Molecular evidence for genetic subdivision of Antarctic krill (Euphausia superba Dana) populations.

Antarctic krill (Euphausia superba Dana) is a key species in the Antarctic food web and occurs on a circumcontinental scale. Population genetic structure of this species was investigated by sequence analysis of the ND1 mitochondrial gene in four population samples collected at different geographical localities around the Antarctic continent. Results indicate the existence of significant genetic differences between samples, and we suggest that oceanographic barriers could be sufficiently strong and temporally stable to restrict gene flow between distinct areas. Moreover, our data indicate that Antarctic krill is not at mutation-drift equilibrium and that the species possibly has a low effective population size as compared to the census size.     

Antarctic krill (Euphausia superba Dana) eat salps

 Feeding behaviour of Antarctic krill (Euphausia superba) on salps was observed in shipboard experiments during the 1994/1995 Kaiyo Maru Antarctic Ocean research cruise. The feeding rate was more than 0.5 salp/krill per day. When offered ethanol extracts of four prey types, salps, phytoplankton, krill and polychaetes, krill preferred the salp extracts. This evidence implies that the substances extracted from salps were most attractive to krill. These results might indicate a tight ecological relationship between krill and salps. Received: 24 May 1995/Accepted: 8 October 1995     

Characterization of proteinases from Antarctic krill (Euphausia superba)

Fractions of three trypsin-like proteinases, TL I, TL II, and TL III, a chymotrypsin-like proteinase, CL, two carboxypeptidase A enzymes, CPA I and CPA II and two carboxypeptidase B enzymes, CPB I and CPB II, from Antarctic krill (Euphausia superba) have been characterized with respect to purity by the means of capillary electrophoresis, CE, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). The masses of the trypsin-like and chymotrypsin-like proteinases were determined to be 25,020, 25,070, 25,060, and 26,260Da for TL I, TL II, TL III, and CL, respectively. The masses of the CPA enzymes are likely 23,170 and 23,260Da, whereas the CPB enzyme masses likely are 33,730 and 33,900Da. The degradation efficiency and cleavage pattern of the trypsin-like proteinases were studied with native myoglobin as a model substrate using CE, MALDI-TOF-MS, and nanoelectrospray mass spectrometry (nESI-MS). The degradation efficiency of the trypsin-like proteinases was found to be approximately 12 and 60 times higher compared to bovine trypsin at 37 degrees C and 1-3 degrees C, respectively. All three fractions of trypsin-like proteinases showed a carboxypeptidase activity in combination with their trypsin activity.     

Identification and quantification of prostaglandins in Antarctic krill (Euphausia superba Dana)

Summary The amount and types of prostaglandins present in Antarctic krill (Euphausia superba Dana.) were estimated. Samples of fresh krill were collected during III Antarctic Cruise of RV Polarstern in November 1984. Prostaglandins were extracted, separated by column and thin-layer chromatography and identified as PGA₂, PGB₂, PGE₂, PGF₂. Quantitative measurements were made by a biological method (Vane cascade), concentrations of the most abundant prostaglandins PGE₂ and PGF₂ being 1.6 and 4 ng/1 g of fresh tissue, respectively. Such low level of prostaglandin would not be harmful when using krill as a food supplement.     

Susceptibility of Antarctic krill (Euphausia superba Dana) to ultraviolet radiation

We irradiated captive juvenile Euphausia superba in the laboratory with lower than spring surface levels of ultraviolet-B, ultraviolet-A and photosynthetically active radiation, in order to examine their response in terms of mortality and generalised activity. Levels of photosynthetically active radiation 3–5 times below surface irradiance caused krill to die within a week, while animals in the dark survived. Addition of ultraviolet-B typical of depths up to 15 m were found to significantly accelerate mortality and lead to a drop in activity in all experiments. A drop in activity in krill exposed to ultraviolet-A wavelengths was evident without an increase in mortality. The protein content of animals from various treatments was found not to vary. Accepted: 10 January 1999     

Genetic homogeneity of krill (Euphausia superba Dana) in the Southern Ocean

Summary Development of a comprehensive picture of the genetic population structure of the Antarctic krill (Euphausia superba) has been hampered by a lack of genetic data from two major areas of the species' distribution, the Bellingshausen Sea and the Ross Sea. Evidence from earlier studies of a discrete Bellingshausen Sea population was based on anomalous allele frequencies in two sample sets that were collected near the west coast of the Antarctic Peninsula rather than in the Bellingshausen Sea proper. In this paper we describe the first biochemical genetic data obtained on krill from the central Bellingshausen Sea and from the Ross Sea. Analyses of eight polymorphic loci in samples from these two areas have failed to provide any evidence of population structuring within the Pacific sector of the Southern Ocean, and have indicated that Pacific sector krill cannot be genetically discriminated from Atlantic sector krill or Indian Ocean sector krill. These findings further support the hypothesis of a single circumpolar breeding population of Antarctic krill.     

