Turnover and vertical transport of zinc by the euphausiid Meganyctiphanes norvegica in the Ligurian Sea

Participatory turnover time is defined as the time required to cycle an element in a system through a given material in that system. The participatory turnover time of ionic zinc by the adult Meganyctiphanes norvegica population in the Ligurian Sea ranged between 498 and 1243 years, depending upon the available food supply, and considering the food chain as the only route for zinc accumulation by the population. A total-impact turnover time was calculated as the sum of the participatory turnover time for live individuals plus the time required for dead euphausiids to lose 90% of their zinc to the water. Carcasses lost zinc to the water slower than either feces or molts, and so established the maximum loss time for all particulate excretion products; nevertheless, total-impact turnover time for zinc did not differ significantly from the participatory turnover time. The net vertical transport of zinc by M. norvegica from the sea surface to any specified depth can be calculated as the sum of the dissolved zinc excreted below the depth plus the concentrations of zinc left in feces, molts, and carcasses after they have sunk to the specified depth. Carcasses sink the fastest and lose the smallest fraction of their zinc concentration during descent; fecal pellets sink the slowest and lose the greatest fraction of their zinc concentration, and molts are intermediate. Nevertheless, feces represents the major route for delivering zinc to the bottom of the Ligurian Sea (2500 m), because concentration of the element in the pellets is so much higher than in carcasses or molts. Excretion of dissolved zinc into the water at the vertical migration depth of the living population during daylight hours was also inconsequential. Feces zinc represented over 80% of the total zinc transported to the sea floor if only marginal food supplies were available to the euphausiids, and over 90% if food was in sufficient supply. M. norvegica can effect a net transport of about 98% of its body zinc concentration below 500 m daily, in conditions of sufficient food supply and assuming that no released products are eaten during descent. If the food supply in the Ligurian Sea is considered only marginal throughout the year, M. norvegica can still effect a daily net transport below 500 m of about 36% of its body concentration, and about 6% of its body concentration will reach 2500 m daily.     

Meganyctiphanes norvegica in the Kattegat

From 1979 to 1983 a population ofMeganyctiphanes norvegica was regularly found in a confined location in the northern Kattegat. The development of this pelagic population was followed over one complete year-cycle, and is discussed in light of published data on landlocked and oceanic populations. In the Kattegat,M. norvegica mature at the age of 9 to 12 months. Copulation starts in january, but fully developed spermatophores are present in males for at least 9 months. Spawning starts in April and continues until October with a marked peak in July. As only 10 to 15% of the gravid females belong to the II-group, it is concluded that recruitment is due to the I-group. Linear growth is fastest in spring and early summer, rises to a second maximum in October and stagnates during winter. The weight development shows some discontinuties caused by spawning or the accumulation of reserves towards winter. Weight decreases during winter. Growth in length and in weight runs largely parallel in both sexes, and can be closely correlated with the prevailing food supply. Diurnal vertical migration could be demonstrated. The results indicate a continuous development, which suggests the existence of an autonomous palagic population, a well suited subject for long-term investigation.     

On the timing of moulting processes in reproductively active Northern krill Meganyctiphanes norvegica

The interactions between moult phasing, growth and environmental cues in Northern krill (Meganyctiphanes norvegica) were examined through analysing populations at seasonal, weekly, and daily timescales. The analyses were carried out on resident populations of krill found in three different neritic locations that experience similar environmental signals (the Clyde Sea, Scotland; the Kattegat, Denmark; Gullmarsfjord, Sweden). Seasonal analyses were carried out on the Clyde Sea population and showed that moulting frequency increased significantly moving from winter to summer. The proportion of moulting females in summer samples was often more than double the proportion of moulting males, suggesting that females had a comparatively shorter intermoult period (IMP). Weekly samples taken from the Kattegat showed a similar pattern. However, although the difference between the proportion of female and male moulters was significant in one week, it was not another, mainly because of the variability in the proportion of female moulters. Such variability in females was equally evident in the daily samples taken at Gullmarsfjord. It suggests that females have a shorter IMP (12.5 days) than males (18.4 days) and are more likely to moult in synchrony. Nevertheless, the daily samples revealed that males are also capable of moult synchronisation, although less frequently than females. Shortened IMPs in females were not a result of the abbreviation of specific moult stages. Accordingly, reproductive activity did not alter the course of the normal moult cycle. There was no significant difference between the total body lengths of males and females indicating that females achieve the same levels of growth despite moulting more frequently and having to provision the energy-rich ovaries. This is in contrast to most other crustaceans where the energy costs of reproduction reduce female growth. The fact that females were less abundant than males, probably by suffering a greater level of mortality, suggests that different behavioural strategies, particularly vertical migration regimes, were adopted by each sex to maximise growth and reproduction.     

