Krill Feeding Research Articles

Glacial melting: an overlooked threat to Antarctic krill.

Strandings of marine animals are relatively common in marine systems. However, the underlying mechanisms are poorly understood. We observed mass strandings of krill in Antarctica that appeared to be linked to the presence of glacial meltwater. Climate-induced glacial meltwater leads to an increased occurrence of suspended particles in the sea, which is known to affect the physiology of aquatic organisms. Here, we study the effect of suspended inorganic particles on krill in relation to krill mortality events observed in Potter Cove, Antarctica, between 2003 and 2012. The experimental results showed that large quantities of lithogenic particles affected krill feeding, absorption capacity and performance after only 24 h of exposure. Negative effects were related to both the threshold concentrations and the size of the suspended particles. Analysis of the stomach contents of stranded krill showed large quantities of large particles ( > 106 μm3), which were most likely mobilized by glacial meltwater. Ongoing climate-induced glacial melting may impact the coastal ecosystems of Antarctica that rely on krill.

Feeding and overwintering of Antarctic krill across its major habitats: The role of sea ice cover, water depth, and phytoplankton abundance

Antarctic krill (Euphausia superba) were sampled in contrasting habitats: a seasonally ice‐covered deep ocean (Lazarev Sea), ice‐free shelves at their northern range (South Georgia) and the Antarctic Peninsula (Bransfield Strait), and shelf and oceanic sites in the Scotia Sea. Across 92 stations, representing a year‐round average, the food volume in krill stomachs comprised 71 ± 29% algae, 17 ± 21% protozoans, and 12 ± 25% metazoans. Fatty acid trophic markers showed that copepods were consistently part of krill diet, not a switch food. In open waters, both diatom and copepod consumption increased with phytoplankton abundance. Under sea ice, ingestion of diatoms became rare, whereas feeding on copepods remained constant. During winter, larvae contained high but variable proportions of diatom markers, whereas in postlarvae the role of copepods increased with krill body length. Overwintering differed according to habitat. Krill from South Georgia had lower lipid stores than those from the Bransfield Strait or Lazarev Sea. Feeding effort was much reduced in Lazarev Sea krill, whereas most individuals from the Bransfield Strait and South Georgia contained phytoplankton and seabed detritus in their stomachs. Their retention of essential body reserves indicates that krill experienced most winter hardship in the Lazarev Sea, followed by South Georgia and then Bransfield Strait. This was reflected in the delayed development from juveniles to adults in the Lazarev Sea. Circumpolar comparisons of length frequencies suggest that krill growth conditions are more favorable in the southwest Atlantic than in the Lazarev Sea or off East Antarctica because of longer phytoplankton bloom periods and rewarding access to benthic food.

Krill feeding on sediment in the Gulf of Maine (North Atlantic)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 455:157-172 (2012) - DOI: https://doi.org/10.3354/meps09632 Krill feeding on sediment in the Gulf of Maine (North Atlantic) Alison C. Cleary*, Edward G. Durbin, Tatiana A. Rynearson University of Rhode Island, Graduate School of Oceanography, Narragansett, Rhode Island, 02882, USA *Email: acleary@gso.uri.edu ABSTRACT: Krill are key members of many marine ecosystems, serving as a critical trophic link between microscopic organisms and large predators such as whales, fish, and seabirds. Krill feeding is thus important to ecosystem carbon cycling. Traditional approaches to determining in-situ krill feeding require a priori assumptions, and may have prey-type detection biases. We took a DNA-based approach to measuring in-situ feeding by northern krill Meganyctiphanes norvegica. The diversity of prey consumed by M. norvegica in situ was analyzed for 80 krill at 8 stations throughout the Gulf of Maine (North Atlantic) using peptide nucleic acid mediated polymerase chain reaction (PNA-PCR) clone library sequencing of 18S rDNA. Relative abundance of the 2 most common prey types was measured with quantitative PCR (qPCR) in the guts of 16 krill. The 245 prey sequences recovered from krill gut contents included copepods, salps, phytoplankton, and a poorly known organism found to be sediment associated. Calanus finmarchicus and the sediment-associated organism were found most commonly, at 7 and 8 stations, respectively, and their 18S rDNA was present in nearly equal quantities in individual krill guts. M. norvegica, like most krill, are typically considered planktivorous; thus krill feeding on sediment organisms represents an unrecognized pathway for carbon flow from the sediment to the pelagic. Calculations suggest that this unrecognized pathway could potentially bring over 100000 t of carbon annually back into the Gulf of Maine pelagic ecosystem, equivalent to 4% of annual primary production, or the energy demands of 80% of the region’s fin whale population. KEY WORDS: Meganyctiphanes norvegica · Gut contents · 18S rDNA · Benthic−pelagic coupling · PNA-PCR Full text in pdf format PreviousNextCite this article as: Cleary AC, Durbin EG, Rynearson TA (2012) Krill feeding on sediment in the Gulf of Maine (North Atlantic). Mar Ecol Prog Ser 455:157-172. https://doi.org/10.3354/meps09632 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 455. Online publication date: May 30, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