Spatial and temporal variability in reproductive timing of Antarctic krill (Euphausia superba Dana)

Spawning dates of Antarctic krill, Euphausia superba Dana, were calculated from larval stage compositions, and corrected using data on maturity stage composition of the adult krill. Both original and literature data obtained from the Antarctic Peninsula-Bellingshausen Sea area and around the Antarctic continent were used. A time series (1975/76–1986/87) for several subareas of the Antarctic Peninsula-Bellingshausen Sea area indicates considerable variation in the krill spawning start, maxima and completion. In particular years (1975/76, 1980/81), krill spawning in the western Atlantic sector began relatively early, was intensive, and completed early. Some years (1977/78, 1981/82) were characterised by long and non-synchronised krill spawning. Compiled data sets for the Atlantic sector (1980/81), the entire Antarctic (1983/84) and the east Indian-west Pacific Antarctic waters (1981–85) reveal some spatial patterns in krill reproductive timing. In relation to spawning timing variation, the habitats of the krill population fall into five categories: (1) areas with an early beginning (late Novemberearly December) and a variable, but normally long, duration (3–3.5 months) of krill spawning; this is generally the southern boundary of the Antarctic Circumpolar Current, (2) areas with an early beginning, but a short duration of krill spawning (Gerlache Strait), (3) areas with a highly variable (within 1–1.5 months) beginning and a relatively long duration (ca. 3 months) of krill spawning (Bransfield Strait, Palmer Archipelago), (4) areas with a late beginning (late December–January) and a long duration of krill spawning (Bellingshausen Sea, D'Urville Sea, and Balleny Islands area), and (5) areas with a delayed beginning, but a very short duration (ca. 1.5 months) of krill spawning (Ross Sea slope, probably the Coastal Current area off the Lasarev Sea shelf and in the south-eastern Weddell Sea. These patterns can be partly explained by peculiarities of the ice regime in particular areas and by routes of krill movement within water circulation systems.     

Purification and Characterization of Cold Adapted Trypsins from Antarctic krill (Euphausia superba)

Three trypsins (TRY-ES) were purified from Antarctic krill (Euphausia superba) by ammonium sulfate precipitation, ion-exchange and gel-filtration chromatography, with relative molecular mass of 28.7, 28.8 and 29.2 kDa respectively. The TRY-ES was inhibited by specific trypsin inhibitors (benzamidine, STI, CHOM and TLCK), with optimum temperature at 40 (Trypsin I), 45 (Trypsin II) and 40 °C (Trypsin III) repetitively. The TRY-ES was stabled between 5 and 40 °C, which was consistent with the red shift in fluorescence intensity peak at 40 °C (Trypsin I) and 45 °C (Trypsin II and Trypsin III) and blue shift at 40 °C (Trypsin II and Trypsin III). The K _cat/K _m values of the TRY-ES was 14.28, 9.46 and 5.93 mM^?1s^?1 respectively, 1.1–10.2 folds higher than trypsins from other crustacean and mammal, which was supported by the differences in thermodynamics parameters, the free energy, enthalpy, and entropy of benzamidine and the TRY-ES system.     

Mature antarctic krill (Euphausia superba Dana) grown from eggs in the laboratory

Fertilized eggs obained from female krill in Antarctic waterswere transported to the Australian Antarctic Division in Tasmania,the eggs were successfully developed to larval, juvenile, subadultand finally mature adult stages in the laboratory. Under experimentalconditions of unlimited food supply, at 0°C and in completedarkness, the length of the life cycle from egg to egg was 3years. ^*Present address: Japan Sea Regional Fisheries Research Laboratory,5939–22, 1 Suido-Cho, Niigata 951, Japan     

Genetic variation and population structure of krill (Euphausia superba dana) from the Prydz Bay region of Antarctic waters

Summary Samples of Antarctic krill E. superba from six locations near Prydz Bay were analysed electrophoretically to detect genetically-based protein variation. Analyses of allele distributions at four polymorphic loci indicate no evidence of significant heterogeneity, a result consistent with the hypothesis that all samples were derived from a single breeding population of krill. The results of this study agree closely with genetic data from other studies on krill in the Atlantic sector of the Southern Ocean, suggesting that krill stocks over at least 6000 km of Antarctic waters are derived from a single interbreeding population.     

Effectiveness of the RMT8 system for sampling krill (Euphausia superba) swarms

Summary The effectiveness of the Rectangular Midwater Trawl (RMT) for sampling krill swarms was studied by observing the behaviour of swarms in the vicinity of the net using echosounders. Large scale avoidance was observed by day but not at night. By day krill layers dispersed to such an extent that few krill were present less than 10 m below the net. Limited avoidance by night indicates that the krill are acting on visual clues of the net's presence.     

