. Nonetheless, when prey target gene copy number in the ambient water is quantified, the qPCR-approach can be compared to other methods, and then used to semi-quantitatively estimate relative copepod grazing on specific prey in situ without involving further incubations. A distinct advantage of a DNA-based molecular approach compared to gut fluorescence and direct microscopic observation, is the ability to detect non-pigmented and macerated prey. Future studies should aim to correct for breakdown in prey DNA and perform extensive calibrations to other methods in order to achieve a quantitative measure of feeding rates in situ.     

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.     

Community-level and species-specific responses of coastal phytoplankton to the presence of two mesozooplanktonic grazers

The response of phytoplankton to the presence of two mesozooplanktonic grazers was studied in a 44-h laboratory experiment from the phytoplankton perspective. The <45-μm-filtered plankton community, as well as females of the copepod Eurytemora affinis and nauplii of the barnacle Balanus improvisus originated from the northern Baltic Sea. The phytoplankton community was dominated by the dinoflagellate Heterocapsa triquetra, which was preferred as food by the mesozooplanktonic grazers, especially E. affinis. Cryptophytes were eaten by the B. improvisus nauplii, while heterotrophic nanoflagellates increased in the presence of B. improvisus nauplii, and colonial cyanobacteria increased in the presence of E. affinis. Because of the strong selective feeding on the dominant, large-sized phytoplankton species, the negative effect of the mesozooplanktonic grazers on total chlorophyll a was stronger than any cascade effect releasing phytoplankton from protozoan grazing.     

Bimodal vertical distribution and diel migration of the copepods Metridia pacifica,M. okhotensis and Pleuromamma scutullata in the western North Pacific Ocean

Diel changes in fine-scale vertical distributions of three calanoid copepods Metridia pacifica, M. okhotensis and Pleuromamma scutullata in the subarctic waters of the western North Pacific were examined. Sampling was carried out in June and August 1983, at two stations in Oyashio water using a Longhurst-Hardy Plankton Recorder (LHPR). Sampling, down to about 1 000 m, was repeated four to five times at intervals of several hours. Vertical resolution was 5 to 40 m. Copepods were concentrated in two strata, the surface (0 to 60 m) and the mesopelagic (200 to 300 m) layers, throughout the day at both stations. Younger M. pacifica (C III and C IV) were dominant in both strata. Although most female C V and adult females demonstrated diel vertical migration at 20 to 30 m h^-1, a significant number of females did not migrate upward but remained in the deep stratum at night. The same trend was evident in M. ohkotensis and P. scutullata. Foregut content observations indicated that feeding activities of the deep mode populations were as high as those of the surface mode, though food of deep individuals was different. Such a bimodal distribution may increase intraspecific diversity of copepod populations and is possibly why metridiid copepods dominate during late summer to winter in the relatively simple ecosystems of high latitudes.     

Trophodynamic function of copepods,appendicularians and protozooplankton in the late summer zooplankton community in the Skagerrak

The study was carried out in the Skagerrak during late summer when population development in the pelagic cycle culminated in the yearly maximum in zooplankton biomass. The cyclonic circulation of surface water masses created the characteristic dome-shaped pycnocline across the Skagerrak. The large dinoflagellate Ceratium furca dominated the phytoplankton biomass. Ciliates and heterotrophic dinoflagellates were the major grazers and, potentially, consumed 43–166% of daily primary production. The grazing impact of copepods was estimated from specific egg production rates and grazing experiments. The degree of herbivory differed between species (14–85%), but coprophagy (e.g. feeding on fecal pellets) and ingestion of microzooplankton were also important. The appendicularian Oikopleura dioica was present in lower numbers than copepods, but cleared a large volume of water. The grazing impact of copepods and O. dioica was estimated to 57±24% and 12±12% of daily primary production, respectively. Sedimentation of organic material (30 m) varied between 169 and 708 mg C m^–2 day^–1, and the contribution from the mesozooplankton (copepod fecal pellets and mucus houses with attached phytodetritus of O. dioica) was 5–33% of this sedimentation. Recycling of fecal pellets and mucus houses in the euphotic zone was 59% and 36%, respectively. However, there was a high respiration of organic material by microorganisms in the mid-water column, and 34% of the sedimenting material actually reached the benthic community in the deep, central part of the Skagerrak.     

Persistent diel rhythms in the chlorophyll fluorescence of marine phytoplankton species

The diel periodicities of in-vivo chlorophyll fluorescence and DCMU-enhanced chlorophyll fluorescence of 47 marine phytoplankton species were examined for 2 d in a 14 h L:10 h D light: dark cycle and then in continous light for another 2 to 3 d. Almost all phytoplankton species exhibit a more rapid increase in in-vivo fluorescence and DCMU-enhanced fluorescence during the light phase than during the dark phase. About one-half of the species examined exhibited persistent diel rhythms in continous light, indicating the operation of a biological clock. No phylogenetic or habitat related trends as to which species exhibited persistent rhythms were apparent. Of the phyla of eukaryotic phytoplankton adequately examined, none lacked biological clocks. Contrary to past hypotheses, some phytoplankton species maintain a persistent diel rhythm in a constant environment while reproducing at a rate greater than one division per day.     

