Integrating conventional microscopy and molecular analysis to analyse the abundance and distribution of four Calanus congeners in the North Atlantic

Analysis of the demographic structure of Calanus species inthe North Atlantic presents particular difficulties due to theoverlapping spatial distributions of four main congeneric species(Calanus finmarchicus, Calanus helgolandicus, Calanus glacialisand Calanus hyperboreus). These species have similar morphologies,making microscopic discrimination only possible between someof the species at late copepodite or adult stages. However,molecular techniques now offer the possibility of screeningsignificant numbers of specimens and unambiguously identifyingthem to species, regardless of developmental stage. Unfortunately,the processing rate of specimens by molecular methods is stilltoo low to offer a realistic alternative to microscopy for analysisof samples from large field surveys. Here, we outline and testan approach involving the use of molecular methodology in conjunctionwith conventional microscopy to assess the species assignmentof developmental stage abundances of Calanus congeners. Ourstudy has highlighted many important methodological issues.First, it cannot be assumed that the species composition ishomogeneous across the development stages; applying proportionalspecies composition of adults to morphologically undistinguishableearlier development stages can result in error. The second importantconclusion is that prosome length may be a highly unreliablediscriminator of C. finmarchicus and C. glacialis.     

Body sizes,development rates,and genome sizes among Calanus species

Data on embryonic and larval development times (D) of Calanus species are analysed using Blehrádek's temperature (T) function, D = a (T – )^b, with b = – 2.05 as in previous studies. Among these species, for embryonic duration varies directly with temperatures in their geographical ranges and a is related to egg diameter. Using and b from embryonic durations, the fitted values of a for older stages are related to body sizes. Roughly estimated nucleus numbers in single adult females of C. finmarchicus, glacialis and hyperboreus were similar at 72 000, 85 000, and 96 000 respectively. Genome sizes (2C) of adult females are ca. 13 pg DNA in C. finmarchicus and pacificus, ca. 17 pg in C. sinicus, ca. 21 pg in C. helgolandicus and marshallae, and ca. 25 pg in C. glacialis and hyperboreus. These correspond roughly to body sizes and temperature-corrected development rates, quite precisely so in the sibling pair C. finmarchicus and C. glacialis, suggesting that, given similar nucleus numbers, there is nucleotypic control of whole-organism characteristics.     

Distribution of <Emphasis Type="Italic">Calanus</Emphasis> populations in a glaciated fjord in the Arctic (Hornsund,Spitsbergen)—the interplay between biological and physical factors

The distribution and diversity of copepods of the genus Calanus were investigated in Hornsund Fjord (on the southwest coast of Spitsbergen) in summer 2001. The Bhattacharya method was used to sort individuals by species based on their prosome length. The established prosome length boundary values for the Calanus copepodid stages coincided with those defined for the Calanus species from Kongsfjorden (on the northwest coast of Spitsbergen). The predominant species in the main and inner fjord basins was Calanus glacialis, whereas Calanus finmarchicus was the prevailing species outside Hornsund. Younger copepodid stages (CI–CIII) of both species concentrated in the surface water layers (0–50∼70 m), while older copepodids (CIV–CVI females) that were ready for wintering stayed in deep layers (50∼70 m to bottom). Calanus hyperboreus was present in low numbers, predominantly as CIV, and in Hornsund deep water layers. The distribution and diversity of Calanus species complied with the notion that the marine fauna in Hornsund is of a more Arctic character than in Kongsfjorden, a fjord 260 km to the north on the west coast of Spitsbergen.     

