Antarctic Krill Euphausia superba

Feeding and Swimming Performances

New Insights with New Methods

1995

Uwe Kils & Hans Peter Marschall

INSTITUTE OF MARINE & COASTAL SCIENCES

RUTGERS STATE UNIVERSITY - NJ USA

BAYER Medical Services - Wuppertal GER

in

Biologie der Polarmeere

Erlebnisse und Ergebnisse

(Biology of the Polar Oceans)

(Adventures and Results)

Hempel I, Hempel G

Fischer Jena - Stuttgart - New York

pages 201-207 (and images p 209-210)

this is a first rough translation of the original German version for teaching purposes made by friends. If you want to help or/and find errors you are heartily welcome to do so (email)

The traditional conception on krill

When in 1974 the revived German Antarctic Program emerged, our general conception was that krill-shrimp float along in huge schools through endless plankton clouds of the Southern Ocean, skimming with its fantastic filtering basket - whose morphology was well described by early publications - even the most minute plankton algae from the seas. Already the old whalers, in their tales, enthused about the deep green krill "guts" - a life "wie im Schlaraffenland" - in the land of milk and honey. In the lecture hall and presentations citing this life style helped us to explain their inconceivable large population size to students and colleagues. We gained unforgettable impressions during our first Antarctic Expedition in 1977: No matter in which direction or how long our research vessel sailed the Scotia Sea, the hauls of our commercial pelagic trawl came back on board tauted with krill.

Traditional methods

At that time, large plankton nets were our traditional gear - the most modern we could get on the markets - with cod-end revolvers or multiple closing mechanisms to learn more about small scale distribution and swarm densities of this important species. Soon the icon of Schlaraffenland became first cracks: Our planktlogists showed that the often cited plankton blessing of the Southern Ocean was not consistent but varied considerably with location and time. And even though the bursting krill nets were quite impressive at first sight, if we calculated the number of the individuals divided by the liters of water filtered, quite mediocre average swarm densities surprised us: only a few dozens of krill per cubic meter of water.

Contradictions and wishes

However, from the first underwater movies of Jacques Cousteau and observing krill near the surface it seemed as if many thousands can school in each cubic meter, also they seemed not simply to float along like plankton, but exercised in astonishing ways and speeds, obviously with a goal and very organized. Consequently there was much discussion whether the low calculations of the net-count averages might have been a consequence of high escape capacity, or maybe because of fishing empty waters besides and above/below an actual school, unavoidable even with the state-of-the-art nets. During these discussions the wish evolved for a new oceanographic tool allowing for in situ exploration of ultra small scale distributions and behavior of single individuals.

New methods

Photo- film- and VIDEO-cameras we stuffed in sealed housings, hooked up with remote controls, communication gear and ultra short XENON lights. They were hauled, towed or dropped through the schools, as very primitive prototypes. Fig. 1 sketches deployments of our first "Optical Plankton Recorder": the streamlined gadget is towed with 3 meters per second through a school, or whizzes -just thrown overboard- with 3 meter per second into the abyss. Each meter of travel it records an image, illuminated by a double-strobe. After 30 meters a Nylon-line limits the fall. Fig. 2 shows in the upper part a krill, hovering obliquely. The tale shows up twice, the left, slightly lighter image was frozen by the second strobe 20 milliseconds delayed (duration of flash 10 microseconds). This individuum just starts an escape flight, the form of the fastest escape strategy, lobstering with a series of tail-flips backwards. The image below captured another animal during a more progressed phase of such an escape-flight: the inflected tail gets just extended again.

Quantification

This primitive gear however allowed already rough evaluations of distance, speed and angle, because the ultra wide aperture lens imaged only a 4 cm thin sheet -between 108 and 112 cm off the frontglass- with steep contrast. Only organisms with good sharpness on both exposures were evaluated - the threshold of edge-contrast and the geometrie was set in a calibration tank with a 3D calibration target.

Fig. 1. Schematic of the deployments of a prototype of our first  "Optical Plankton Recorder (OPR)". The system can be trawled behind the ship or operated "free-falling". A long focal length lens images sharp only the areas one meter off the frontglass (pink). Illumination with double-flashes. Modified after KILS 1979.

The size of the animals in Fig. 2 is about 55 mm, and the lower animal darts with more than 650 mm per second away. This krill-escape-flight was already scanned in 1978 and is probably the first quantitative in situ image ever published from the open ocean allowing for an evaluation of dynamics - a field today growing into a major topic within the international initiative GLOBEC (Global Ocean Ecosystem Dynamics).