한국 근해의 난바다곤쟁이 Euphausia pacifica의 지방 조성에 의한 섭식 생태 비교

Dietary lipid biomarkers (fatty acids, fatty alcohols, and sterols) in adult specimens were analyzed to compare and understand the feeding ecology of the euphausiid, Euphausia pacifica, from three geographically and environmentally diverse Korean waters (Yellow Sea, East China Sea, and East Sea). Total lipid content of E. pacifica from Korean waters was about 10% dry weight (DW) with a dominance of phospholipids (>46.9% of total lipid content), which are known as membrane components. A saturated fatty acid, C16:0, a monounsaturated fatty acid, C18:1(n-9), and two polyunsaturated fatty acids, C20:5(n-3) and 22:6(n-3), were most abundant (>60% of total fatty acids) in the fatty acid composition. Some of the fatty acids showed slight differences among regions although no significant compositional changes of fatty acids were detected between these regions. Phytol, originating from the side chain of chlorophyll and indicative of active feeding on phytoplankton, was detected all samples. Trace amounts of various fatty alcohols were also detected in E. pacifica. Specifically, krill from the Yellow Sea showed relatively high amounts of longchain monounsaturated fatty alcohols (i.e. 20:1 and 22:1), generally found in herbivorous copepods. Three different kinds of sterols were detected in E. pacifica. The most dominant of these sterols was cholest-5-en-<TEX>$3{\beta}$</TEX>-ol (cholesterol). The lipid compositions and ratios of fatty acid trophic markers are indicative of herbivory in E. pacifica from the Yellow Sea and East Sea (mainly feeding on dinoflagellates and diatoms, respectively). The lipid compositions and ratios of fatty acid trophic markers are indicative of carnivory or omnivory in E. pacifica from the East China Sea, mainly feeding on microzooplankton such as protozoa. In conclusion, lipid biomarkers provide useful information about krill feeding type. However, further analyses and experiments (i.e. gut content analysis, in situ grazing experiment, etc.) are needed to better understand the feeding ecology of E. pacifica in various marine environments.

Detection of zooplankton items in the stomach and gut content of larval krill, Euphausia superba, using a molecular approach

The usefulness of a molecular approach based on polymerase chain reaction (PCR) was investigated to identify and quantify the feeding of larval krill on zooplankton organisms in the Lazarev Sea during winter in 2006. Different primers and probes of dominant copepod species (Oithona sp., Ctenocalanus citer, copepodid stages of Metridia gerlachei and Calanoides acutus), co-occurring with larval krill under sea ice during winter, were developed for quantitative PCR (qPCR) and their species specificity was tested on target and non-target species. The qPCR results showed that larval krill were exclusively feeding on Oithona sp. This result was confirmed by microscopic analysis of stomach and gut contents of larvae from the same stations.

Interannual variation in the diet of non-breeding male Antarctic fur seals, Arctocephalus gazella, at Isla 25 de Mayo/King George Island

The diet of non-breeding male Antarctic fur seals, Arctocephalus gazella, was investigated at Stranger Point, King George Island, through the analysis of scats during three consecutive summer seasons (1996, 1997, 1998). Overall, fish and krill were the most frequent prey occurring, respectively, in an average of 82.9% and 78.8% of samples (n = 131), followed by penguins (22.8%) and cephalopods (17.8%). Myctophids constituted almost 90% of the fish predated, with Electrona antarctica and Gymnoscopelus nicholsi being the most abundant and frequent species consumed. All fish taxa identified were krill feeding species suggesting that seals foraged mainly on a krill and a fish community associated with krill aggregations. However, a seasonal change was observed in the relative proportions of the different prey taxa, with a progressive decrease with time in the occurrence of krill and a concomitant increase of fish, penguins and squid. Possible influence of the strong 1997/98 ENSO event is discussed.

