Ecological investigations on the zooplankton community of Balsfjorden, northern Norway: population dynamics of the euphausiids Thysanoessa inermis (Kroyer), Thysanoessa raschii (M.Sars) and Meganyctiphanes norvegica (M.Sars) in 1976 and 1977

The population dynamics of the euphausiids Thysanoessa inermis(Kröyer), T. raschii (M.Sars), and Meganyctiphanes norvegica(M.Sars) have been followed in Balsfjorden in 1976 and 1977.Seasonal variations in length-frequency distributions, growthin carapace length, sex-ratio, and spermatophore productionand attachment are presented and discussed in relation to changesin hydrography and phytoplanlcton standing-crop. An annual generationof T. inermis and T. raschii was spawned in April and May. Eggsand larvae of M. norvegica were not found in Balsfjorden, indicatingthat recruitment occurs from outside the fjord. T. inermis andT. raschii reached maximum carapace lengths of 7–8 mmand 6–7 mm respectively and had life-spans of c. 2 years3 months. M. norvegica had a life-span of c. 2 years 6 monthsand reached a maximum carapace length of c. 12.5 mm. In bothT. inermis and T. raschii 0-group underwent the greatest lengthincreases from May to October, I-group from March to Augustand II-group from April to June. The population structure, growthpatterns and growth periods of M. norvegica were difficult todiscern. The phytoplanlcton cycle appears to be the dominantfactor regulating both growth and spawning of the Thysanoessaspp in Balsfjorden, while temperature has no obvious influence.     

Feeding behaviour in Meganyctiphanes norvegica (M. Sars) (Crustacea: Euphausiacea)

Meganyctiphanes norvegica (M. Sars) will feed upon the centric diatom Thalassiosira weissflogii (Grunow) G. Fryxell & Hasle but cannot fulfil its energy requirement for metabolism on this food. Its daily metabolic requirement can be exceeded when the euphausiid feeds upon the copepods Calanus finmarchicus Gunner or Centropages typicus Krøeyer, but not when feeding upon the smaller copepods Pseudocalanus spp. or Acartia spp. When feeding upon a natural copepod assemblage Meganyctiphanes norvegica requires high concentrations of copepods to achieve its metabolic requirements, suggesting that the euphausiid may exploit vertically patchy concentrations of prey. Short-term predation rates on Pseudocalanus spp. were also used to estimate feeding rates. Feeding in Meganyctiphanes norvegica appears to be adapted to a spatially variable food supply and rapid exploitation of food sources concentrated into patches or layers. The filter area of the feeding basket of M. norvegica is proportionally smaller than the filter area of Euphausia superba Dana, but has the same allometric length exponent. The filter area probably reflects the difference between the primarily carnivorous diet of Meganyctiphanes norvegica and herbivorous diet of Euphausia superba.     

Ecological investigations on the zooplankton community in Balsfjorden,Northern Norway: seasonal changes in the lipid class composition of Meganyctiphanes norvegica (M. Sars), Thysanoessa raschii (M. Sars), and T. inermis (Krøyer)

The lipid class composition of three krill species, Meganyctiphanes norvegica (M. Sars), Thysanoessa raschii (M. Sars), and T. inermis (Krøyer), was determined throughout the year in Balsfjorden, Northern Norway. Meganyctiphanes norvegica had triglycerides as its major neutral lipid, Thysanoessa raschii had mainly triglycerides with small amounts of wax esters, whereas T. inermis had mainly wax esters with lesser amounts of triglycerides. Neutral lipids were generally accumulated by the krill during spring-early summer, maintained at high levels through to mid-winter and then declined to minimal levels during early spring. Differences in rates of deposition and mobilization of neutral lipid between males and females occurred in Meganyctiphanes norvegica and Thysanoessa raschii but not in T. inermis. T. raschii accumulated lipid somewhat later in the year than T. inermis but earlier than Meganyctiphanes norvegica. A major rôle for lipid in the elaboration of gonads is indicated. The accumulation of either wax esters or triglycerides by euphausiids is considered in terms of the feeding habits and ecological niche differentiation of the species.     