Functional feeding morphology of the euphausiid Nyctiphanes australis

The structure of the feeding basket, mandible and stomach armature of the krill Nyctiphanes australis (G. O. Sars, 1883) was examined by scanning electron microscope with the aim of predicting its diet. N. australis were collected during February 1986 and October 1986 to February 1987 in Otago Harbour, New Zealand (45° 50 S; 170° 37 E). Predictions based on the functional morphology were tested by examining stomach contents with SEM. Intersetule distances of the feeding basket (1 to 7.5 m) are finer than in other krill species, suggesting that N. australis can efficiently collect nanoplankton-sized particles (2 to 20 m). The mandibular edge index (0.74) matched the edge index of Meganyctiphanes norvegica. This indicates, in contrast to the fine feeding-basket setulation, that N. australis has a mandible resembling that of predominantly carnivorous krill species. The ratio of mandibular palp length to mandible width is 3.2±0.2, or relatively longer than the elongate palp thought to be indicative of herbivorous habit in Euphausia superba. The fine structure of the molar shows specialized surfaces which differ from those of other krill species. The internal armature of the stomach is heavily spinose, as is common in krill of herbivorous habit. Layers of various-shaped spines at differing densities were observed. The functional morphology suggests that N. australis is an opportunistic omnivore. The stomach contents tentatively support this prediction, containing fragments of phytoplankton and detrital material. However, stomach contents were generally amorphous, making identification of dietary components in the field difficult.     

Diel vertical migrations and nocturnal feeding of a dense coastal krill scattering layer (Thysanoessa raschi and Meganyctiphanes norvegica) in stratified surface waters

To study the nocturnal feeding of euphausiids during vertical migrations and its impact on the phytoplankton, a phytoplankton-rich water mass (drogue marked) drifting over a dense krill scattering layer (acoustic 104kHz) in the lower St. Lawrence estuary was monitored over 46 h in July 1982. Phytoplankton >20 m was abundant and mostly concentrated at the bottom of the photic layer above the pycnocline. Less than 42% of the particulate carbon was due to phytoplankton. The krill scattering layer was about 2 to 3 km in width, elongated along the 100-m bathymetric contour, and absent when the bottom was shallower than 50 m. Its upper day depth was 50 m. At deeper depths, its vertical distribution frequently changed from unimodal to polymodal shapes and viceversa, often with large concentrations of zooplankton just above the bottom. Typical vertical migrations were observed on both days. At night the scattering layer had a lower scattering strength. Most of it was below the thermocline but net catches showed that large concentrations of euphausiids (up to 57 individuals m^-3) crossed it. Stomach pigment content of Thysanoessa raschi was generally low, but mean stomach fullness was always high. They were more opportunistic than herbivorous. From stomach fullness and the presence of a food bolus in mouth parts, feeding in surface waters appeared to be intensive, but gut content indicated that food was not processed there. It is therefore suggested that individuals underwent vertical interchanges across the thermocline while feeding during the night. Meganyctiphanes norvegica had significant herbivorous activity during the night. The grazing pressure impact of the scattering layer on phytoplankton was negligible.Contribution to the program of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Observations sur le développement larvaire de Meganyctiphanes norvegica (Crustacea: Euphausiacea) au laboratoire

Larvae of Meganyctiphanes norvegica (M. Sars), caught with a plankton net, were reared in the laboratory from the calyptopis phase onwards, under different temperature and trophic conditions. Eyestalk ablation was performed on the first furciliae. Data are presented on growth, moulting rate and ontogenesis of the larvae as a function of the varying experimental conditions, and on larval morphology, pigmentation and diet. Recent modifications of larval nomenclature in euphausiids are discussed.