Vertical distribution of zooplankton biomass and species in northeastern Baffin Bay related to temperature and salinity

Summary Vertical distributions of various species and stages of zooplankton at different times of the day were determined by stratified sampling with the BIONESS in northeastern Baffin Bay during early August. The water column was divided into an upper subarctic zone (>0° C, salinity < 32), a lower subarctic zone (< 0° C, salinity 32 to 34) and a deep zone of Atlantic water (>0° C, salinity 34). The upper subarctic zone was dominated by two species of pteropod molluscs; the lower subarctic water was dominated by the copepods, Calanus finmarchicus, C. glacialis and C. hyperboreus whose copepodite stages showed depth distributions that were different from one another, with the copepodite stage 5 and adult females generally shallower than the younger stages. All stages of all Calanus species were in the zone of primary production (10 to 50 m) while the copepods Pseudocalanus, Metridia and Oithona were generally found below this zone. Only C. finmarchicus and C. glacialis showed evidence of diurnal migration, migrating to the surface waters when the sun was at its lowest position on the horizon (i.e. at 0100 h).     

Lipids and life strategies of Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus in late autumn, Kongsfjorden, Svalbard

Stage IV and V copepodites were the dominant forms of Calanus finmarchicus, C. glacialis and C. hyperboreus in Kongsfjorden in late September 1997. Stage IV and V copepodites of C. glacialis and C. hyperboreus were rich in lipid, largely wax esters, and were well fitted to overwinter. Stage IV copepodites of C. finmarchicus were also rich in wax esters, but stage V copepodites of C. finmarchicus were less wax ester-rich. Large size increments between stage IV and V copepodites and between stage V copepodites and females were noted in C. finmarchicus. A very large increment between stage IV and V copepodites was noted for C. glacialis but the size difference between stage V copepodites and females was very small in this species. Particularly large increments were noted between stage IV and V copepodites of C. hyperboreus and also between stage V copepodites and females of this species. The very large, wax ester-rich C. hyperboreus is well adapted to survive the most extreme variations in the Arctic, in Arctic basin waters, whereas the smaller, wax ester-rich C. glacialis is adapted to survive less extreme Arctic variations, as in Arctic shelf waters. The smallest of the three, C. finmarchicus, is best adapted to survive the more predictable waters of the North Atlantic and the Barents Sea. Accepted: 3 January 2000     

An evaluation of factors affecting vertical distribution among recruits of Calanus finmarchicus in three adjacent high-latitude localities

The vertical distributions of populations of Calanus finmarchicus are described in three different fjord areas near Tromse, northern Norway during May 1986. These localities (Malangen, Grøtsund and Balsfjorden) had characteristic differences in temperature, phytoplankton and population density of copepods. They probably are representative annual situations during the spring and summer period for coastal and fjord areas in northern Norway. Copepodite stage I and II C. finmarchicus are found in the surface waters (0–30 m) during a 24 h cycle, while the other stages appear to have a different diel depth distribution in Malangen. Pronounced differences in the depth distribution of the various copepodite stages and adult females were found in Grøtsund and Balsfjorden during the same period of the day on 20 and 21 May. The tendency for vertical overlap among CI–CV was clearly less pronounced in an environment with low phytoplankton standing stock and high population density of copepods. The patterns of vertical distribution are analysed by multidimentional scaling (MDS) and it is evident that the distribution pattern of C. finmarchicus is different at each locality. These preliminary results, are discussed in relation to ontogenetic vertical migration and aspects of resource partitioning and the possible importance of vertical separation for reducing competitive interactions between the different life stages of C. finmarchicus.     

The effects of temperature on metabolic rates of different life stages of Calanus glacialis in the Barents Sea

Summary The effects of temperature on rates of respiration, excretion and gut evacuation were examined for copepodite stages and adult female Calanus glacialis collected in areas close to the ice-edge during the arctic summer in the Barents Sea. The various life history stages responded differently to acute temperature changes above the in situ temperature (ca.-1.7°C). Respiration rates of early copepodite stages (C I to C IV) were very variable whereas excretion rates declined with increasing temperature in the range from-1.7°C to +5°C. Rate of oxygen consumption of adult females were independent of temperature between-1.7° and +5°C, but increased as temperature increased from +5° to 10°C. Rates of excretion of copepodite stage V and adult females were independent of temperature in the range from-1.7° to +2°C, whereas excretion of copepodite stages III and IV was negatively related to temperature in the range from-1.7° to +5°C. In C IV, C V and adult females the instantaneous rate of gut evacuation increased with increasing temperature. The different response patterns of metabolic rates of small copepodite stages, copepodite stage V and adult females C. glacialis to acute temperature changes suggest that the capacity for adjustment of ammonia excretion is better developed in C V's and adult females than in the younger life stages.     