Relationships between stomach contents and vertical migration in Meganyctiphanes norvegica, Thysanoessa raschii and T. inermis (Crustacea Euphausiacea)

Stomach contents of Thysanoëssa raschii, T. inermis andMeganyctiphanes norvegica collected in the Gulf of St. Lawrenceshowed the Thysanoëssa species fed primarily during thenight in the upper 75 m of water. M. norvegica fed on copepodswhile at their daytime depth of about 125 m and on phytoplanktonand copepods during the night above 75 m depth. M. norvegicafed on a wider range of organisms than the other two speciesduring the three sample periods of spring, fall and early winter.There was no difference in the minimum size of the cells ingestedbetween any of the species. No evidence was found of any ofthe species feeding on bottom deposits during the sample periods.     

Zooplankton in the Ligurian Sea: Part II. Exploration of their physical and biological forcing functions during summer 2000

A survey of the biological and physical oceanography of theLigurian Sea was conducted in the late summer of 2000. Forty-onestations were sampled for nutrients, oxygen, fluorescence andhydrographic information. Acoustic backscatter measurementswere used to estimate abundance of small (<5 mm) zooplanktonbiovolume versus depth and the distribution of northern krill,Meganyctiphanes norvegica. Net-tow and underwater video datawere collected to identify the zooplankton present. These datawere used to analyze the Ligurian Sea ecosystem for physicaland biological linkages that control zooplankton abundance anddistribution. Results are compared with those from a similarstudy conducted in 1999. Hydrographic sampling showed a domeof dense water in the southwestern middle of the basin. Thehighest chlorophyll a (Chl a) concentrations were measured inthis area, while small zooplankton biovolume was evenly distributedthroughout the survey. Integrated values of Chl a and smallzooplankton biovolume in 2000 were greater than in 1999. Meganyctiphanesnorvegica, siphonophores and salps were the dominant componentsof the macrozooplankton population in the upper 200 m. In thesampled depth strata, siphonophore abundance did not changeduring the day, while M. norvegica were only caught at night.Acoustic backscatter data show that higher densities of M. norvegicaoccurred in deeper water and in the western and southwesternareas of the Ligurian Sea.     

Advection of euphausiids in a Norwegian fjord system subject to altered freshwater input by hydro-electric power production

Due to hydro-electric power production, the freshwater inputto the Sandsfjord system has increased during winter, whilethe freshwater input to Jøsenfjorden has been removed.In studies conducted during autumn and winter, both Meganyctiphanesnorvegica and Thysanoessa raschii were most common in Jøsenfjorden,and T.raschii was almost wanting in the Sandsfjord system. Theeuphausiids generally swam to the surface layer at night, butavoided low salinity water in the uppermost metres of the Sandsfjordsystem. Still, in most locations 50% of M.norvegica was foundin the upper 10 m. Their vertical distribution overlapped withfreshwater-driven currents, and estuarine circulation was apparentlyprominent in governing the horizontal distribution within theSandsfjord system. At the entrance to the fjord, M. norvegicawas distributed deeper than within the fjord, reducing the importanceof transport with estuarine circulation relative to currentsdriven by wind at the coast. Estimates of advective rates ofM.norvegica suggest that exchange with coastal waters was moreimportant for the standing stock than local growth within thefjords. In Jtfsenfjorden, maxima of T.raschii were always foundin a zone of shallowing bottom topography (bottom depth 130m), where this species possibly maintained its horizontal positionby means of vertical migrations.     