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Observations sur le développement larvaire de Meganyctiphanes norvegica (Crustacea: Euphausiacea) au laboratoire

     

The three-dimensional prey field of the northern krill, Meganyctiphanes norvegica, and the escape responses of their copepod prey

In the north Atlantic, Meganyctiphanes norvegica feeds predominantly on copepods, including Calanus spp. To quantify its perceptual field for prey, and the sensory systems underlying prey detection, the responses of tethered krill to free-swimming Calanus spp. were observed in 3D using silhouette video imaging. An attack–which occurred despite the krill’s being tethered—was characterized by a pronounced movement of the krill’s antennae towards the target, followed by a propulsion and opening of the feeding basket. Frequency distributions of prey detection distances were significantly different in the light vs. the dark, with median values of 26.5 mm and 19.5 mm, respectively. There were no significant differences in the angles at which prey were detected by krill (relative to the predator’s longitudinal body axis) in the light vs. the dark. Prey detections were symmetrically distributed on either side of the predator, in both light and dark. However, significant asymmetry was found in the dorsal–ventral direction with 80% of the prey detections located below the midline of the krill’s body axis and, given the placement and orientation of the compound eyes, presumably outside its visual field of view. This indicates that, at least under these conditions, vision was not the main sensory modality involved in the detection of active prey by M. norvegica. However, under some circumstances, vision may provide supplemental information. Avoidance responses of copepod prey were nearly twice the velocity of their nominal background swimming speed (153 ± 48 and 85 ± 75 mm s⁻¹, respectively), on average taking them 43 ± 16 mm away from the predator. This is far beyond the krill’s perceptual range, suggesting that the escape reaction provides an effective deterrent to predation (although perhaps less so for free-swimming krill). This information can be used to parameterize models that assess the role of krill as predators in marine ecosystems.     

Metabolic properties of Northern krill, Meganyctiphanes norvegica, from different climatic zones. I. Respiration and excretion

Adaptive processes linked to overall metabolism were studied in terms of oxygen consumption and ammonia excretion in each of three self-contained krill populations along a climatic gradient. In the Danish Kattegat, krill were exposed to temperatures which ranged from 4°C to 16°C between seasons and a vertical temperature gradient of up to 10°C during summer. In the Scottish Clyde Sea, water temperatures varied less between seasons and the vertical temperature gradient in summer was only 3°C. Temperatures in the Ligurian Sea, off Nice, were relatively constant around 12-13°C throughout the year, with a thin surface layer (20-30 m) of warm water developing during summer. The trophic conditions were rich in the Kattegat and, particularly, in the Clyde, but comparatively poor in the Ligurian Sea. Oxygen consumption increased exponentially with increasing experimental temperature, which ranged from 4°C to 16°C. Overall respiration rates were between 19.9 and 89.9 µmol O₂ g^-1 dry wt h^-1. Krill from the Kattegat, the Clyde Sea, and the Ligurian Sea all exhibited approximately the same level of oxygen consumption (30-35 µmol O₂ g^-1 dry wt h^-1) when incubated at the ambient temperatures found in their respective environments (9°C, 5°C, and 12°C). This indicates that krill adjust their overall metabolic rates to the prevailing thermal conditions. The exception to this were the respiration rates of Ligurian krill from winter/spring, which were about twice as high as the rates from summer krill despite the fact that the thermal conditions were the same. This effect appears to result from enhanced somatic activity during a short period of increased food availability and reproduction. Accordingly, krill appears to be capable of adapting to both changing thermal and trophic conditions, especially when nutrition is a limiting factor in physiological processes.     

