Trophic behavior of Euphausia superba Dana in laboratory conditions

Summary Experiments conducted at US Palmer Station, 1980 on antarctic krill, Euphausia superba, attempted 1) to quantify and describe behavioral features of ingestion, food clearance and egestion, and 2) to test the hypotheses that feeding and swarming are mutually exclusive events, and that feeding is a cyclical, diel phenomenon. Ingestion was quantified in a large flow-through aquarium. Food clearance and egestion were estimated visually and fluorometrically in E. superba individually kept in one liter jars. Ingestion was directly proportional to chlorophyll concentration (0.65–11.5 gChl/l) and did not change significantly as a function of krill density (200–9000 krill/m³). Ingestion and egestion did not show significant diel trends or mean day-night differences in a 16 h light-8 h dark cycle. Our results suggested that feeding may occur in nature: at swarm densities, within a wide range of phytoplankton concentrations, and may be sustained throughout the diel cycle. We propose that feeding and swarming are co-occurring events. The theoretical basis at the individual and group levels, and its implications, are briefly discussed.In memory of Mary Alice McWhinnie (1922–1980)This research was supported by the University of Chile, National Science Foundation and Chilean Antarctic Institute     

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.     

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.     

Population age structure in males and juveniles of the Antarctic Krill,Euphausia superba Dana

Summary Further results are reported on the age structure of populations of the Antarctic krill, using the age pigment lipofuscin. Study of the males shows a five-year adult life span as in females in an earlier study. Only a single age group was found in the juvenile stage.     

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.     

Changes in the biochemical composition of Euphausia superba Dana embryos during early development

Summary The biochemical composition of developing Euphausia superba Dana embryos has been examined at three stages: fresh spawned, gastrula, and limb bud. Fresh spawn embryos had 31% lipid, 1.0% carbohydrate, and 57% protein on a gram dry weight basis. Throughout development lipids were utilized more slowly than in other crustacean embryos for a total utilization of 37.5%. Overall, 35.6% of the starting protein was utilized. On a weight basis, twice as much protein than lipid was used during development, and it appeared that, energetically, protein and lipid contributed equally to the energetics of the developing embryo. Carbohydrate was evidently a minor substrate in early development, although the level increased 38% during development. Average water content was 86% in fresh spawned embryos and 88% in the gastrula stage. The average dry weight of the embryos throughout development was 30 g. The features of planktonic embryos are contrasted with demersal embryos and the atypical metabolic pattern of krill embryos is discussed.MOA dedicates this paper to the memory of Dr. Mary Alice McWhinnie, for her encouragement during my years at DePaul University and for the opportunity to participate in her research     

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     

Benthic feeding by krill, Euphausia superba Dana, in coastal waters off West Antarctica and in Admiralty Bay, South Shetland Islands

Diatoms in the food of krill caught in the summer in coastal waters close to the South Shetland Islands and in the summer and three winters in Admiralty Bay, King George Island were investigated. The diatoms were used as indicators of sources of krill food. Principal component analysis divided the diatom taxa into two groups, one being pelagic diatoms and from the food of krill in coastal waters, and the other being benthic diatoms and taxa from the food of krill in Admiralty Bay. All pelagic taxa of diatoms were consumed by krill. In the summer in coastal waters, benthic diatoms in krill food originated from the sublittoral. In the summer in Admiralty Bay, besides phytoplankton, diatoms were noted from the sublittoral and littoral in the food of krill. However, in the winter mostly sublittoral and epiphytic diatoms were present in krill food. Accepted: 20 March 2000     

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.     

Larval development of Euphausia superba Dana, 1852 and a phylogenetic analysis of the Euphausiacea

Literature data and new investigations by SEM of selected ontogenetic stages of the Antarctic Krill, Euphausia superba Dana, 1852 revealed morphological characters that are either missing in, or significantly changed towards, the adult. Besides adult features, such ontogenetic characters enabled us to propose a hypothesis of the phylogenetic relationships of and within the Euphausiacea on the basis of a computer aided cladistic analysis. These are of the form (Bentheuphausia amblyops + Euphausiidae = ('Thysanopoda' + Nematobrachion + Euphausiinae = (Meganyctiphanes norvegica + Euphausiini, new name + Nematoscelini, new name = (Nyctiphanes + Nematoscelina, new name)))). From this analysis the taxon names 'Euphausiina', 'Nematoscelini', and 'Nematoscelina' are introduced for in-groups of the taxon Euphausiacea as representing monophyletic units. The position of a set of ontogenetic characters remains relatively uncertain due to the still unknown larval development of Bentheuphausia amblyops (G. O. Sars, 1883).     

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.     

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     

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.     

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.     

Microscopic anatomy,functional morphology,and ultrastructure of the stomach of Euphausia superba Dana (Crustacea,Euphausiacea)

Summary Microscopic anatomy, functional morphology, and ultrastructure of the stomach of the antarctic krill Euphausia superba Dana were investigated by means of serial sections, scanning and transmission electron microscopy, and video technique. A separation of the stomach into an anterior part, called cardia, and a posterior part, called pylorus, became evident. Protrusions of the stomach into the midgut form the third region, called the funnel. The interior of the cardia is dominated by the two lateralia, originating from the side walls of the stomach. At their undersurface, they bear the primary filter. It separates the dorsal food channel from the ventral filtration channel, which is divided into two channels by a ventromedian ridge, the anteromedianum. Within the pylorus, the inferolateralia act in sealing the food channel from the filtration channel. In contrast to many other Malacostraca, the inferomedianum bears no secondary filter. During live observations, the stomach of Euphausia superba shows distinct pumping phases. A comparison of the structure of the stomach with data obtained from other Crustacea will lead to a better understanding about the relationships between the Malacostraca.