The effect of lunar eclipse on the vertical migration behaviour of Meganyctiphanes norvegica (Crustacea: Euphausiacea) in the Ligurian Sea

The vertical migration of a zooplankton community dominated by the euphausiid Meganyctiphanes norvegica was monitored between 16 and 23 September 1997 with a 153 kHz Acoustic Doppler Current Profiler (ADCP) and a MOCNESS net. The sampling period covered a phase in the lunar cycle when the rise of the moon (full moon) coincided initially with sunset and then became progressively later. On 16 September 1997, a lunar eclipse occurred 45 min after sunset, lasting for 2 h. At dusk, the ADCP observed the upward vertical migration of two principal backscattering bands 10 min apart with vertical velocities of up to 7 cm s^-1. After a period at the surface, a more diffuse band subsequently sank at a slower rate (1-2 cm s^-1) to a depth of 75-100 m. Net samples showed that the earlier band consisted mainly of the pteropod Cavolinia inflexa, whilst the later band was mostly euphausiids, predominantly M.norvegica. This species was also the major constituent of the band that sank. The timing of upward migration was relatively constant over the sampling period, but there was an increasing delay of the secondary sinking until 21 September. This showed as a strong correlation between the onset of sinking and the time of moonrise. The lunar eclipse on 16 September perturbed this pattern, such that animals did not sink soon after their arrival at the surface, as occurred on 17 September but remained at the surface until the end of the umbra. This suggests that M.norvegica can perceive moonlight and that this influences vertical migration. Evidence that the behaviour is not solely mediated by this erogenous factor, however, is seen in the pattern that emerged after 21 September, when midnight sinking occurred at a relatively constant time after sunset and before moonrise. These observations support the hypothesis that moonlight is a Zeitgeber for an endogenous rhythm that synchronizes secondary sinking behaviour with the lunar cycle.      

Gamete ultrastructure in the arctic holothurian Molpadia borealis M. Sars, 1859 (Holothuroidea: Molpadiidae)

This study examines the gonad condition and gamete morphology of the Arctic deep-sea holothurian Molpadia borealis M. Sars, 1859 (Molpadiidae) collected from the Kara Sea in September 2006. The intensive process of gametogenesis observed in the holothurian gonads was close to completion, suggesting upcoming spawning. The sperm ultrastructure in M. borealis is similar to that in most Holothuroidea. This species has classic flagellated sperm, echinosperm, which are typical of animals with external insemination. The sperm head has a 2.5-??m spherical nucleus with a proximal acrosome consisting of a spherical acrosomal vesicle surrounded by periacrosomal material. A single circular mitochondrion located in the sperm mid-piece surrounds the proximal and distal centrioles, which are arranged at an obtuse angle to each other. The eggs of M. borealis are approximately 300 ??m in diameter. This suggests indirect development with a planktotrophic larva. This type of development of M. borealis is probably related to life in high latitudes in the deep sea.     

Redescription of Scapholeberis echinulata Sars 1903 (Crustacea,Cladocera)

Scapholeberis echinulata Sars is rediscribed, based on type material of Sars (S. echinulata), Daday (s. erinaceus) and the author's own samples. A differential diagnosis with Scapholeberis spinifera Nicolet and S. mucronata O. F. Müller, S. microcephala Sars, S. kingi Sars and S. aurita Fischer is given.     

Fluoride in tissues of Krill Euphausia superba Dana and Meganyctiphanes norvegica M. Sars in relation to the moult cycle

Summary The fluoride content of whole animals and different tissues of the euphausiid species Euphausia superba and Meganyctiphanes norvegica was analyzed by two different and improved methods of isolation and determination. In contrast to other authors our findings show that the internal organs (muscle, hepatopancreas and hemolymph) contain less than 6 ppm d.w. fluoride this being the same order of magnitude as for vertebrates. The high concentrations reported by other authors must be mainly due to contamination of the soft tissue during storage (post-mortem migration of fluoride from shell) and/or contamination caused by minute fractions of cuticle during dissection. Over 99% of the total fluoride content is located in the cuticle (i.e. integument) of the euphausiids (2600 ppm/d.w. in E. superba and 3300 ppm/d.w. in M. norvegica in pleon cuticle). Analysis of F^- levels in relation to the moulting cycle showed that the uptake in both euphausiids occurs at a comparable and fast rate during the same physiological phase shortly after moult, parallel to the general construction of the cuticle. The internal organs show homeostasis in respect to fluoride. Accordingly, no internal deposition takes place, and F^- is reaccumulated from the external medium at each moult.This work was supported by grants from the DFG No. Ad 24/9 and Bu 548/1     