Colorimetry of the hepatopancreas in Antarctic krill,Euphausia superba

Summary Hepatopancreas color of freshly caught as well as starved Euphausia superba was examined by a chroma meter to express it quantitatively and compare it with plant pigment contents. There was a high positive correlation (r= 0.77) between plant pigment contents of hepatopancreas and the purity (vividness) of hepatopancreas color, and a negative correlation between the pigment contents and the luminosity (brightness) of the color. During the starvation experiment the purity decreased from 0.480 to 0.198 during the first 6 days in accordance with the decrease in plant pigment contents from 4.7 to 0.9 g krill^–1. An examination of hepatopancreas color by a chroma meter proved an easy and quantitative way of knowing the feeding condition of E. superba.     

Laboratory observations of moulting,growth and maturation in Antarctic krill (Euphausia superba Dana)

Summary Observations of intermoult period, growth and maturation were made on krill which were transported from Antarctic waters and maintained in the laboratory in Australia over a three year period. The mean intermoult period (IP) for each of 10 specimens, with initial body lengths of 24.7=46.8 mm, kept at -0.5° C varied from 22.0 to 29.8 days (overall mean = 26.6 days). These measurements of IP are significantly longer than those obtained in some previous studies. Differences in experimental temperatures, light, body sizes and growth patterns of the specimens between studies are unlikely to be causes of these dissimilar results. The pattern of changes in body length (BL) varies from one individual to the next. The greatest increase in BL over a series of 4–5 moults ranged from 0.024 to 0.070 mm/day, which is equivalent to 0.0020 to 0.0086/day in body weight, assuming exponential growth. This maximum growth rate is about half the rate predicted from the growth scheme of Mauchline (1980) for wild krill. Comparison of growth data for other euphausiids suggests that Mauchline's scheme produces anomalous growth rate. The slower growth rate observed in the present study would extend the estimated life span of krill from 3–4 years, as calculated by Mauchline (1980), to 4–7 years. If krill undergo body shrinkage during the Antarctic winter the estimated life span might be even longer. Examination of the external sexual characters of moults showed both progression and regression of maturity stage in association with changes in BL.     

Apparent independence of the spawning and moulting cycles in female Antarctic krill (Euphausia superba Dana)

Summary Sixty female Antarctic krill (Euphausia superba Dana) spawned in shipboard experiments and the interval between egg-laying and ecdysis was noted. The number of eggs laid per female ranged from 263–3662, most females produced only one batch of eggs before moulting, and the post-spawn ovaries of all females contained few, if any, mature oocytes. As reported in other studies, the total number of eggs produced per female was not well correlated with body size. Females appeared to spawn at all times during the moulting cycle and although no diurnal rhythm in spawning was observed, moulting occurred mainly at night-time despite the animals being kept in near-constant darkness. No evidence of synchronous moutling was detected.     

Diurnal variations in mean volume backscattering strength of an Antarctic krill (Euphausia superba) patch

A series of acoustic transects and quantitative net hauls wasmade in a large patch of Antarctic krill (Euphausia superba)just north of South Georgia. The krill went through a diurnalpattern of dense swarming in daytime and dispersion after dark.This was accompanied by an 8 dB change in mean volume backscatteringstrength.     

Notes of food selection in the Antarctic krill, Euphausia superba

Analysis of the gut contents of Euphausia superba and of the phytoplankton composition in the area of the animals catchment served as a basis for answering the question whether or not E. superba is capable of food selection. It was found that the diatom Thalassiosira is preferred by E. superba as a food item and small pennate diatoms, Nitzschia, and Navicula are only tolerated as food. Other algae taxa, such as Tropidoneis, Dactyliosolen, Chaetoceros, and Gyrodinium, are clearly avoided. With respect to phytoplankton cell size, E. superba prefers cells with a length of 20–40 μm and cells larger than 70 μm are rejected. Trophic conditions only slightly affect the food selection of E. superba. Received: 4 September 1995/Accepted: 30 June 1996     

Genetic variation and population structure of krill (Euphausia superba) in the Atlantic sector of Antarctic waters and off the Antarctic peninsula

Summary Samples of Antarctic krill Euphausia superba from 16 locations in the Weddell Sea, Scotia Sea and on the west coast of the Antarctic Peninsula were analysed for protein variation using enzyme electrophoresis techniques. Analysis of allele distributions indicate that samples from all locations are part of a single breeding population of krill. Significant departures from random mating expectations were observed in the distribution of electrophoretic phenotypes of the enzymes esterase and glucose phosphate isomerase. Possible explorations of these results are advanced, but they do not materially affect conclusions concerning population structure. The spatial and temporal distribution of electrophoretically detected genetic variation in Atlantic and Pacific sector krill stocks is discussed by comparing the results of this study with those of previous authors working with E. superba.     

Some aspects of feeding in the Antarctic krill,Euphausia superba

Polar Biology - The interaction between feeding activity, swarming behaviour and vertical migration in Antarctic krill, Euphausia superba Dana, was examined by a combination of laboratory...