Antarctic Ecosystem: Are Deep Krill Ecological Outliers or Portents of a Paradigm Shift?

Serendipitous observations of Antarctic krill feeding at abyssal depths may revolutionise our view of the ecology of this supposed surface-dwelling animal that is key to the function of the Southern Ocean ecosystem.

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.

Simulated light regimes affect feeding and metabolism of Antarctic krill, Euphausia superba

The effect of different light regimes on physiological parameters (feeding activity, oxygen consumption, and activity of the metabolic enzyme malate dehydrogenase [MDH]) of Antarctic krill, Euphausia superba, was studied over 12 weeks under laboratory conditions. Krill were exposed to light‐cycle regimes of variable intensity to simulate Southern Ocean summer, autumn, and winter conditions, respectively, using: (1) continuous light (LL; 200 lux max), (2) 12‐h light and 12‐h darkness (LD 12 : 12; 50 lux max), and (3) continuous darkness (DD). In all experimental groups, the food concentration was kept at high levels (~ 800 µg C L−1). Krill exposed to LL and LD 12 : 12 showed an increase in all measured parameters over the experimental period. Physiological parameters of krill held under LD 12 : 12 showed a more consistent increase and remained below those of krill held under LL. No change was recorded for krill exposed to DD; clearance rates and daily C rations did not respond to the high food availability, and oxygen consumption rates and MDH activity were significantly lower (p &lt; 0.05) than those of krill exposed to summer light condition. Thus, changes in the environmental light regime have an important effect on physiological parameters of krill, such as feeding and metabolic rates, and may indicate an inherent overwinter adaptation strategy triggered by the Antarctic light regime.

The cold side of life

Barren land covered with huge ice shelves, swept by merciless winds and entrapped by deadly cold seas: that is a common perception of the polar regions. However, this picture could not be more wrong, as both the Arctic and Antarctic are teeming with life that is extraordinarily adapted to the harsh conditions and which often exists at the edge of survival (Fig 1,Fig 2,Fig 3,Fig 4). There has been a growing interest in these psychrophilic organisms, and how they evolved to grow and proliferate at permanently cold temperatures (D'Amico et al , 2006). Yet, it is not only the life sciences that can gain important insights from studying biological communities at the poles. Understanding how polar ecosystems respond to rapid climate change is of global significance; there is evidence that organisms, oceans and the atmosphere in the polar regions interact strongly, influencing each other in complex ways and with potentially major consequences for climate. Figure 1. A jellyfish offshore of McMurdo Station, Ross Island. Photograph by Steve Clabuesch/US National Science Foundation. Figure 2. Chinstrap penguins are about 68 cm (27 inches) tall and weigh about 4 kg (9 pounds). They are social animals and number about 5 million. Photograph by Zee Evans/US National Science Foundation. Figure 3. A crab‐eater seal lounges on an ice floe near the Antarctic Peninsula. Crab‐eaters are the most numerous seals in the world, with a population of more than 15 million. Although they do not eat crabs, they do eat krill and other crustaceans. They will reach a length of over 8 ft (2.44 m) and a weight of up to 500 lb (227 kg). Photograph by Jeffrey Kietzmann/US National Science Foundation. Figure 4. A snailfish, Paraliparis devriesi . Common inhabitants of deeper Antarctic waters, snailfish are characterized by a layer of jelly between the muscle and the …

Seasonal variation of crude digestive protease activity in Antarctic krill Euphausia superba

Crude krill digestive protease (CKP) was extracted on board a fishing vessel from fresh whole krill caught around the Antarctic Peninsula, and its protease activity was measured by using the azocasein method. CKP activity was high from December to February (in summer) and was low from March to August (in winter). CKP activity and mean feed intake index had a high correlation coefficient by Pearson’s correlation coefficient test (P<0.05). These results suggest that the seasonal variation of CKP activity coincides with the seasonal prevalence of some phytoplanktons as feed for krill. In summer, phytoplankton is plentiful so that krill feed on them and the krill protease activity remains high, but in winter phytoplankton abundance is low and krill do not have to keep their proteolytic activity high.