Distribution of dominant calanoid copepod species in the Greenland sea during late fall

Summary Between 6 November and 12 December 1988, vertical distributions of Calanus finmarchicus, C. hyperboreus, C. glacialis and Metridia longa were studied at three stations in the Arctic water of the Greenland Sea Gyre (GSG) and compared with two stations in the Atlantic water (AW) of the Westspitsbergen Current. Nine depth strata down to 3,000 m were sampled. C.finmarchicus was most abundant in AW, C. hyperboreus in GSG, M. longa showed no preference and C. glacialis was rare everywhere. Stage composition differed with species and water mass. Vertical distribution varied also with water mass in all species but C. hyperboreus, which was always centered between 1,000 and 1,500 m. The other species were concentrated in the upper 300 m in AW and between 1,000 and 1,500 m in GSG, although not all stages followed this trend. Ontogenetic vertical migration leads to significant dislocations of organic matter from the euphotic zone to great depth in the Greenland Sea. In egg production experiments, C. hyperboreus spawned up to 149 eggs female^–1 day^–1. Implications of stage composition, sex ratio, and gonad maturation on copepod life cycles are discussed.     

Protein polymorphisms in six species of the genus Calanus

Isoelectric focusing was used to study total protein patterns and allozyme variations of six species of the genus Calanus. Each species could be characterized by total protein patterns. The results of the allozyme study indicated, in agreement with previous morphological studies, that the six Calanus species belong to three different groups: the C. finmarchicus group C. finmarchicus, C. glacialis and C. marshallae the C. helgolandicus group (C. helgolandicus, C. pacificus), and C. hyperboreus, which stands apart. There is no indication that there are more loci coding for the proteins studied in species with larger genome sizes. Nor is the degree of enzyme polymorphism related to genome size in these species.     

Density- and sound speed contrasts in sub-Arctic zooplankton

Summary The sound speed was determined for Meganyctiphanes norvegica, for a mixture of Thysanoessa raschii and Thysanoessa inermis and for a mixture of Calanus finmarchicus and Calanus hyperboreus. The sound speed contrasts ranged from 1.014 to 1.044. Seasonal variations in specific density were measured for Thysanoessa inermis, Thysanoessa raschii, Meganyctiphanes norvegica, Calanus finmarchicus and Calanus hyperboreus. The density of 20 mm T. inermis was lowest in November (1.052 g/cm³) and highest in February–March (1.065 g/cm³). For a 20 mm T. raschii the minimal density was determined in December (1.059 g/cm³) and the maximum in February–March (1.074 g/cm³). M. norvegica individuals of 35 mm also had their lowest density in December (1.060 g/cm³), but reached their maximum density in July (1.076 g/cm³).The density of the euphausiids was found to be size dependent. The density increases as the size decreases. C. finmarchicus and C. hyperboreus had densities less than seawater (1.026 g/cm³) during most of the year. Just before spawning the density increased to 1.028 g/cm³ and 1.036 g/cm³ for C. finmarchicus and C. hyperboreus respectively. The seasonal variations of the density were closely related to the lipid content of the animals.     

