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.     

Breeding antarctic krill in captivity

Antarctic krill were maintained in large aquaria at Port of Nagoya Aquarium, Japan, under controlled photoperiod and were fed on phytoplankton and enriched animal feed. Maturation and spawning were induced after the light?:?dark (L?:?D) cycle was increased from 8?:?16 or 12?:?12 to 24?:?0, or when the L?:?D cycle was held constant at 14?:?10. This study is one of the first studies that demonstrate initiation of maturation and spawning events of krill under controlled photoperiod. Out of three experimental batches of krill, a total of 28 spawning events were observed. The mean number of eggs per event was 1424 with a range between 139 and 3458. The mean hatching success per batch was 19.1%. The relation between photoperiod and maturity/spawning is discussed. Furthermore, hatching is compared to previous studies and the reason for the low success is discussed.     

Breeding antarctic krill in captivity

Antarctic krill were maintained in large aquaria at Port of Nagoya Aquarium, Japan, under controlled photoperiod and were fed on phytoplankton and enriched animal feed. Maturation and spawning were induced after the light : dark (L : D) cycle was increased from 8 : 16 or 12 : 12 to 24 : 0, or when the L : D cycle was held constant at 14 : 10. This study is one of the first studies that demonstrate initiation of maturation and spawning events of krill under controlled photoperiod. Out of three experimental batches of krill, a total of 28 spawning events were observed. The mean number of eggs per event was 1424 with a range between 139 and 3458. The mean hatching success per batch was 19.1%. The relation between photoperiod and maturity/spawning is discussed. Furthermore, hatching is compared to previous studies and the reason for the low success is discussed.     

Changes in muscle tissue of shrinking Antarctic krill

We examined the mechanism by which Antarctic krill, Euphausia superba, shrink, and suggest that cellular changes occurring during shrinkage may provide a means for identifying krill that have undergone shrinkage. We compared the muscle tissue of juvenile, adult and shrunken adult krill to identify changes in cell number associated with maturity and shrinkage. Comparison of the absolute number and density of nuclei in abdominal segments of juvenile, adult and shrunken adult krill revealed differences related to maturity and shrinkage. Shrunken adult krill had nearly twice as many nuclei per unit area than adult krill that had not shrunk. This suggests that krill shrink by a reduction in cell volume, rather than cell loss. This simply detected variation in muscle cell nucleus density may be useful in distinguishing shrunken adult krill from juveniles, and contribute to our knowledge of age structure in natural populations.     

Antarctic krill stock distribution and composition in the Elephant Island and King George Island areas,January–February, 1988

Information is provided on the distribution, size and maturity composition of Antarctic krill (Euphausia superba) stocks in the Elephant Island and King George Island areas, and at repeatedly sampled sites to the north of each island, during January–February, 1988. The overall distributional patterns of different sizes and maturity stages demonstrated a seasonal progression of those observed in the Antarctic Peninsula region during November–December, 1987 by Siegel (1989). The krill sampled at each island site represented different size-maturity groups and demonstrated different horizontal and vertical distributional characteristics. These distributional differences may be related to the demographic differences and/or hydrographie differences between the two sites.     

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.     

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     

Adult antarctic krill feeding at abyssal depths

Antarctic krill (Euphausia superba) is a large euphausiid, widely distributed within the Southern Ocean [1], and a key species in the Antarctic food web [2]. The Discovery Investigations in the early 20(th) century, coupled with subsequent work with both nets and echosounders, indicated that the bulk of the population of postlarval krill is typically confined to the top 150 m of the water column [1, 3, 4]. Here, we report for the first time the existence of significant numbers of Antarctic krill feeding actively at abyssal depths in the Southern Ocean. Biological observations from the deep-water remotely operated vehicle Isis in the austral summer of 2006/07 have revealed the presence of adult krill (Euphausia superba Dana), including gravid females, at unprecedented depths in Marguerite Bay, western Antarctic Peninsula. Adult krill were found close to the seabed at all depths but were absent from fjords close inshore. At all locations where krill were detected they were seen to be actively feeding, and at many locations there were exuviae (cast molts). These observations revise significantly our understanding of the depth distribution and ecology of Antarctic krill, a central organism in the Southern Ocean ecosystem.     