Fig. 2. Quantitative images of krill during escape-flights, illuminated with double-flashes. The upper animal is just closing the filtering basket, the lower individual has folded the filtering basket close to the body during the high-speed-phase of the flight, protecting the delicate structures and reducing friction. Direction of flight is to the right. Scanned from fisheries research vessel "Walther Herwig" in Austral Summer 1977/1978. Modified after KILS 1981

New insights

The high swimming speeds we published were often doubted by colleagues, also the oblique hovering angle, which was in conflict with all krill-drawings or -photos existing in those years - however those were taken probably all from conserved animals. Later both results could be confirmed manifold and independently by laboratory experiments and by advanced versions of "Optical Plankton Recorders" (fig.3). Our quantitative in situ optics granted us new results of the dynamics of krill: They can maintain cruising speeds of 10 -20 cm per second over long periods, they can escape forwards with up to 40 cm per second, and backwards over a short distances lobster away with 70 cm per second.

And krill does not float along in the manner of good plankton: due to a high weight and lacking of a swim-bladder it must burn considerable amounts of food in order not to stall into the abyss: Only well nourished animals are capable to devote enough energy struggling day and nights.

Also, filtration is not so easy: The "net" (image see fig. 2 in article BUCHHOLZ ) is not just leisurely snugged along - then the viscous water could not even pass the minute openings of only a few micrometers. With powerful pumping excursions smallest packages of water are entrapped, then by compression of the basket squeezed through a fascinating filtering net. A design no technician would be able to copy til today. Also, the school-density calculations derived from our quantitative optics came out orders of magnitude higher than the net-gear-estimations: up to 8000 animals per cubic meter.

Fig. 3. Quantitative images of an advanced version of our OPR. Visible is the oblique position of the animals in space. Right images are taken from the side, left image from below. Scanned from research vessel "METEOR" during Austral Summer 1980/81 in exceptionally clear water. The upper animal has just closed the filtering basket, the middle animal half, the lower animal fully opened. Below the tail the metachron beating swimming-legs are visible.

Our new perspectives of krill

So, slowly, some of our perspectives on krill drifted from "plankton" towards nekton, from a passive floating to an active swimming organism. And, after the returnees from winter-expeditions reported extremely low plankton concentration, waters as clear as glass, with visibilities over 70 meters, our image of "Schlaraffenland" faded more and more. We were pretty helpless, questioned by our students "How do krill get over the winter?" , or even "Where are and what nourishes these billions of animals in winter?"

The first plankton nets, painstakingly deployed under the pack-ice, returned empty. However in the wake of icebreaking ships it was frequently observed that krill washed up onto tilted floes. Therefore we tried to scrape with newly constructed nets directly under the ice - also with poor success.

But when finally - after many, many  frustrating attempts - we succeded to maneuver our high definition cameras with a remotely operated underwater-robot downunder the ice (MARSHALL 1988), we looked into a system of caves and crevices, buzzing with krill (fig.4). and many were eagerly fiddling with the ice: scraping and raking with the tips of their filter-legs they tempered in rhythmic motions with the crystals.

Fig. 4. Krill under the ice (partial image, click into image or go to even higher resolutions). Many animals move around between the crevices and crystals, some working on the ice, harvesting diatoms. A SLR -single lens reflective - camera was maneuvered remotely by our underwater robot (remotely controlled vehicle, ROV) into the pack-ice. From onboard the research vessel the operator can view directly through the viewfinder of the photocamera via a VIDEO chip, optionally via a high power light amplifier, to follow the scenery. With joy-sticks the propellers of the ROV are controlled. On interesting events the high resolution photo-camera is triggered. By synchronously recording the video signal we later can analyze motion.

Now the function of several strange constructed rows of setae of the filtering basket at the very distant limbs of the filter legs (fig. 5) became sense: with these combs the krill is harvesting microalgae and microorganisms off the ice, like a lawn mower. The lower few centimeters of the ice we found high food concentrations (see article of SPINDLER), while the water just a foot below is nearly void of food during winter. Laboratory experiments later with artificial "ice-lawn" we grew on glassplates discovered an astonishing, unknown feeding capacity of krill: a "lawn" of a foot square is grazed down in a systematic pattern within 10 minutes.

Fig. 5. Head of krill imaged on high definition film (click in the image to go to higher resolutions or on the eye etc. to investigate deeper). Shown about one quarter of an animal with a total length of 58 mm. In the lower right (at 5 o'clock) on two end limbs of the filtering legs two rows of rake setae are visible, because the illumination angle was adjusted to an angle, at which this micro-grid reflects the light. Remarkable are the large, black compound eyes, carried on a stalk, granting high positioning capabilities and free rotation of 360 degrees. Directly below the eye is one of the ten light-organs visible (red spot on the stalk), capable of emitting a yellow-greenish luminance. In the lower part the filtering basket (in a closed status) is visible, formed by the 6 thoracopods as mainframes, carrying again rows of setae to form a micro-net. The body of the animal is transparent, with the "gastric mill" (behind the eye) and at the left side the hepatopancreas ("gut") shining through. In the gastric mill the diatom shells are crushed and the digestible components moved into the hepatopancreas. The characteristic red appearance krill derives from pigments, contained in star-shaped, contractile chromatophores. This animal was imaged - alive - in our newly developed micro-aquarium onboard the research vessel, with ultra short XENON illumination - a newly developed software package optimized the raytracing for best resolution and depth of focus.