Genetic differentiation in a pelagic crustacean (Meganyctiphanes norvegica: Euphausiacea) from the North East Atlantic and the Mediterranean Sea

Meganyctiphanes norvegica (M. Sars)is a pelagic crustacean that plays a key role in marine food webs of North Atlantic Ocean and marginal seas. We studied eight population samples collected in the European Atlantic and Mediterranean Sea. By means of single strand conformation polymorphism analysis (SSCP) and direct sequencing, we investigated a segment of 158 base pairs of the mitochondrial gene coding for the subunit 1 of NADH dehydrogenase. We found 12 sequence variants among the 385 individuals studied. Analysis of molecular variance (AMOVA) showed that 14.75% of the total genetic variability was explained by differences between populations, thus indicating absence of panmixia for these populations. Pairwise comparisons revealed three distinct genetic pools: the first one represented by Cadiz Bay, the second one by the Ligurian Sea, and the third one included all the NE Atlantic samples. We also investigated one population from the Alboran Sea (within the Mediterranean basin, east of the Strait of Gibraltar). This population was found to be genetically intermediate between the NE Atlantic samples and the Ligurian sample, suggesting that the restriction to the gene flow is not associated with the Strait of Gibraltar, but possibly with the Oran–Almeria oceanographic front. The present work indicates that M. norvegica, although endowed with a high dispersal capacity because of its pelagic habit, can develop separate breeding units inside the same oceanic basin (the Atlantic). Furthermore, the Ligurian sample should be considered as a distinct evolutionary entity, separated from the Atlantic population. Received: 2 May 1999 / Accepted: 26 November 1999     

Metabolic properties of Northern krill, Meganyctiphanes norvegica, from different climatic zones. II. Enzyme characteristics and activities

Activities and characteristics of two metabolic key enzymes, citrate synthase (CS) and pyruvate kinase (PK), were studied in the Northern krill, Meganyctiphanes norvegica, with respect to adaptive properties under different thermal conditions. Krill were sampled during late winter/spring and summer from the constantly warm Ligurian Sea (12-13°C below the thermocline), the colder but also comparatively constant Clyde Sea (7-8°C), and the variable Kattegat (2-16°C). Both enzymes showed distinct tissue- and organ-specific activities, which were highest in the pleopods - the principal locomotive organs. The fourth and fifth abdominal segments, however, were used for routine investigation due to lowest variability. Specific activity of CS and PK did not differ between seasons in krill from the Kattegat or the Clyde Sea. In the Ligurian Sea, in contrast, specific CS activities were significantly lower during summer. Analysis of individual data illustrated a decrease of CS activity with size and an increase of PK activity with size. Taking these allometric effects into account, as emphasized by calculating the ratio between both enzymes, variation of CS and PK activities turned out to be solely dependent on body size, which differed between locations and seasons. Ligurian krill from the summer, however, were unique in that they showed a lower CS/PK ratio than would be predicted by the scaling effect. Thermal characteristics of each enzyme were similar between locations and seasons. During the winter, in Kattegat and Clyde Sea krill, K_m values (Michaelis-Menten constant) of CS towards acetyl-coenzyme A exhibited an almost constant level over the experimental temperature range of 4-16°C. During summer, however, K_m values were lower at 8°C in the Clyde Sea and at 12°C in the Kattegat. In Ligurian krill from the summer, K_m values were consistently lower than those of winter krill over the entire experimental temperature range. In conclusion, Kattegat and Clyde Sea krill show only minor adaptations to their respective thermal environments in terms of CS and PK characteristics. Ligurian krill, in contrast, exhibited decreased specific CS activity during summer, which might be compensated by elevated enzyme-substrate affinity as indicated by lower K_m values. Since temperature was constant during both seasons, this effect cannot be explained as a reaction to thermal conditions. Consequently, oligotrophic conditions in the Ligurian Sea during summer may entail a reduction in the somatic performance of krill, which is reflected by lower CS activity.     