Spatial distribution of digestive enzyme activities of Calanus finmarchicus and C.hyperboreus in Fram Strait/Greenland Sea

Digestive enzymes of copepodite V (CV) Calanus finmarchicusand C.hyperboreus from two different depths were compared duringMIZEX 1984 (June/July) at stations in the ice and open water.CV of both species from 500–200 m showed reduced enzymeactivity, indicating that they were in a resting stage. In moultingexperiments at the end of June using CV from 100–0 m moultingwas delayed and began only after 3 weeks in C.finmarchicus andafter 3 months in C.hyperboreus. These results suggest thatthe deep CV populations are the seed of the new overwinteringstock. In surface CV C.finmarchicus and C. hyperboreus enzymeactivities were generally much higher than in deep CV. In neitherspecies were enzyme activities correlated with chlorophyll concentrations.Activities in C.finmarchicus reflected overall phytoplanktondistribution and were highest in the marginal ice zone, whereasthey decreased under the ice except for polynya stations onthe East Greenland Shelf. In surface C.hyperboreus digestiveenzymes were not correlated with those of C.finmarchicus, pointingto different diets or regulatory mechanisms. Enzyme activitywas lowest in the marginal ice zone and increased under theice. High activities were found at polynya stations and otherclose pack ice. The utilization of ice algae by C.hyperboreuscould explain these discrepancies in digestive enzyme activitiesof both species.     

Assimilation and biosynthesis of lipids in Arctic Calanus species based on feeding experiments with a C labelled diatom

The dominant Arctic Ocean and North Atlantic copepods Calanus hyperboreus, Calanus glacialis, and Calanus finmarchicus were collected in the Greenland Sea and fed ¹³C labelled diatom Thalassiosira weissflogii to follow the transfer and assimilation of carbon, lipid, and individual fatty acids and alcohols. The diatom was grown with ¹³C for 3 to 5 days and fed then to the copepods. During the feeding period of 14 days, total carbon increased in the copepodite stages V of C. hyperboreus and C. finmarchicus, whereas carbon remained almost constant in C. glacialis females. However, total lipid increased in all species and stages. Highest lipid accumulation occurred in C. hyperboreus in which nearly all lipids were exchanged already after 11 days of feeding. In the other species lipid accumulation made up between 22% (C. finmarchicus) and 45% of total lipid (C. glacialis). The proportion of wax esters was high ranging from 76% of total lipid in C. glacialis to 92% in C. finmarchicus. The fatty acid composition of the alga was dominated by 16:1(n-7), 16:0, 20:5(n-3), and 22:6(n-3). The composition of the copepods was similar because of feeding already on diatoms in the field. In addition, the monounsaturated fatty acids and alcohols, 20:1(n-9) and 22:1(n-11), were major components of the copepod lipids. During the feeding period the highest ¹³C labelling was always found in the C₁₆ polyunsaturated fatty acids and in the 16:1(n-7) alcohol. Because these components occurred only in trace amounts in the copepods they totally originated from the diet explaining the high labelling. It is noteworthy that the 16:1(n-7) alcohol originated only from the corresponding dietary and not from the abundant internal fatty acid. The long-chain monounsaturated fatty acids and alcohols, 20:1(n-9) and 22:1(n-11), are not existent in phytoplankton and have to be produced de novo. They were less labelled in the smaller species but highly ¹³C enriched in C. hyperboreus. Although dietary fatty acids were generally retained by the copepods it seems that fatty acids or even lipids were selectively accumulated and turned over due to bodily requirements, and thus, essential polyunsaturated fatty acids were preferentially retained. During feeding mixing, accumulation, and exchange of internal and dietary fatty acids and alcohols occurred as well as utilisation of lipids from both sources for metabolic requirements. The differences in lipid assimilation fit to the different life strategies of the copepods.     

Warming of Subarctic waters accelerates development of a key marine zooplankton Calanus finmarchicus

