How Krill feeds

by Uwe Kils

An astonishing phenomenon of the ecosystem in the Southern Ocean is that the size steps between the trophic levels are extreme: microalgae - krill - whale.

Fig. 1: In situ image of a fast swimming krill. Under the body the swimming legs (pleopods) are beating heavily, the filtering basket, formed by the thorecopods, is opened. In front of the feeding basket a red object is visible, probably a copepod. In the lower right corner a spitball is visible and the left lower corner a fecal string (copied from another area of a much larger area of the image) (Image: U. Kils).

A 6 cm long krill (fig.1) filtrates 30 micrometer small planktonalgae, Fragilariopsis kerguelensis, directly out of the open water. The krill has developed an extremely fascinating and in nature unique filtering apparatus: The anterior six legs (thoracopods) are very long and are carried in difolded status below the body obliquely anterior/ventral, forming a finemeshed filtering basket.

The feeding setae carry two rows of of 2nd degree filter setae covering in a v-shape the distance (fig.2).

Fig. 2: Electron microscopic image detail feeding basket. On the 1st-degree filter setae in v-shape are 2nd degree filter setae. The small ball, probably a bacteria, is only 1 micrometer in size. When you want to display the whole net in this magnification one would have to patch this image 7500 times (Image: F. Alberti and U. Kils).

On these are again filter setae 3rd degree arranged so that the distance left open in some areas is only 1 micrometer (fig. 3).

Fig. 3: Again magnified. The gap between the 3rd degree setae is only 1 micrometer. (Image: F. Alberti and U. Kils).

Under normal plankton concentrations the krill pushes its open feeding basket over some decimeters with high velocity of about 10 cm per second through the water, opened only a small slit on its front end. The planktonmass collected on its inner side is then under repeatedly opening and closing of the feeding basket "combed" towards the mouth opening. This is carried out with some rows of strong comb setae, interlocking in right angle into the filtering setae of the anterior thoracopods. Under high planktonconcentrations the krill is hovering at the spot and pumps continuously with its feeding basket. There has been a lot of discussion whether the water in spite of its high viscosity can actually penetrate the finest filtering areas; probably the pumping and the high travel velocity deliver the necessary pressure for this.

The feeding basket sometimes is also utilized to capture larger plankton organisms: In the in situ image of fig. 1. a red organism directly between the antennae can be observed, probably a copedpod. In aquaria krill has been observed feeding on dead animals of the same species.

When it became possible, with underwater cameras, to look into the folded environments of the under-ice surface of the pack ice another feeding strategy of krill became known: Many animals swam in upside down position obliquely under the ice, others were working the side and bottom areas of ice caves.

We had already observed with microscopes and electron microscope six rows unusually strong, conical setae at the distal parts of the anterior thoracopods (fig. 4).

Fig. 4: Three comb setae at the distal part of a thoracopod, strong formed setae, used by krill to scrape off ice algae from the undersurface of ice floes (Image: F. Alberti and U. Kils).

In aquaria krill scraped with these rakelike structure on glass plates on which we had grown beforehand microalgae (diatoms) and was capable to scrape clear a DIN A4 (one square foot) sized area within 10 minutes (Marschall 1988). This was performed in astonishingly systematic pattern, with back and forth swings in the form of a lawn mower, while the feeding basked was pumping continuously and the tips of the thoracopods were swung close to the glass plate. It was clearly visible in these macro video registrations in aquaria that a green aggregation of diatoms was forming in front of the comb setae. The pumping probably delivers an underpressure in the area in which the opened feeding basket is in contact with the ice - otherwise the relatively heavy krill would not be able to hold its position upside-down under the ice, because its propulsion is located ventral under the body and is capable only to propel water obliquely ventral downwards. On the in situ registration krill has never been observed in an upside down position in the free water, only in direct contact with the ice. Krill turned only in the last millimeters directly before contacting with the ice when approaching the ice from below. Therefore krill can not only harvest the organisms growing on the ice surface but is probably sucking out the ice algae living in the ice lakunes, coloring the ice in the lower centimeters green. So krill utilizes, especially in the spring, when 19 million square kilometers large pack ice areas (2 times the area of the USA inclusive Alaska) is melting, a considerable component of the ice ecosystem as food.

In the three following aspects the autecology of feeding of the antarctic krill has influence on processes in global scales:

1) On the high resolution film- and video registrations became visible, that often a ball of plankton aggregation is spit out obliquely anterior/ventral with high speed (see fig. 1 lower right corner, copied from a different part of the image). These aggregates of many thousands of intact plankton algae are sinking with much higher speed than a single cell.

2) Every few minutes the krill ejects a fecal string (see fig. 1 lower left corner, copied from a different part of the image). In these many still intact or only slightly broken algae cells with a lot of carbon and chlorophyll are enclosed. Also these fecal strings sink very fast into the abyss (see article Bathmann).

Where krill lives over several thousands of meters of water depths both processes cause, that in areas with high krill activity large quantities of carbon is carried into the deep ocean where it stays for about 1000 years.

3) Recently it became known that the comparatively messy feeding of krill leads to the fact, that a large amount of DMS (Dimethylsulfid) from the algae cells is set free (Kasamatsu et al. 2004). DMS is propagating the formation of clouds and is therefore relevant for the climate and feedbacks. When plankton is eaten by salps instead no DMS is formed.

Literature

• Kils, U., (2000) IMAGES: Krill stuff. ed. Kayser J., Science 290 (5496) - Netwatch online publication ecoscope.com - enhanced IT tools and translation of Kils, U., Marschall, P. 1995 HYPERLINK