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     

Preliminary study on chromosomes of Antarctic krill,Euphausia superba Dana

Summary Meiotic chromosomes of antarctic krill, Euphausia superba Dana, collected at Admiralty Bay, are described and the method used to obtain this genetic material is reported. Forty seven dividing meiotic nuclei at diakinesis and metaphase I from seminal glands of four males were examined, all with n= 17 bivalents. The results were compared with those obtained by other workers using krill specimens from the opposite side of the Antarctic continent and apparent differences in chromosome numbers were observed. Possible reasons for these differences and the potential for using cytogenetic techniques to study populations of krill are discussed.     

Collagenolytic serine proteinase from Euphausia superba Dana (Antarctic krill).

1. A serine proteinase isolated from E. superba shows collagenolytic properties: it acts on collagens from Achilles tendon (type I and V) and reconstituted fibrils of calf skin collagen under conditions that do not denature the substrates. 2. At 25 degrees C and pH 7.5 the enzyme both splits the calf skin collagen in solution to the fragments TCA and TCB and catalyses the conversion of dimeric molecules to monomeric chains. 3. The enzyme exhibits strong chymotrypsin-like and lower trypsin-like activities. 4. All the enzyme activities are inhibited to the same degree by diisopropylfluorophosphate (DFP), phenylmethylsulphonyl fluoride (PMSF), N alpha-tosyl-L-lysine chloromethyl ketone (TLCK), soybean trypsin inhibitor (SBTI), chicken ovomucoid (CHOM), chymostatin and leupeptin. None of the activities is inhibited by chelating agents and L-cysteine. 5. pH-Optima of the proteinase in protein substrates hydrolysis (6.0-6.2) are lower than those of synthetic substrates cleavage (7.8-8.0 in the case of BzTyrOEt and 8.7-8.9 for BzArgOEt). 6. Four from nine cysteine residues present in the enzyme molecule possess free thiol-groups. Since the enzyme is inhibited by p-chloromercuribenzoate (pCMB), N-ethylmaleimide (NEM) and iodoacetic acid (IAA), the role of its thiol-groups has been discussed.     

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     

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.     

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.     

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.     

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.     

The influence of feeding on the metabolic activity of Antarctic krill (Euphausia superba Dana)

Summary The influence of feeding on the metabolic activity of juvenile krill was assessed from 24h experiments in which krill were incubated with various concentrations of diatoms (Chaetoceros calcitrans, Phaeodactylum tricornutum, Thalassiosira eccentrica, Fragilariopsis vanheurkii), newly hatched Artemia nauplii and latex beads. Krill fed on the larger food more efficiently, with reluctant feeding on latex beads. Feeding of krill expressed as clearance rates was poorly correlated with their oxygen uptake rates. Instead, a positive correlation was found between the oxygen uptake rates and ingestion rate (except for latex beads). The result implies that the specific dynamic action is the major cause of the increased oxygen uptake of krill. Krill fed diatoms increased both ammonia and phosphate excretion with increasing ingestion rate, but only phosphate excretion was increased in parallel with ingestion rate for those fed Artemia nauplii. Assuming the daily ration of krill in the field is 5% of the body weight, and the major food source is phytoplankton, oxygen uptake, ammonia excretion and phosphate excretion rates of wild krill are estimated to be 1.6, 4.5 and 7.8, respectively, times the rates of non-feeding krill in 24h laboratory experiments. Krill offered various kinds of food showed different metabolic quotients (O/N, N/P and O/P ratios). While no functional relationship was seen between the metabolic quotient and the ingestion rate of krill fed Artemia nauplii, those fed Fragilariopsis showed a progressive decrease in O/N, N/P, and O/P ratios as their ingestion rates increased.     

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.     

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.     

Trace element concentrations in Antarctic krill,Euphausia superba

Whole Antarctic krill, Euphausia superba collected along the Western Antarctic Peninsula, were analyzed for 14 elements. Average element abundances (in parentheses) in g/g, in descending order, were as follows: P(9940), Cu (80.5), Zn (43.5), Fe (28.0), Se (5.80), Ba (3.78), Mn (1.98), As (1.92), Ag (1.71), Ni (0.54), Cr (0.30), Cd (0.29), Pb (0.22), and Hg (0.025). Inverse relationships were found between krill length and Hg concentration as well as between As and P levels. A geographic trend of increasing Mn and P levels from southwest to northeast along the Antarctic Peninsula was Sound. Results were compared to earlier data for evidence of metal concentration changes due to anthropogenic activity over the last 15 years.     

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.     

The association of Antarctic krill Euphausia superba with the under-ice habitat

The association of Antarctic krill Euphausia superba with the under-ice habitat was investigated in the Lazarev Sea (Southern Ocean) during austral summer, autumn and winter. Data were obtained using novel Surface and Under Ice Trawls (SUIT), which sampled the 0-2 m surface layer both under sea ice and in open water. Average surface layer densities ranged between 0.8 individuals m(-2) in summer and autumn, and 2.7 individuals m(-2) in winter. In summer, under-ice densities of Antarctic krill were significantly higher than in open waters. In autumn, the opposite pattern was observed. Under winter sea ice, densities were often low, but repeatedly far exceeded summer and autumn maxima. Statistical models showed that during summer high densities of Antarctic krill in the 0-2 m layer were associated with high ice coverage and shallow mixed layer depths, among other factors. In autumn and winter, density was related to hydrographical parameters. Average under-ice densities from the 0-2 m layer were higher than corresponding values from the 0-200 m layer collected with Rectangular Midwater Trawls (RMT) in summer. In winter, under-ice densities far surpassed maximum 0-200 m densities on several occasions. This indicates that the importance of the ice-water interface layer may be under-estimated by the pelagic nets and sonars commonly used to estimate the population size of Antarctic krill for management purposes, due to their limited ability to sample this habitat. Our results provide evidence for an almost year-round association of Antarctic krill with the under-ice habitat, hundreds of kilometres into the ice-covered area of the Lazarev Sea. Local concentrations of postlarval Antarctic krill under winter sea ice suggest that sea ice biota are important for their winter survival. These findings emphasise the susceptibility of an ecological key species to changing sea ice habitats, suggesting potential ramifications on Antarctic ecosystems induced by climate change.     

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...     

Determining the sex of Antarctic krill Euphausia superba using carapace measurements

A discriminant function using simple carapace measurements corretly determined the sex of 87% of male and 89% of female Antarctic krill Euphausia superba. This facilitated the use of sex-specific regression models which increased the accuracy of total length estimates. Received: 5 May 1997 / Accepted: 30 August 1997     

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 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.