Comparison of feeding habits of myctophid fishes and juvenile small epipelagic fishes in the western North Pacific

To examine the potential trophic competition between myctophids and small epipelagic fishes in the nursery grounds in spring, we compared the stomach contents of dominant myctophids (Symbolophorus californiensis, Ceratoscopelus warmingii and Myctophum asperum; n = 179) and juvenile epipelagic fishes (Japanese sardine, Sardinops melanostictus, Japanese anchovy, Engraulis japonicus, chub mackerel, Scomber japonicus, and spotted mackerel, S. australasicus; n = 78) that were simultaneously collected at nighttime with a midwater trawl net around the Kuroshio-Oyashio transition zone in the western North Pacific. It was clear that the neritic copepod Paracalanus parvus s.l. was the most abundant species in NORPAC samples (0.335 mm mesh size) taken at the same stations. Diets of dominant myctophid fishes differed from those of the juvenile epipelagic fishes; Japanese sardine and anchovy mostly preyed upon P. parvus s.l. (23.6% of stomach contents in volume) and Corycaeus affinis (16.1%), respectively. Both chub and spotted mackerels mainly preyed upon the seasonal vertical migrant copepod, Neocalanus cristatus (15.9 and 14.7%, respectively). On the contrary, myctophid fishes probably do not specifically select the abundant neritic copepods. Namely, S. californiensis mostly preyed upon a diel vertical migrating copepod, Pleuromamma piseki (22.7 and 30.6% in stomach of juvenile and adult, respectively), while C. warmingii and M. asperum preyed on Doliolida (43.0% in stomach of juvenile C. warmingii), appendicularians (11.0% in stomach of juvenile M. asperum), and Ostracoda (6.3% in stomach of adult C. warmingii). Feeding habits of myctophid fishes seem adapted to their prey animals; low rate of digested material (less than 30% in volume) in stomachs of S. californiensis may be linked to the movement of P. piseki, hence S. californiensis can easily consume this copepod at night since they are more concentrated at night than daytime. High rate of digested material (over 40%) of M. asperum and adult C. warmingii suggest that they feed not only at night but also during the daytime in the midwater layer. Thus, myctophid fishes actually fed in the surface layer but less actively than the small epipelagic fishes. These results suggest that the potential for direct food competition between myctophids and small epipelagic fishes is low in the nursery ground, but there remains a possibility of indirect effects through their prey items, since the above gelatinous animals feed on common prey items as juveniles of Japanese sardine and anchovy.     

Plankton of the fladen ground during FLEX 76 III. Vertical distribution,population dynamics and production of Calanus finmarchicus (Crustacea: Copepoda)

Samples taken in the northern North Sea with the Continuous Plankton Recorder (CPR), the Undulating Oceanographic Recorder (UOR), the Longhurst Hardy Plankton Recorder (LHPR) and by our colleagues from other participating Institutes during the Fladen Ground Experiment (FLEX 76) were used to describe the vertical distribution and population dynamics of Calanus finmarchicus (Gunnerus) and to provide estimates of the production and carbon budget of the population from 19 March to 3 June, 1976. Total production of the 19 March to 3 June, 1976. Total production of the nauplii and copepodite stages (including adults), during the exponential growth phase in May, was estimated to be in the range of 0.49 to 0.91 g C m^-2 d^-1 or 29.0 to 55 g dry wt m^-2 (14.5 to 27.8 g C m^-2) for the three successive 10 d periods in May. Two gross growth efficiencies (K ₁) (20 and 34%), together with the lower value of C. finmarchicus production, were used to calculate the gross ingestion levels of algae as 2.45 and 1.44 g C m^-2 d^-1 (73.5 and 43.2 g C m^-2 over the May period). These ingestion levels, together with the algae ingested by other zooplankton species, are greater than the estimated total phytoplankton production of 45.9 g C m^-2 over the FLEX period. A number of factors are discussed which could explain the discrepancies between the production estimates. One suggestion is that the vertical distribution of the development stages of this herbivorous copepod and their diel and ontogenetic migration patterns enable it to efficiently exploit its food source. Data from the FLEX experiment indicated that the depletion of nutrients limited the size of the spring bloom, but that it was the grazing pressure exerted by C. finmarchicus which was responsible for the control and depletion of the phytoplankton in the spring of 1976 in the northern North Sea.JONSDAP Contribution No. 51     