Recent observations confirm the rising temperatures of Atlantic waters transported into the Arctic Ocean via the West Spitsbergen Current (WSC). We studied the overall abundance and population structure of the North Atlantic keystone zooplankton copepod Calanus finmarchicus, which is the main prey for pelagic fish and some seabirds, in relation to selected environmental variables in this area between 2001 and 2011, when warming in the Arctic and Subarctic was particularly pronounced. Sampling within a 3‐week time window each summer demonstrated that trends in the overall abundance of C. finmarchicus varied between years, with the highest values in “extreme” years, due to high numbers of nauplii and early copepodite stages in colder years (2001, 2004, 2010), and contrary to that, the fifth copepodite stage (C5) peaking in warm years (2006, 2007, 2009). The most influential environmental variable driving C. finmarchicus life cycle was temperature, which promoted an increased C5 abundance when the temperature was above 6°C, indicating earlier spawning and/or accelerated development, and possibly leading to their development to adults later in the summer and spawning for the second time, given adequate food supply. Based on the presented high interannual and spatial variability, we hypothesize that under a warmer climate, C. finmarchicus may annually produce two generations in the southern part of the WSC, what in turn could lead to food web reorganization of important top predators, such as little auks, and induce northward migrations of fish, especially the Norwegian herring.     

Pristane in the marine environment

Summary 1. Methods are described for the sensitive determination of pristane and similar hydrocarbons in individual planktonic organisms and for the isolation of pristane from copepod oil.2. Pristane (2, 6, 10, 14-tetramethylpentadecane) occurs in unusually high concentrations (1–3% of the body lipid) inCalanus finmarchicus, C. glacialis, andC. hyperboreus, and at lower concentrations in a wide range of planktonic animals from the Gulf of Maine and continental slope waters.3. A predator,Paraeuchaeta norvegica, contains pristane at an intermediate level between that ofCalanus and the other herbivores, probably as a result of feeding onCalanus.4. On the basis of relative abundance and structural similarity, phytol is suggested as the precursor of pristane in herbivorous zooplankton.5. Pristane content is unaltered or increased during the metabolism of the major deposit lipids ofCalanus hyperboreus during starvation in the laboratory or in nature. Because of its low density, pristane may contribute to the bouyancy ofCalanus, especially when the other lipids are metabolized.6. Pristane may prove useful as a biochemical integrator for the total assimilation of phytoplankton byCalanus.7. Some planktonic organisms may be characterized by the presence of specific compounds, e. g. pristane inCalanus and several unknown compounds inRhincalanus.8. Specific products of organisms of limited geographical occurence may prove useful as biochemical tags of water masses.

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Pristan in der marinen Umwelt

Kurzfassung Pristan (2, 6, 10, 14-Tetramethylpentadecan) kommt in niedriger Konzentration in manchen planktonischen Tieren des Golfs von Maine, USA, und der Gewässer des kontinentalen Schelfes vor. Dieser Kohlenwasserstoff leitet sich vermutlich vom Phytol in der Nahrung der planktonischen Herbivoren ab. Seine Konzentration ist ungewöhnlich hoch inCalanus finmarchius, C. glacialis undC. hyperboreus, wo er möglicherweise als auftriebsregulierende Substanz in Hungerzeiten wichtig ist. Arten, wieParaeuchaeta norvegica, welcheCalanus verzehren, stellen sekundäre Pristanquellen innerhalb der marinen Nahrungskette dar. Die proCalanus-Individuum angetroffene Pristanmenge könnte als Indikator für die gesamte Nahrungsassimilation dienen. Pristan und andere artspezifische Stoffe können in das die Individuen umgebende Wasser gelangen und dann als biochemische Markierungs-Substanzen dienen.

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Contribution No. 1438 of the Woods Hole Oceanographic Institution.     

Reproduction of Calanus glacialis in the Laptev Sea, Arctic Ocean

Abundance and reproductive biology (gonad maturation and egg production) of the Arctic copepod Calanus glacialis were studied in the Laptev Sea and adjacent Arctic Ocean in September 1993 and from July to September 1995. Both abundance and reproductive activity were subject to strong spatial and seasonal variability, which was related to the ice cover, feeding conditions and circulation pattern. Maximum abundance of the C. glacialis population was generally confined to the outer shelf and slope with depths between 50 and 1000 m. During both cruises, highest egg production rates and largest number of young copepodite stages were observed in the eastern Laptev Sea, where the development of the C. glacialis population seems to follow the opening of the “Siberian Polynya”. In the western part, which is usually covered by pack ice, females were all immature, and no young stages were found. However, females responded quickly to a temporary opening of the ice there in 1995 and spawned. Starvation experiments showed that food-independent reproduction fuelled by internal energy resources was at least partly responsible for relatively high egg production rates at low ambient food concentrations. Egg production rates in starved females were considerably higher than those previously reported. Accepted: 14 June 2000     