Feeding and energy budgets of Antarctic krill Euphausia superba at the onset of winter—I. Furcilia III larvae

Physiological condition and feeding behavior of furcilia larvae were investigated in autumn (April 1999) in the southwestern Lazarev Sea prior to the critical overwintering period. Furcilia stage III (FIII) larvae were most abundant, so only these were used for all analyses (dry mass [DM], elemental and biochemical composition, gut content) and experiments (metabolic and ingestion rates, selective feeding behavior). Chlorophyll a (Chl a) concentrations in the mixed layer were ≪0.1 µg L-1. Respiration rates of freshly caught FIII larvae were between 0.4 and 1.2 µl O2 mg-1 DM h-1, similar to larvae fed for 7 d on high food concentrations (4 µg Chl a L-1). Excretion rates ranged between 0.01 and 0.02 µg NH4 mg-1 DM h-1. Their atomic O:N ratio of 72 indicated that lipids were the main metabolic substrate of FIII larvae in the field. The daily C ration ranged from 0.4% at the lowest food concentration of 3 µg C L-1 to 28% at the highest enriched food concentration of 216 µg C L-1, whereas clearance rates decreased with increasing food concentrations. In natural seawater, 115 ml mg-1 C h-1, and in natural seawater enriched with ice biota, 24 ml mg-1 C h-1, the clearance rates on specific phytoplankton taxa revealed no significant difference across a food size range of 12–220 µm. The study suggests that during periods of low food supply in the water column, larvae have to exploit ice biota to cover their metabolic demands.

Occurrence of Antarctic krill ( Euphausia superba) concentrations in the vicinity of the South Shetland Islands : relationship to environmental parameters

The influences of abiotic and biotic parameters on the occurrence of Antarctic krill ( Euphausia superba) concentrations were studied in the waters north of the South Shetland Islands, a major krill fishing ground in the Antarctic, during the 1990/1991 austral summer. From early to mid-summer krill density increased and showed distinct offshore-inshore differences in abundance and maturity stages. In mid-summer, krill density was low in the oceanic zone (8 g m -2) and higher in the slope frontal zone (36 g m -2), being highest along the shelf break (131 g m -2) in the inshore zone. Krill were in the reproductive stage in the oceanic and frontal zones, but non-reproductive in the inshore zone. Water circulation, food and frontal features were considered important environmental factors determining the occurrence of high krill concentrations in these waters. Drifting buoys indicated that the surface water circulation in areas of krill concentrations was characterized by a sluggish current with eddies along the shelf break in the inshore zone and a shear current in the frontal zone, thus enhancing the residence time for krill. Diatom abundance in both the frontal and inshore zones increased from early to mid-summer, improving krill feeding conditions. Gravid females were closely associated with the frontal zone, showing that the latter may also be a favored spawning area because of the higher probability of survival for embryos and larvae, in addition to enhanced opportunities for their transport to nursery grounds by prevailing currents. In the waters north of the South Shetland Islands, krill appeared to be highly adapted for exploitation of the rich food resources of the frontal/inshore zone, taking advantage of oceanographic features that led to their retention in these zones.

Interannual Variation in the Diet of the Antarctic Prion Pachyptila desolata at South Georgia

SummaryThe diet of the Antarctic Prion Pachyptila desolata was examined using food samples regurgitated by adult birds during five breeding seasons at Bird Island, South Georgia. In all years the diet was mainly crustaceans, with a small proportion of myctophid fish and trace amounts of cephalopods. Antarctic krill Euphausia superba was the dominant prey item in three years and was replaced by calanoid copepods, especially Rhincalanus gigas and Calanoides acutus, in two years of low krill abundance. Differences in the prey species taken and observations of foraging behaviour suggest that in years of low krill availability Antarctic Prions forage closer inshore, taking copepods by filtering surface water through their palatal lamellae. By switching to feeding on copepods Antarctic Prions are apparently able to maintain a comparable level of reproductive success, unlike most other krill feeding species which suffer much reduced reproductive performance in years of reduced krill availability.