Ovarian development and spawning in relation to the moult cycle in Northern krill, Meganyctiphanes norvegica (Crustacea: Euphausiacea), along a climatic gradient

Adaptive processes linked to reproduction were studied comparatively for three populations of Northern krill, Meganyctiphanes norvegica (M. Sars, 1857), sampled during winter and summer cruises in the Clyde Sea (W Scotland), the Kattegat (E Denmark), and the Ligurian Sea (NW Mediterranean). The aim was to investigate the functional relationship between egg production and moulting under contrasted climatic and environmental conditions. A staging system for female sexual development established for live krill was complemented by a histological study of the ovary at various developmental steps. During the reproductive season, all adult female krill were engaged in cyclical egg production. During experiments, female krill released one batch of mature oocytes in one or two spawning events. The ovary of postspawn female krill still contained developing oocytes for another egg batch. In the non-reproductive period, all female krill had a resting ovary. Ovarian structure and pattern of egg production were identical in the three populations, but seasonal timing of egg production was different. The model proposed for the Ligurian population of the annual cycle of ovarian development can be extended to the other two populations, taking into account the seasonal characteristics of each site. Random field samples were staged simultaneously for moult cycle and for sexual development. Moult stages and the seasonal variation of the intermoult period were studied for the Kattegat population using multi-year data and compared to data obtained during summer/winter cruises in the Clyde and the Ligurian Sea. At the three sites, intermoult period was shorter and temperature-dependent during the reproductive period, concurrent with the season of greatest food availability. During most of the year and the period of sexual rest, moulting activity was reduced. The relationship between spawning and the moult cycle was studied comparatively for the three populations. Eggs were released during the premoult phase of a “spawning moult cycle”, in one or two spawnings associated with apolysis and Moult Stage D1, respectively. Yolk accumulation for the next egg batch was completed during an alternating “vitellogenic moult cycle”. A model for the timing of cyclical egg production in relation to moulting, as proposed for the Kattegat, can be extended to the other populations, taking into account intermoult period variation with temperature. Temperature appeared to be the principal environmental factor controlling growth (through moulting) and egg production during the reproductive season, in connection with favourable trophic conditions. Received: 22 December 1997 / Accepted: 29 August 1998     

Ecological investigations on the zooplankton community in balsfjorden,northern Norway: Lipids and fatty acids in Meganyctiphanes norvegica,Thysanoessa raschi and T. inermis during mid-winter

Total lipid of Meganyctiphanes norvegica (M. Sars) contained 53% triacylglycerols and traces of wax esters, that of Thysanoessa raschi (M. Sars) contained 44% triacylglycerols and 10% wax esters and that of T. inermis (Krøyer) contained 28% triacylglycerols and 40% wax esters. The triacylglycerols of M. norvegica were relatively rich in 20:1 and 22:1 fatty acids and its traces of wax esters resembled those of calanoid copepods. The triacylglycerols of both Thysanoessa species were deficient in 20:1 and 22:1 fatty acids but were richer in 16:1(n-7) and 18:1 (n-7) acids than those of M. norvegica. The wax esters of T. raschi contained phytol as almost the only fatty alcohol and were rich in 16:0 and 18:1 (n-9) fatty acids. The wax esters of T. inermis contained mainly 16:0 and 14:0 fatty alcohols with lesser amounts of phytol and their dominant fatty acid was 18:1, especially the (n-9) isomer. The triacylglycerols of T. inermis had 18:4 (n-3) as the major polyunsaturated fatty acid. From these and other aspects of fatty acid and fatty alcohol analyses it is concluded that a major foodstuff of M. norvegica in Balsfjorden is wax ester-rich calanoid copepods. T. raschi and especially T. inermis are concluded to have much more preference for phytoplanktonic food. Results are discussed in terms of current knowledge of the lipid chemistry of krill in the northern and southern hemispheres.     

Distribution and feeding of the carnivorous copepod Paraeuchaeta norvegica in habitats of shallow prey assemblages and midnight sun

Paraeuchaeta norvegica was found to be widely distributed in the Norwegian Sea. They were least abundant in north-western areas, but otherwise no clear horizontal patterns appeared with respect to latitude, longitude or water mass. Females and males had similar vertical distributions. The highest concentrations of adults occurred at 400-500 m depth; they largely avoided the upper 50-100 m, even at night. Stages CIV and CV lived shallower in the water column than the adults, with the highest concentration between 100 and 300 m. Stages CII-CIII were most abundant at 50-100 m, while CI was distributed slightly deeper (maxima at 100-200 m). Potential prey were most abundant in the upper 100 m; i.e. shallower than P. norvegica. Numbers of fecal pellets produced by freshly collected adult females were relatively low (estimated at 0.7 pellets per individual on average for the entire sea), with maximal numbers for individuals captured in shallow waters. This suggests food limitation during summer, when food is concentrated in upper waters, and short and light nights limit nocturnal access to the shallow food resources. Pellets mainly contained copepod remains.     