Differences in food delivered to chicks by males and females of little auks ( Alle alle ) on South Spitsbergen

Sex differences in the quality and quantity of food loads brought to little auk (Alle alle) chicks were investigated in a large colony in Hornsund (South Spitsbergen). Adults returning to the colony were caught in mist-nets and food loads were taken from their gular pouch. The sex of each bird was determined by means of molecular methods. Females brought significantly more food per single load than males in terms of wet weight (30% more on average), number of prey items (46%) and energy contents (39%). However, there was no difference between the sexes in the size of prey taken. Energy-rich Calanus glacialis, originating from cold Arctic waters, was the most frequent prey item and made up the majority of food loads brought by both males and females (75 and 72%, respectively). This indicates that both sexes forage mainly in Arctic waters. However, differences in the proportion of cold water Calanus species (C. glacialis and C. hyperboreus), warm water Calanus species (C. finmarchicus), as well as other taxa, between males and females may suggest different ways of exploiting the feeding area.     

In a comfort zone and beyond—Ecological plasticity of key marine mediators

Copepods of the genus Calanus are the key components of zooplankton. Understanding their response to a changing climate is crucial to predict the functioning of future warmer high‐latitude ecosystems. Although specific Calanus species are morphologically very similar, they have different life strategies and roles in ecosystems. In this study, C. finmarchicus and C. glacialis were thoroughly studied with regard to their plasticity in morphology and ecology both in their preferred original water mass (Atlantic vs. Arctic side of the Polar Front) and in suboptimal conditions (due to, e.g., temperature, turbidity, and competition in Hornsund fjord). Our observations show that “at the same place and time,” both species can reach different sizes, take on different pigmentation, be in different states of population development, utilize different reproductive versus lipid accumulation strategies, and thrive on different foods. Size was proven to be a very mutable morphological trait, especially with regard to reduced length of C. glacialis. Both species exhibited pronounced red pigmentation when inhabiting their preferred water mass. In other domains, C. finmarchicus individuals tended to be paler than C. glacialis individuals. Gonad maturation and population development indicated mixed reproductive strategies, although a surprisingly similar population age structure of the two co‐occurring species in the fjord was observed. Lipid accumulation was high and not species‐specific, and its variability was due to diet differences of the populations. According to the stable isotope composition, both species had a more herbivorous diatom‐based diet in their original water masses. While the diet of C. glacialis was rather consistent among the domains studied, C. finmarchicus exhibited much higher variability in its feeding history (based on lipid composition). Our results show that the plasticity of both Calanus species is indeed impressive and may be regulated differently, depending on whether they live in their “comfort zone” or beyond it.     

Chick diet of dovekies Alle alle in Northwest Greenland

This paper reports the results of an investigation of dovekies (Alle alle) breeding at Hakluyt Island in the Thule District, Northwest Greenland. The paper presents a comprehensive examination of the chick diet of dovekies in the species' most important breeding area. Food delivered to nestlings consisted primarily of copepods Calanus hyperboreus and C. glacialis in the copepodite stages IV and V. Themisto libellula, other amphipods, decapods and fish larvae in small numbers were found in the chick diet. The zooplankton in the chick diet is presented both in terms of numbers and dry weight. The characteristic copepod of high-arctic waters, C. hyperboreus, was found to be the single most important species in the chick diet, contributing approximately 54% of the dry food weight. The food samples revealed low diversity, and the three species C. hyperboreus, C. glacialis and T. libellula made up 91.5% of the total zooplankton dry weight. The results from this paper are compared with previous dovekie-chick diet studies. Accepted: 14 June 2000