Experiments on the physiology of southern and northern krill, Euphausia superba and Meganyctiphanes norvegica, with emphasis on moult and growth - a review

Physiological data are needed for life history studies on krill, and as parameters for input into energy budgets and models. In conjunction with moult and growth data, these may also prove useful for assessing the fishable biomass of krill. Here, the development of physiological concepts in experimental krill research is briefly evaluated, with emphasis on the gaps to be filled. Krill growth is very flexible, as well as strongly temperature and nutrition dependent. The polar Antarctic krill Euphausia superba grows as fast as the boreal species Meganyctiphanes norvegica, at least during the first 2.5 years, and the species are comparable in terms of physiological plasticity. Accordingly, as krill appear to adjust quickly to specific laboratory conditions, short-term experiments are essential if field conditions are to be reflected as closely as possible. Furthermore, direct comparisons between laboratory experiments and swarming studies in the field are advantageous. For these, M. norvegica is particularly well-suited, as swarms can be followed over longer times and more easily than in E. superba. For example, processes of moult and reproduction were found to be highly coordinated in swarms and populations of Northern krill. For this species a conceptual model of reproduction was developed based on a combination of short-term laboratory observations coupled with field data on moult and ovary stages. In further physiological experiments krill should be studied as groups when swarming. Using proxies, that is applying physiological and/or biochemical methods side by side, is a promising way to enhance the reliability of life history data.     

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.     

Partial characterization of chitin degrading enzymes from two euphausiids,Euphausia superba and Meganyctiphanes norvegica

Summary Chitinase and N-acetyl--D-glucosaminidase have been demonstrated in Meganyctiphanes norvegica and in Euphausia superba and partly characterized. The enzymes from both species have broad pH-optima (maxima around pH 5.0) and temperature optima between 40 and 50°C. The enzymes are relatively stable; even at about 45°C half of the enzyme activity is retained after 30 min incubation. The presence of fluoride does not affeet enzymatic activity. Chitinase activity appears in three different molecular masses, N-acetyl--D-glucosaminidases in two different forms. pH and temperature optima, thermal stability and kinetic properties of the two enzymes are strikingly similar in the polar E. superba versus the boreal euphausiid M. norvegica. Enzyme activity in the lower temperature range is still high, whereas activation energies are low in both euphausiids. This suggests a functional adaptation to a low temperature range in seawater.     

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.     

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.     

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.     

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     

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.     

Modelling individual growth of the Antarctic krill Euphausia superba Dana

Summary Growth of the Antarctic krill, Euphausia superba, is not easily determined from net catches nor from laboratory experiments. Therefore, in support of these methods, a phenomenological model was constructed which in its present state describes the growth of a single krill specimen under periodically limiting food conditions with summer seasons of variable lengths. Published data of krill body length vs. age and of the annual cycle of primary production of algae in the Drake Passage were used to formulate equations and to calculate growth curves. At 1,000 days after hatching, the model predicts a body length of 63 mm, growth being delayed by 380 days compared with constant, optimal feeding conditions. Final length, weight and time delay are related to the amount of food supplied and compared with published population growth curves.     

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     

Histopathology of Antarctic krill, Euphausia superba, bearing black spots

Small black spots have been noticed on the cephalothorax of Antarctic krill, Euphausia superba, since January, 2001. To study the nature of the black spots, the krill were sampled in the winter of 2003, 2006, and 2007 in the South Georgia region, the Antarctic Ocean. Histological observations revealed that the black spots were melanized nodules that were composed of hemocytes surrounding either bacteria or amorphous material. In the 2007 samples, 42% of the krill had melanized nodules. Most of the nodules had an opening on the body surface of the krill. A single melanized nodule often contained more than one type of morphologically distinct bacterial cell. Three bacteria were isolated from these black spots, and classified into either Psychrobacter or Pseudoalteromonas based on the sequences of 16S rRNA genes. More than three bacterial species or strains were also confirmed by in situ hybridization for 16S rRNA. The melanized nodules were almost always accompanied by a mass of atypical, large heteromorphic cells, which were not observed in apparently healthy krill. Unidentified parasites were observed in some of the krill that had melanized nodules. These parasites were directly surrounded by the large heteromorphic cells. Histological observations suggested that these heteromorphic cells were attacking the parasites. These results suggest the possibility that the krill had been initially affected by parasite infections, and the parasitized spots were secondary infected by environmental bacteria after the parasites had escaped from the host body.     

The overwintering of Antarctic krill, Euphausia superba, from an ecophysiological perspective

A major aim of this review is to determine which physiological functions are adopted by adults and larvae to survive the winter season with low food supply and their relative importance. A second aim is to clarify the extent to which seasonal variation in larval and adult krill physiology is mediated by environmental factors with a strong seasonality, such as food supply or day light. Experimental studies on adult krill have demonstrated that specific physiological adaptations during autumn and winter, such as reduced metabolic rates and feeding activity, are not caused simply by the scarcity of food, as was previously assumed. These adaptations appear to be influenced by the local light regime. The physiological functions that larval krill adopt during winter (reduced metabolism, delayed development, lipid utilisation, and variable growth rates) are, in contrast to the adults, under direct control by the available food supply. During winter, the adults often seem to have little association with sea ice (at least until early spring). The larvae, however, feed within sea ice but mainly on the grazers of the ice algal community rather than on the algae themselves. In this respect, a miss-match in timing of the occurrence of the last phytoplankton blooms in autumn and the start of the sea ice formation, as has been increasingly observed in the west Antarctic Peninsula (WAP) region, will impact larval krill development during winter in terms of food supply and consequently the krill stock in this region.     

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.     

Purification and characterization of a proteinase from Euphausia superba Dana (Antarctic krill)

The thiol-dependent serine proteinase (inhibited by DFP, PMSF, pCMB and iodoacetate) was isolated from the whole krill specimens and from the content of the krill digestive tract. The enzyme was purified to homogeneity using a seven-step procedure. Its specific activity with denatured haemoglobin as a substrate was about 6.0 unit/mg. The molecular weight of the enzyme, as determined by gel exclusion chromatography was 33 000 and by polyacrylamide gel electrophoresis with SDS 31 600 (12.5% gel) and 27 000 (7.5% gel). The enzyme is an acidic glycoprotein (pI below 2.9) containing about 5% of carbohydrate. The pH optimum of the enzyme with haemoglobin was 6.0 at the optimal temperature of 40 degrees C in 15-min reaction. The enzyme showed the esterase activity (hydrolysis of BAEE) and was inactive with carbobenzoxy- and benzoyl-dipeptides with the following C-terminal amino acids: Phe, Tyr, Lys, Gly and Leu.     

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.