These observations triggered the installation of a new school for "Eislückenfauna" in the following years, covered in this book on a special section (see article SPINDLER)

Resume

During the "summer" krill is -in spite of its very low body temperature- a surprisingly mobile, busy and alert animal. Much energy is burned for pumping plankton through the filtering basket, for fueling strong swimming legs scanning vast seas for ever new plankton clouds, and for the continuous struggle against everthreatening stalling. Disturbances trigger astounding escape capacities.

During winter, when the habitat is covered with endless pack-ice, the krill-herds graze down the folded "anti-benthos" under the floes. The caves and crevices, too,  grant shelter against numerous predators. So, too, these icy critters did hide their secrets of winter-life from our traditional science gear for many years.

Maybe the biggest sorrow of the krill is more like "how do we get over the next summer?" because then from northern latitudes cruise again the great whales into the luscious Antarctica  - and maybe soon men with their super-nets?

Vantage and Respect

More about the new optical in situ methods, granting us today also astonishing insights into the dynamic and beauty of other schooling critters, can be found in SCHULZE et. al. (1992) or KILS (1992), more about morphology and dynamic of krill in BERGSTROEM et al. (1990), HAMNER (1988), ALBERTI & KILS (1980), KILS & KLAGES (1979), KILS (1979, 1982, 1983, 1986) und MARSCHALL (1988).

With each new observation of these so successful species in its mystic element ocean our fascination and respect for life did grow. Today, one very characteristica of our new gadgets granted for us - but, more important, for our children - the most happiness: Nearly all critters we explored stayed alive - "... und wenn sie nicht gestorben sind, so leben sie noch heute" - and they lived happily ever after.

Literature

ALBERTI G, KILS U (1980) The Filtering-basket of Euphausia superba. Coun Meet Int Coun Explor Sea 1980/L54:1-7

BERGSTROM BI , HEMPEL G , MARSCHALL HP, NORTH A, SIEGEL V & STROEMBERG JO (1990) Spring distribution, size composition and behavior of krill, Euphausia superba in the western Weddell Sea. Polar Rec 26:85-89

HAMNER WM (1988) Behavior of plankton and patch formation in pelagic ecosystems. - Bull Mar Sci 43:752-757

KILS U, KLAGES N (1979) Der Krill. Naturwiss Rundsch 32:397-402

KILS U (1979) Swimming Speed and Escape Capacity of Antarctic Krill, Euphausia superba. Meeresforsch 27:264-266

KILS U (1979) Schwimmverhalten, Schwimmleistung und Energiebilanz des Antarktischen Krills, Euphausia superba. Ergebnisse der 2. deutschen Antarktis-Expedition des FFS Walther Herwig im Südsommer 1977/78, Ber Inst Meereskd Kiel 65:1-147

KILS U (1982) Swimming behavior, swimming performance and energy balance of Antarctic krill, Euphausia superba. - BIOMASS Sci Ser, Texas 3:1-121

KILS U (1982) Size Dissociation in Krill Swarms. In: Rheinheimer G, Flugel H, Lenz J, Zeitzschel B (eds) Lower Organisms and Their Role in the Food Web. Kiel Meeresforsch 5:262-263

KILS U (1982) The Unique Position of Krill in the Antarktic System. In: O'Quinn (ed) Joint Oceanographic Assembly Abstracts. Dalhousie University Halifax 1982:18

KILS U (1983) Swimming and Feeding of Antarktic Krill, Euphausia superba - some Outstanding Energetics and Dynamics, some Unique Morphological Details. Ber Polarforsch Sonderh 4:130-155

KILS U (1986) Verhaltensphysiologische Untersuchungen an pelagischen Schwärmen, Schwarmbildung als Strategie zur Orientierung in Umweltgradienten, Bedeutung der Schwarmbildung in der Aquakultur. Ber Inst Meeresk, Kiel 163:1-168

KILS U (1992) The ecoSCOPE and dynIMAGE: Microscale tools for in situ studies of predator-prey interactions. Arch Hydrobiol Beih 36:83-96

SCHULZE P, STRICKLER R, BERGSTROEM B, BERMAN M, DONAGHAY P, GALLAGER S, HANEY J, HARGREAVES B, KILS U, PAFFENHOEFER G, RICHMAN S, VANDERPLOEG H, WELSCH W, WETHEY D, YEN J (1992) Video systems for in situ studies of zooplankton. Arch Hydrobiol Beih 36:1-21

MARSCHALL HP (1988) The overwintering strategy of Antarctic krill under the pack-ice of the Weddell Sea. Polar Biol 9: 129-135


This is the smallest and latest version (in Hempel's hand) with 20 meter cable (in Uwe's hand)

on the danish island of Fanoe in summer 1998


copyright fischer verlag and uwe kils

edited and served by uwe kils

scienceplus natureplus kinder university e. superba krill.rutgers.edu http://krill.rutgers.edu