Lipogenesis in the arctic euphausiid Thysanoessa inermis

Biosynthesis of lipids by Thysanoessa inermis collected from Balsfjorden, northern Norway, in May 1980, was examined in vitro. The highest concentration of lipid within the krill was in the hepatopancreas, and this organ was the most active in esterifying free fatty acids into wax esters. The hepatopancreas (i.e., thoracic contents) incorporated (¹⁴C) glucose, (¹⁴C) alanine and ³H₂O into wax esters, with the fatty alcohol moieties being labelled more than the fatty acids. (¹⁴C) fatty acid was incorporated preferentially into the fatty acid moieties of wax esters, this incorporation being markedly stimulated by free fatty alcohol. It is concluded that the fatty alcohols of wax esters are preferentially biosynthesized de novo from dietary protein and carbohydrates, whereas the fatty acids derive preferentially from dietary lipid. On the basis of ³H incorporated from ³H₂O, the hepatopancreas in a 50 mg II-group (2 yr old) individual of T. inermis is capable of biosynthesizing de novo, approximately 0.1 mg of lipid (as fatty acids) per day at 5°C.     

Distribution of euphausiid assemblages in the Mediterranean Sea

Eleven species of euphausiids from 24 Isaac-Kidd Midwater Trawl (IKMT) night collections taken at stations throughout the Mediterranean Sea were counted. The frequency of occurrence and dominance of individual species and percent similarity faunal analysis of the euphausiid community were used to describe changes in faunal composition between geographical areas and differences in vertical distribution. Although most species were widespread, three distinct patterns of abundance were apparent: Euphausia krohnii, Nematoscelis megalops, Meganyctiphanes norvegica, and Stylocheiron abbreviatum predominated in western basin areas west of the Tyrrhenian Sea; Euphausia hemigibba, Thysanopoda aequalis, and Stylocheiron longicorne predominated in the Tyrrhenian Sea and eastern Mediterranean Sea; Euphausia brevis and Stylocheiron suhmii predominated in the eastern Mediterranean Sea. Percent similarity analysis of data from the IKMT samples and data from Ruud (1936) indicates the Tyrrhenian Sea fauna at the time of the collections was more similar to eastern Mediterranean areas than to most other areas in the western basin, although the degree of similarity was dependent, to some extent, on the depth at which the samples were collected. The composition of euphausiids living above 150 m at night in this area was more similar to eastern basin areas, while the composition of deeper living forms was more similar to those of the rest of the western basin. Comparison of euphausiids collected at three points over a 60 year time-span in the Balearic Sea shows the similarity in composition to be greater within the area over time than between adjacent areas in the western Mediterranean Sea.Contribution No. 2732 from the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA. This research was supported by the Atomic Energy Commission Contract AT (30-1)-3862, ref. NYO-3862-49, National Science Foundation Grant GA 29303 and Office of Naval Research Contract N00014-71-C-0284.     

Respiration and chemical composition of the bathypelagic euphausiid Bentheuphausia amblyops

Bentheuphausia amblyops is a cosmopolitan bathypelagic euphausiid with a vertical range of from 280 to 7 000 m. Determinations of proximate and elemental composition show that B. amblyops has a more robust structure (lower water content and higher protein content) than is typical of other bathypelagic Crustacea. B. amblyops is a strong swimmer and is capable of employing its thoracic legs in raptorial fashion. Discrete depth trawls taken between depths of 400 and 2 500 m on a diel basis show no evidence of vertical migration. There was no significant difference in oxygen consumption within the species environmental temperature range (1.5° to 7.5°C), which may be partially ascribed to a limited temperature effect and partially to variability in spontaneous activity at the different temperatures. Between 7.5° and 9.5°C there was a marked temperature effect on metabolism (Q₁₀=6.4). The observed vertical distribution and metabolic response to elevated temperature preclude the migrations to the surface that have been postulated for B. amblyops. The robust composition of the species, its behavioral characteristics, and published diet information strongly suggest that the species is omnivorous with a strong predatory component in its foraging strategy.     