Natural diets of vertically migrating zooplankton in the Sargasso Sea

The feeding preferences of three common diel vertically migrating zooplankton were investigated from December 1999 to October 2000 at the U.S. JGOFS Bermuda Atlantic Time-Series Study (BATS) station in the Sargasso Sea. Gut content analysis of the copepods Pleuromamma xiphias (Giesbrecht) and Euchirella messinensis (Claus) and of the euphausiid Thysanopoda aequalis (Hansen) indicated that all three species fed on a wide variety of phytoplankton, zooplankton, and detrital material. Diet changes generally reflected seasonal trends in phytoplankton community structure. However, species-specific feeding preferences and differences in feeding selectivity among the three species were evident, and in general agreement with feeding habits predicted from the analysis of mouthpart morphology. The euphausiid T. aequalis fed equally on more different food types compared to both copepod species. The copepod P. xiphias consumed a diverse assemblage of phytoplankton from late winter through the summer (78-93% of gut items, by number, were phytoplankton) and based its diet more strongly on carnivorous feeding in autumn and early winter (31% and 61% of gut items were phytoplankton, respectively). E. messinensis showed the greatest feeding specialization, with a strong preference for pennate diatoms in winter and spring and for coccolithophorids during late summer and fall (constituting 67-93% of gut items by number). All three species consumed diatoms more than other phytoplankton taxa, even though diatoms form only a small fraction of the phytoplankton biomass in the Sargasso Sea. Although the majority of gut items identified were phytoplankton cells, the relative biomass contribution of these small cells may be lower than that of zooplankton and detritus. Zooplankton on which the three species primarily preyed were protozoans and crustaceans, but also included other metazoans such as chaetognaths and cnidarians. Marine snow was also an important component of the diet in all three species, with typically >50% and rarely <20% of the gut content being olive-green debris. Marine snow from larvacean houses was found in the guts of all three species, while E. messinenis appeared to selectively consume marine snow aggregates enriched with bicapitate Nitzschia spp. Large cyanobacteria (>4 µm in diameter) found in guts were also likely consumed with marine snow. The species-specific differences in the diets of these three migrating species suggest that an individual species approach is important in determining how feeding habits affect the structure of pelagic food webs and carbon cycling in the sea. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-002-0815-8.     

Metabolism and body composition of a copepod (<Emphasis Type="Italic">Neocalanus cristatus</Emphasis>: Crustacea) from the bathypelagic zone of the Oyashio region,western subarctic Pacific

Metabolism [respiratory oxygen consumption, electron-transfer-system (ETS) activity] and body composition [water, ash, carbon (C), nitrogen (N), carbon/nitrogen (C/N) ratio] of stage C5/C6 Neocalanus cristatus from 1000 to 2000 m depth of the Oyashio region, western subarctic Pacific, were determined during the period of July 2000 through June 2003. Compared with the C5 specimens from shallow depths (<250 m), those from 1000 to 2000 m were characterized by quiescent behavior, reduced respiration rates (30% of the rates at active feeding), very low water content (61–70% of wet weight), but high C content (56–64% of dry weight) and C/N ratios (7.2–10.6, by weight). Artifacts due to the recovery of live specimens from the bathypelagic zone appeared to be unlikely in this study, as judged by the consistent results between re-compression (100 atm) and non-compression (1 atm) respiration experiments, and between ETS activities and respiration rates directly measured. In addition, the respiration rates of C6 males and females of N. cristatus from the same 1000–2000 m depth were two to three times higher than the rates of C5 individuals, but were similar to the rates of a bathypelagic copepod, Paraeuchaeta rubra. Combining these results with literature data, C budgets of: (1) diapausing C5 specimens, weighing 6–10 mg dry weight; (2) molt to C6 females; and (3) the complete the life span were established, taking into account assorted losses in respiration during diapause at stages C5 and C6, molt production and egg production. Respiratory C losses by C5 and C6 specimens were estimated on the basis of body N as adjusted metabolic rates [AMR; µl O₂ (mg body N)^–0.843 h^–1], then N budgets were also computed subtracting N lost in the form of cast molts and eggs from the initial stock. Calculations revealed that allocation of the C stock was greatest to egg production (34–57%), followed by respiration (27%) and cast molts (3%), leaving residual C of 13–36% in spent C6 females. The present results for N. cristatus from the North Pacific are compared with those of Calanus spp. in the North Atlantic.Communicated by O. Kinne, Oldendorf/Luhe     

Plankton of the fladen ground during FLEX 76 II. Population dynamics and production of Thysanoessa inermis (Crustacea: Euphausiacea)

Samples taken in the northern North Sea with the Continuous Plankton Recorder (CPR), the Undulating Oceanographic Recorder (UOR) and the Longhurst-Hardy Plankton Recorder (LHPR) during the Fladen Ground Experiment in 1976 (FLEX 76) are used to describe the vertical distribution and population dynamics of Thysanoessa inermis (Krøyer) and to provide estimates of the production and carbon budget of the population from 19th March to 3 June 1976. Spawning occurred in late April and early May, in near synchronisation with the start of the spring bloom of phytoplankton. Eggs, nauplii and calyptopes reached maximum abundance in succession, and furciliae were numerous when sampling ceased in early June. Adults increased in length from a mean of 12.1 mm in mid-March to 17.5 mm in early June and the estimated production was 2.40 mg m^-3 over the 74 d period. Total carbon ingested by the population of T. inermis was estimated to be 10 mg C m^-2 d^-1 in the upper 100m which was only 1.5% of the daily primary production of 0.68 gC m^-2 measured over the FLEX period 26 March to 4 June 1976. The grazing by T. inermis on the phytoplankton population was assumed to have little effect on the control and depletion of the spring phytoplankton bloom during FLEX 77.JOSDAP Contribution No. 50