Gut evacuation rates and pigment destruction in the Antarctic krill Euphausia superba

Krill grazing data collected during cruises in the region of the Antarctic Polar Front (S.A. “Agulhas” Voyage 70) and the South Georgia shelf (R.V. “Africana” Voyage 119) during the austral summer of 1993 were analyzed to estimate the variability of crucial parameters of the gut fluorescence technique in relation to food availability and krill feeding history. Gut evacuation rates (k) and passage or throughput times (1/k) varied in the ranges of 0.101 to 0.424 h-1 and 2.3 to 9.9 h and were strongly correlated (p 0.05) with ambient food concentration. However, the pigment lost per individual krill was strongly correlated with the total amount of pigment ingested (p<0.001, r2=0.99). We suggest that both gut evacuation rates and pigment destruction efficiency may be realistically estimated only when krill is allowed to continue ingesting particles uninterruptedly. Charcoal particle concentration should be equivalent to the in situ wet weight of total seston per unit volume. An objective criterion for the standardization of the measurement and calculation of k values is also proposed.

Short-term variations in particulate matter sedimentation off Kapp Norvegia, Weddell Sea, Antarctica: relation to water mass advection, ice cover, plankton biomass and feeding activity

A multi-cup sediment trap was deployed at 250m in the shelf area off Kapp Norvegia, Weddell Sea (630 m water depth) to determine the relative importance of water mass advection, sea ice movement, phytoplankton biomass and plankton feeding. Short-term fluctuations in sedimentation were determined using a sampling frequency of 2.7 days over 54 days during January and February 1988. Three periods of enhanced sedimentation were associated with water mass exchange, settling of diatoms following break-up of ice cover and release of fecal matter by krill feeding on particulate matter derived from phytoplankton and ice algae. An initial sedimentation pulse (28 Jan) was mainly due to sinking pelagic diatoms and krill fecal strings containing algae released from sea ice passing over the trap position. The δ13C-composition of the sedimented organic carbon was about-24‰. The isotope ratio decreased sharply by about 5.5‰ at the end of the first pulse indicating the source of sinking matter becoming pelagic diatoms of the retreating ice-edge. At this time the diatom Corethron criophilum contributed a very high proportion of the organic flux causing an increase of the opal/Corg ratios. The second pulse (6 Feb) was due to empty diatom frustules, minipellets and small planktonic aggregates. Much of the organic carbon was transported by round fecal pellets. During the third pulse (14 Feb), round fecal pellets transported even more; the percentage of C. criophilum to the diatom organic carbon flux was more than 80% (>2mg C m−2 day−1).

Grazing rates of Nyctiphanes australis (Euphausiacea) in the laboratory and Otago Harbour, New Zealand, measured using three independent methods

Functional morphology of Nyctiphanes australis suggests these krill should be able to graze on nanoplankton-sized (2–20 μm) phytoplankton. We measured the functional response of N. australis grazing monocultures of the diatoms Chaetoceros gracilis (5 μm diameter valves) and Thalassiosira weissflogii (10–15 μm valves) to test this prediction. Clearance rates of krill feeding on C. gracilis, measured using controlled dilution of in vivo fluorescence (CDIV) and the gut pigment technique (GP) were low (0.015–0.7 ml krill −1 min −1). Ingestion rates measured by CDIV increased linearly with concentration up to very high pigment levels (36 μg Chl α 1 −1), whereas the GP technique showed evidence of saturation. Twenty-four hour starvation prior to CDIV experiments increased the clearance rate by a factor of 1.25 at concentrations above 4 μg Chl α 1 −1. The krill grazed T. weissflogii at a lower rate than C. gracilis. Clearance rates measured using CDIV (0.18 ml krill −1 min −1) were comparable to rates measured from in vivo fluorescence while controlling algal concentration (CAC) at 14 μg Chl a 1 −1 ( ±10%) for 24 h (0.1–0.5 ml krill −1 min −1). Rates measured by GP near this concentration were lower (0.025 ml krill −1 min −1). Agreement between the three laboratory methods was sufficient to validate the CDIV and CAC techniques, and confirm low grazing rates on nanoplankton diatoms. Clearance rates of N. australis fed nanoplankton-sized phytoplankton cultures in the laboratory were lower than krill in Otago harbour by a factor of 2–18 times using the same (GP) technique. This suggests that the source of gut pigment in the field comes either from larger algae, or from the guts of copepods or other prey.