Zooplankton prey selection by juvenile fish in Nova Scotian Shelf basins

The stomach contents of fish <100 mm standard length (SL)and fish >100 mm SL collected during June and October intwo deep basins on the Scotian Shelf, showed that juvenile co(and silver hake fed primarily on the two most abundant speciesof zooplankton in the basins: Calanus finmarchicus and Meganyctiphanesnorvegica. These zooplankton were abundant in the basin: duringthe 2 months, forming high concentrations at depths below 200m during the day. In June fish (<100 mm SL) concentratedin the upper 50 m during both day and night. Cod and wolffislcollected between 195 and 240 m had a large percentage of C.finmarchicusin their stomachs suggesting that these fish exploited the highconcentrations of C.finmarchicus below 200 m depth There wasno evidence that Calanus hyperboreus was eaten by any juvenilefish species. Calanus finmarchicus was the most common preyof cod feeding in the top 50 m, but the percentages o C.finmarchicusstages IV and V were lower in the stomachs than in the watercolumn. Cod showed ; strong preference for M.norvegica, withthis prey species having a modified Ivlev's index of 1 indicatingthat cod were highly selective for this species. There was noevidence that C.hyperboreus. stage IV was eaten by any of thespecies offish. Silver hake and dogfish (>100 mm SL) bothfeed oi M.norvegica and C.finmarchicus in the deep regions ofthe basins. In October, silver hake was the most common juvenilefish in the basins, feeding primarily on small stages of M.norvegica.The populations of fish (>100 mm SL) in the basins were mainlyspecies known to feed heavily or euphausiids. The two basinsare unique regions of the shelf because of the large populationsof Calanus copepods and M.norvegica they contain all the yeararound, thereby providing an atrractive feeding ground for manyspecies of fish, particularly silver hake.     

Food ingestion and digestive transit time in the euphausiid Meganyctiphanes norvegica as a function of animal size

Laboratory experiments with Meganyctiphanes norvegica of differentsizes (7 to 70 mg dry) fed on Artemia salina show that the dailyspecific ingestion rate varies with animal weight accordingto a power law with a negative exponent of -0.58. This relationis used to calculate the production rate of faeces which ishigher in the smaller individuals when expressed as a percentageof animal weight; a comparison of the M. norvegica data withpublished data for Calanus helgolandicus indicates that thisrate is higher for the smaller species. Under the experimentalconditions used the digestive transit time of food in M. norvegicais appreciably shorter in smaller animals (15 minutes versus30 minutes). The data, combined with other data previously obtainedfor the same ipecies, show that the metabolism of active, well-fedindividuals is about three times higher than the standard metabolism.     

Separation of two Neotropical species: Macrothrix superaculeata (Smirnov, 1982) versus M. elegans Sars, 1901 (Macrothricidae, Anomopoda, Cladocera)

The morphology of two Neotropical taxa, Macrothrix elegans Sars, 1901 and M. superaculeata (Smirnov, 1982) (Macrothricidae, Anomopoda, Cladocera) was redescribed, based on type materials (a lectotype of M. elegans was selected here), and additional samples from the Americas. Previous conclusion about synonymy of both species was erroneous, because it was based on limited material from South America. M. superaculeata differs from M. elegans in the presence of a sharp spine at postero-dorsal angle of valves; a more fine ring around dorsal head pore; thinner hexagonal reticulation of valves; the presence of setules on basal segment of postabdominal seta; armature of exopod on antenna II, and some features of thoracic limbs. Previously, the discriminative features of the two species were not formulated accurately, and it was a reason of several misidentifications. Actually, M. superaculeata is found only in a limited set of localities from the Amazon basin, while M. elegans is one of the most common anomopod species in all Neotropics, from Argentina to Mexico.     