Ocean-bottom krill sex

For the first time the entire sequence of the mating behaviour of Antarctic krill (Euphausia superba) in the wild is captured on underwater video. This footage also provides evidence that mating can take place near the seafloor at depths of 400-700 m. This observation challenges the generally accepted concept of the pelagic lifestyle of krill. The mating behaviour observed most closely resembles the mating behaviour reported for a decapod shrimp (Penaeus). The implications of the new observation are also discussed.     

Related antipodes: a comparative study on digestive endopeptidases from Northern krill and Antarctic krill (Euphausiacea)

The Antarctic krill, Euphausia superba, and the Northern krill, Meganyctiphanes norvegica, are closely related species but occupy significantly different trophic and climatic environments. E. superba holds a key position as a phytoplankton grazer in the Southern Ocean. The omnivorous M. norvegica is an important member of plankton communities in the Northeast Atlantic. Both species expressed high proteolytic activities which were dominated by serine proteinases. In the stomachs of Antarctic krill, activities of total proteinase, trypsin, and chymotrypsin were significantly higher than in Northern krill. In the midgut glands, however, total proteinase and trypsin activities were similar in both species, but chymotrypsin activity was significantly higher in Antarctic krill. Moreover, Antarctic krill expressed four trypsin isoforms while only one isoform appeared in Northern krill. Chymotrypsin was present in either species as one single isoform. Antarctic krill adapted to the low and patchy distribution of food by elevated enzyme activities and the expression of trypsin isoforms with slightly different catalytic properties. Presumably, these enzymes facilitate in concerted action the efficient utilization of proteins from phytoplankton, the major food. Northern krill, in contrast, seems not to be equipped to face food limitation. It expresses a “simple” or “basic” set of digestive enzymes for utilizing abundant and easily digestible prey.     

Working with living krill - the people and the places

The fascination of Antarctic scientists with Antarctic krill and their capabilities has a long and varied history, and prompted many scientists to maintain and manipulate krill under laboratory conditions. Starting in the Discovery era with Mackintosh at the King Edward Point labs on South Georgia, 1930, scientists have collected krill from sailing vessels, small boats, inflatable zodiacs and large ice-breaking vessels. Krill have been maintained in small and large jars, deep rectangular tanks, large round tanks and in flow-through and recycling systems. They have been maintained both on board research vessels and in laboratories, in flowing seawater systems at ambient conditions and in temperature-controlled environmental rooms. A few researchers have transported living krill back to their home laboratories, for example tropical laboratories in Japan (Murano) and Australia (Ikeda), temperate laboratories (Nicol) in Australia, a northern European laboratory in Germany (Marschall) and a sunny maritime laboratory in California (Ross and Quetin). The goals have been varied: short-term experiments to understand in situ physiological rates, long-term experiments to test the effects of manipulations or controlled changes in environmental conditions, and behavioral responses. We take you on a brief historical tour as we trace the lineage of modern day research on living Antarctic krill.     

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.     

Discovery of ciliates reproducing in the gut of Antarctic krill

During a recent Antarctic research cruise (December 1994/February 1995), dissection of fresh Antarctic krill (Euphausia superba) on board ship revealed live motile ciliates in the gut of krill. Further observation of gut samples by scanning electron microscopy indicated that the ciliates were symbionts located within the gut. We inferred that the ciliates may have enabled the krill to digest a wider range of food items, and as a consequence, this had became an important strategy for Antarctic krill's survival in the Antarctic ecosystem. Received: 1 August 1996 / Accepted: 1 December 1996     

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.     

A photographic documentation of the development of Antarctic krill (Euphausia superba) from egg to early juvenile

Antarctic krill (Euphausia superba) is a key species in Antarctic marine ecosystems, as well as an important species in the Southern Ocean fishery. Here, we provide the first detailed photographic documentation of embryonic and larval development of Antarctic krill over a 5-month developmental period under controlled laboratory conditions. Developing embryos and larvae were photographed every 3 h and every 5 days, respectively. Our results indicated a developmental time of approximately 6 days for embryos and 138 days for larvae (0.5 °C). This study provided baseline biometry information for future investigations of Antarctic krill development under changing environmental conditions.     