Larval development of the euphausiid Euphausia pacifica in the Yellow Sea

The complete larval development of Euphausia pacifica in the Yellow Sea is described and the stages are compared with larvae of E. nana. Euphausiid larvae examined in the present study were collected at 30 stations in the Yellow Sea in 1989. During the stages of Calyptopis III to Furcilia II, E. pacifica larvae are smaller than E. nana larvae but they are larger from Furcilia III onward. There are geographical variations in body size of Calyptopis III among the southern California waters, the eastern Japan waters, the Japan Sea, and the Yellow Sea; body size is smallest in the Yellow Sea and Japan Sea, while it is largest in southern California waters and intermediate in eastern Japan waters. Segmentation of antennal endopods was observed in the 56 to 51 furcilia forms in the Yellow Sea population, suggesting that this characteristic is not as reliable for identification of furcilia stages as the pleopods and terminal telson spines. Thus, we propose here that there are six furcilia stages of E. pacifica, instead of the seven previously reported by Boden (1950).     

Investigations on some tropical euphausiid species of the Indian Ocean

In spite of the great ecological importance of euphausiids in large areas of the sea, very little is known about their early developmental stages. During the 40th and 41st cruises of RV Vityaz (USSR) in the western part of the Indian Ocean, new information was obtained on the development of 2 species common in that area: Euphausia diomedeae and Slylocheiron carinatum. In E. diomedeae, the time between fertilization and hatching of the nauplius amounts to about 16 h; the metanauplius stage lasts 2 days, the calyptopis stage I also 2 days, and the calyptopis stage III up to 4 days, at water temperatures ranging from 22° to 26°C. The developmental stages of both species are described and illustrated.     

Drivers of euphausiid species abundance and numerical abundance in the Atlantic Ocean

Mid-ocean ridges are common features of the world’s oceans but there is a lack of understanding as to how their presence affects overlying pelagic biota. The Mid-Atlantic Ridge (MAR) is a dominant feature of the Atlantic Ocean. Here, we examined data on euphausiid distribution and abundance arising from several international research programmes and from the continuous plankton recorder. We used a generalized additive model (GAM) framework to explore spatial patterns of variability in euphausiid distribution on, and at either side of, the MAR from 60°N to 55°S in conjunction with variability in a suite of biological, physical and environmental parameters. Euphausiid species abundance peaked in mid-latitudes and was significantly higher on the ridge than in adjacent waters, but the ridge did not influence numerical abundance significantly. Sea surface temperature (SST) was the most important single factor influencing both euphausiid numerical abundance and species abundance. Increases in sea surface height variance, a proxy for mixing, increased the numerical abundance of euphausiids. GAM predictions of variability in species abundance as a function of SST and depth of the mixed layer were consistent with present theories, which suggest that pelagic niche availability is related to the thermal structure of the near surface water: more deeply-mixed water contained higher euphausiid biodiversity. In addition to exposing present distributional patterns, the GAM framework enables responses to potential future and past environmental variability including temperature change to be explored.     

Production of the euphausiid Nyctiphanes australis in Storm Bay,south-eastern Tasmania

The euphausiid Nyctiphanes australis Sars is an important food item for a wide range of seabirds and commercially exploited fish in coastal waters of south-eastern Tasmania. During most of the year, N. australis forms the major component of the zooplankton biomass in Storm Bay, particularly during the summer months when it swarms in dense aggregations. Production was calculated using methods devised for individuals with continuous recruitment. Estimates of production, integrated for the whole of Storm Bay (approx 18.5km³) were from 78.3 to 84.8 mg m^-3 yr¹. The P:B ratio obtained for the period December 1979 to March 1981 was about 14. The production of exuviae was also calculated based on certain assumptions about moulting rate and dry weight of the moult, supported by some experimental data. The production of exuviae was 41.03 mgm^-3yr^-1 or 7.6 times the mean biomass. This gives a total production of flesh and exuviae of 2 212 tonnes dry weight per year for Storm Bay.