Occurrence of Diaphanosoma excisum (Sars) on a sandy beach at Balramgari (Orissa), India

The occurrence of Diaphanosoma excisum (Sars), a freshwater cladoceran, in benthic samples of an intertidal sandy beach is reported. Population density was seasonal. A relatively high density was recorded from June to September (south-west monsoon season) with a maximum (46 ind 10 cm^–2) in September at a depth of 10–15 cm of sediment. A sudden decline occurred during north-east monsoon (October to January), and in the fair season (February to May), the cladocerans disappeared. Mean density varied (P<0.001) with sediment depth and season and showed a contagious dispersion. Abundance was negatively correlated with salinity (r = –0.76) but positively with POC (r =0.79) and mean grain size of the sediment (r = 0.93). The density of D. excisum was highest in fine sand.     

On the physical oceanography of the Kattegat and Clyde Sea area, 1996–98, as background to ecophysiological studies on the planktonic crustacean, Meganyctiphanes norvegica (Euphausiacea)

 The euphausiid, Meganyctiphanes norvegica (M. Sars), has an extremely wide distribution around the North Atlantic, from Canada to the Arctic to the western Mediterranean. It appears to be adaptable to life under quite different oceanographic conditions, yet no morphological features have been identified that distinguish any of the populations occurring in different areas between which exchange must be supposed to be restricted. The genetic, physiological, behavioural and ecological investigations that are being carried out on M. norvegica in the Kattegat, the Clyde Sea area and the Ligurian Sea (the EU MAST III PEP Project) in order to study its adaptability must be closely related to the physical environment within which the respective populations occur. This paper presents a summary of the hydrographic conditions found in the Kattegat and the Clyde Sea area in summer and in winter, during each research cruise, and places them in the context of current understanding of the hydrography in each area. An account of the hydrography of the Ligurian Sea is in preparation (Dallot et al., in preparation). The data will be used in the further analysis of genetic, physiological and behavioural adaptability of M. norvegica, and will be available for use as appropriate to supplement other hydrographic studies in the regions concerned. Received: 2 February 1999 / Accepted: 6 April 1999     

The correlation of downwelling irradiance and staggered vertical igration patterns of zooplankton in Wilkinson Basin, Gulf of Maine

Field studies on the characteristics of light that influencevertical migrations in the mesopelagic realm are sparse, dueto the difficulty in simultaneously monitoring changes in speciesdistributions with changes in downwelling irradiance. Usingthe Johnson-Sea-Link submersible as a platform, in situ measurementsof the changes in downwelling irradiance at sunset were madesimultaneously with observations on changes in animal distributionpatterns in Wilkinson Basin, Gulf of Maine. The results indicatethat the vertical migrations for several species of large zooplanktonare staggered, with euphausiids (Meganyctiphanes norvegica)migrating first, cydippid ctenophores (Euplokamus) migratingnext, and two species of caridean shrimp (Dichelopandalus lepiocerusand Pasiphaea multidentata) migrating last Data collected ondaytime dives indicate that the daytime depth distribution isnot solely responsible for the migration order, and that differentspecies may be responding to different cues, or have differentthresholds for the same cue.     

Vertical distribution and feeding pattern of Euphausiacea (Crustacea) in the eastern Banda Sea (Indonesia) during the SE and NW monsoons

The vertical distribution of Euphausiacea (Crustacea) in theeastern Banda Sea (Indonesia) during the SE and NW monsoon seasonswas studied. In August 1984 and February/ March 1985, stratifiedday and night sampling was carried out between 0 and 500 m atfour stations. A total of 31 species was found, of which mostwere present in both monsoon seasons. Species with a high presencein samples and performing diurnal vertical migration in bothmonsoon seasons were Euphausia diomedeae, Euphausia pseudogibba,Thysanopoda monacantha, Thysanopoda iricus-pidaia and Nematoscelismicrops. Species with a high presence in samples showing nodiurnal vertical migration in both monsoon seasons were Euphausiasimilis, Nematobrachion boopis. Nematoscelis tenella, Stylocheironmaximum and Thysanopoda orientalis. The density of Euphausiaceaat the sampled stations was higher, but more heterogeneous,in the nutrient-enriched SE monsoon period than in the relativelynutrient-poor NW monsoon season, when densities were lower andmore similar. The vertical distributions and diurnal verticalmigration of species did not show a unanimous and strong responseto seasonal hydrographic differences. The day–night depthdistribution pattern of the Euphausiacea population in the upper500 m is largely the same in both seasons, with comparable migrationranges. Some species showed a somewhat upward shifted depthdistribution in the nutrient-enriched period. For mesopelagicand bathypelagic species especially, food resources were thenfavourable in the upper layers, as indicated by rare recordsof Thysanopoda crisiata. Analyses of stomach contents snowedmainly a nocturnal feeding pattern for the diurnal verticalmigrating species and a continuous feeding pattern for the non-migratingspecies.     