Relative changes in krill abundance inferred from Antarctic fur seal

Antarctic krill Euphausia superba is a predominant species in the Southern Ocean, it is very sensitive to climate change, and it supports large stocks of fishes, seabirds, seals and whales in Antarctic marine ecosystems. Modern krill stocks have been estimated directly by net hauls and acoustic surveys; the historical krill density especially the long-term one in the Southern Ocean, however, is unknown. Here we inferred the relative krill population changes along the West Antarctic Peninsula (WAP) over the 20th century from the trophic level change of Antarctic fur seal Arctocephalus gazella using stable carbon (δ(13)C) and nitrogen (δ(15)N) isotopes of archival seal hairs. Since Antarctic fur seals feed preferentially on krill, the variation of δ(15)N in seal hair indicates a change in the proportion of krill in the seal's diets and thus the krill availability in local seawater. For the past century, enriching fur seal δ(15)N values indicated decreasing krill availability. This is agreement with direct observation for the past ～30 years and suggests that the recently documented decline in krill populations began in the early parts of the 20th century. This novel method makes it possible to infer past krill population changes from ancient tissues of krill predators.     

Super-aggregations of krill and humpback whales in Wilhelmina Bay, Antarctic Peninsula

Ecological relationships of krill and whales have not been explored in the Western Antarctic Peninsula (WAP), and have only rarely been studied elsewhere in the Southern Ocean. In the austral autumn we observed an extremely high density (5.1 whales per km(2)) of humpback whales (Megaptera novaeangliae) feeding on a super-aggregation of Antarctic krill (Euphausia superba) in Wilhelmina Bay. The krill biomass was approximately 2 million tons, distributed over an area of 100 km(2) at densities of up to 2000 individuals m(-3); reports of such 'super-aggregations' of krill have been absent in the scientific literature for >20 years. Retentive circulation patterns in the Bay entrained phytoplankton and meso-zooplankton that were grazed by the krill. Tagged whales rested during daylight hours and fed intensively throughout the night as krill migrated toward the surface. We infer that the previously unstudied WAP embayments are important foraging areas for whales during autumn and, furthermore, that meso-scale variation in the distribution of whales and their prey are important features of this system. Recent decreases in the abundance of Antarctic krill around the WAP have been linked to reductions in sea ice, mediated by rapid climate change in this area. At the same time, baleen whale populations in the Southern Ocean, which feed primarily on krill, are recovering from past exploitation. Consideration of these features and the effects of climate change on krill dynamics are critical to managing both krill harvests and the recovery of baleen whales in the Southern Ocean.     

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.     

Effects of simulated light regimes on gene expression in Antarctic krill (Euphausia superba Dana)

A change in photoperiod has been implicated in triggering a transition from an active to a quiescent state in Antarctic krill. We examined this process at the molecular level, to identify processes that are affected when passing a photoperiodic threshold. Antarctic krill captured in the austral autumn were divided into two groups and immediately incubated either under a photoperiod of 12 h light:12 h darkness (LD), simulating the natural light cycle, or in continuous darkness (DD), simulating winter. All other conditions were kept identical between incubations. After 7 days of adaptation, krill were sampled every 4 h over a 24 h period and frozen. Total RNA was extracted from the heads and pooled to construct a suppression subtractive hybridisation library. Differentially expressed sequences were identified and annotated into functional categories through database sequence matching. We found a difference in gene expression between LD and DD krill, with LD krill expressing more genes involved in functions such as metabolism, motor activity, protein binding and various other cellular activities. Eleven of these genes were examined further with quantitative polymerase chain reaction analyses, which revealed that expression levels were significantly higher in LD krill. The genes affected by simulated photoperiodic change are consistent with known features of quiescence, such as a slowing of moult rate, a lowering of activity levels and a reduction in metabolic rate. The expression of proteases involved in apolysis, where the old cuticle separates from the epidermis, showed particular sensitivity to photoperiod and point to the mechanism by which moult rate is adjusted seasonally. Our results show that key processes are already responding at the molecular level after just 7 days of exposure to a changed photoperiodic cycle. We propose that krill switch rapidly between active and quiescent states and that the photoperiodic cycle plays a key role in this process.