Overwintering development, growth, and feeding of larval Euphausia superba in the Antarctic marginal ice zone

Seasonal extent of pack ice has enormous influence on the Antarctic ecosystem. An investigation of the marginal ice zone in winter took place in the Scotia‐Weddell Seas, a major nursery ground for the Antarctic krill Euphausia superba. Krill larvae were abundant at the ice edge and on the undersurfaces of ice floes where the pack ice provided greater concentrations of food and a superior refuge compared to the water column. Larval development and growth, which is dependent on food supply, progressed steadily from June through August. In early June, furcilia stages F3–F5 were most abundant but, by August, F6s and juveniles predominated. Krill molted about every 20 d and growth rates (0.07 mm d−1) were similar to reported summer rates. Gut fullness indicated that 98% of the larvae were feeding both day and night. Ingestion rates based on gut pigments, however, were inadequate to meet respiratory requirements. Dietary analysis revealed that in addition to diatoms, larvae ingest protozoans and possibly detritus. If heterotrophic C is considered, larval krill feeding on sea ice biota could ingest sufficient C to support observed growth. Seasonal pack ice coverage over nursery grounds thus plays an important role in the overwintering of larval krill.

Biomechanics of Filter Feeding in the Antarctic Krill Euphausia Superba: Review of Past Work and New Observations

Recent observations of filtration in live euphausiids and unresolved historic controversies about filter feeding in euphausiids based mostly on inferences from preserved specimens suggest that reconsideration of the biomechanics of filter feeding is in order. Explanations of filter feeding in reviews by Mauchline and Fisher (1969) and Mauchline (1980) are incorrect. Here I review previous work and present new observations on the morphology and mechanics of the thoracic endopodites and exopodites of Euphausia superba. Movements of the endopodites are rapid and stereotyped. As the feeding basket expands laterally a pressure gradient is created which sucks water and particles into the basket from the front. Water and particles never enter the basket from the sides, below, or behind. Once inside the feeding basket, particles are retained on the filter and water is squeezed out laterally between the setae. During expansion of the basket the exopodites act as flapper valves which inhibit lateral entry of water, but during the compression stroke the exopodites lift for approximately 100 ms and permit expulsion of water. Comb setae loosen food particles trapped on the filter setae and the tips of the filter setae brush them to the mandibular palps. When phytoplankton is scarce, krill feed steadily with metronomic sweeps of the basket. When phytoplankton is abundant, they exhibit discontinuous feeding patterns. Filter feeding by compression pumping may occur in all those euphausiids that have thoracic appendages like those of E. superba and which filter feed on

Omnivorous feeding behavior of the Antarctic krill Euphausia superba

Feeding experiments were conducted at Palmer Station from December 1985 to February 1986 to examine the potential role of copepod prey as an alternative food source for Euphausia superba. Copepod concentration, copepod size, phytoplankton concentration, the duration of krill starvation and the volume of experimental vessels were altered to determine effects on ingestion and clearance rates. Krill allowed to feed on phytoplankton and copepods in 50-litre tubs showed greatly increased feeding rates relative to animals feeding in the much smaller volumes of water traditionally used for krill-feeding studies. Clearance rates on copepods remained constant over the range of concentrations offered, but clearance rates on phytoplankton increased linearly with phytoplankton concentration. Feeding rates increased when larger copepods were offered and when krill were starved for two weeks prior to experiments. Clearance rates of krill feeding on copepods were higher than, but not correlated with, their clearance rates on phytoplankton in the same vessel. E. superba may have a distinct mechanism for capturing copepods, perhaps through mechanoreception. Although our observed clearance rate of 1055 ml krill-1 h-1 indicates that krill can feed very efficiently on copepod prey, such feeding would meet less than 10% of their minimum metabolic requirements at the “typical” copepod concentrations reported for Antarctic waters. However, substantial energy could be gained if krill fed on the patches of high copepod concentrations occasionally reported during the austral summer, or if krill and copepods were concentrated beneath the sea ice during the winter or spring months. Our results, indicating efficient feeding on zooplankton and higher clearance rates on phytoplankton than previously believed, represent a step towards balancing the energy budget of E. superba in Antarctic waters.