Diel vertical migrations ofMeganyctiphanes norvegica in the Kattegat: Comparison of net catches and measurements with Acoustic Doppler Current Profilers

Diel vertical migration of a stable and well-defined population of Nordic krill,Meganyctiphanes norvegica (Crustacea, Euphausiacea) was investigated during eight days in August 1989, in the Läsö-Deep, East of the Danish island Läsö. Net catches with a multi-net (MOCNESS) and measurements with a moored and a shipboard Acoustic Doppler Current Profiler (ADCP) were compared. Backscattered energy as a measure for biomass gave good correlations to the dry weight ofM. norvegica and smaller zooplankton from net catches. Diel migratory patterns matched well, as determined, parallel with both methods. Migratory vertical velocity was determined with ADCP at 2–3 cm sec^?1. The potential for the use of ADCPs for biological investigation is discussed. Vertical migration was dependent on environmental parameters. The krill did not cross a temperature barrier of 14°C, although rich food sources were situated beyond it. Differences in salinity did not play a role. Currents were involved in plankton distribution. Light was an important Zeitgeber (synchronizer) and determined the density of the krill aggregations. Feeding behaviour did not interfere with the light-induced migratory pattern of Nordic krill at the Läsö-Deep.     

The annual cycle and production of Arctodiaptomus laticeps (Sars) in Lake Malsjoen, Norway

The annual cycle and production of Arctodiaptomus laticeps (Sars)was studied in the small Lake MålsjÖen, the waterof which is characterized by CaO 11 mg/l, pH 7.0, and much humus.Reproduction takes place at the turn of February-March. Thenauplii stay in the water until the ice breaks, in mid-May.Thereafter the metamorphosis takes place very rapidly and CIV reach their maximum in mid-June. The C V stay in the wateruntil late September, when adults suddenly occur. The adultslive under the ice during winter, the mortality being almostzero until the first egg-bearing females occur in late February.Three successive generations had identical cycles. Less than10% of a generation's production was eggs and nauplii, 50% establishedthe biomass of C IV in June, the rest of the production maintainedthe biomass in July-October. The yearly production was 550 mgm^–2 dry weight in 1970 and 340 in 1971, establishing amean biomass of 226 and 119 mg m^–2 for the productiveperiod 25 May-5 October.     

Bathymetric distribution of medusae in the open waters of the middle and south Adriatic Sea during spring 2002

Twenty-eight species of hydromedusae and one scyphomedusa werecollected from the middle and south Adriatic in Spring 2002.Haliscera bigelowi was recorded for the first time in the AdriaticSea and Dicodonium adriaticum for the southern Adriatic. Thecosmopolitan species Rhopalonema velatum, Aglaura hemistomaand Persa incolorata averaged 88% of total medusan abundance.The Mediterranean species Rhabdoon singulare and Solmissus albescenswere numerous at the deeper stations. These species were foundeither in the southern or in middle Adriatic, and their abundanceis greater than in other regions of the Mediterranean Sea. Speciesdiversity and abundance generally were highest in the upper200 m and decreased with depth. Diel vertical migration patternsincluded some species that nocturnally sought shallower or deeperlayers, and some species migrated in both directions. A fewspecies did not show any vertical migration. Although lightlikely triggers vertical movements, data suggest that foragingstrategies also influence the vertical migration of medusae.