The investigations of the flow field while swimming (fig.l0, ll, 12) end the analysis of the pleopod stroke (fig.21) showed that the propulsion force produced by the krill is not, as was to be expected, directed directly backwards, but rather diagonally downwards. The influence of gravitational acceleration on the krill should be investigated as a possible cause for this phenomenon.
The center of mass was determined thus: dead krill was held under water by two opposing needle points, so that it could turn easily in this bearing. The points of contact of the needles were changed until the krill no longer turned, that is until it hung "in balance". The center of force designates the center of the estimated surface on which the exo- and endopodite of the five pleopod pairs can have an influence. The determination of the swimming angle (between back line and horizontal) was arrive4 photometrically from the above described continuous imaging and the macro slow motion pictures, which also made possible measurements of the swimming speed. Only animals, whose height from the tergit-underside to the back line of the first abdominal segment did not exceed 15 % of the animal's length, were evaluated: these animals were really taken from the side, so that errors resulting from horizontal angle deviations could be prevented. The oxygen experiments are described in KILS, 1979.
In fig.22 the center of mass and the center of force are shown. Since the krill, as will be shown later, has non negligible weight even under water, and since center of mass and center of force are not found at the same place, the following theoretical consideration can be put forward: if the krill wants to hover stationary in the water, it could only do this in a position in which the center of mass is located directly vertically above the center of force (as drawn in fig.22) - in any other position an unbalanced force would be left over and would accelerate the animal forwards or backwards.
This hovering condition would be filled by a swimming angle (back line to the horizontal) of ca. 55 degree. The water would have to be propelled vertically downwards, i.e., at an angle of 35 degree to the dorsal line. A first support towards this hypothesis can be found in the actually observed flow field of the weak pleopod stroke (fig.10), with an average flow angle of ca. 40 degree. (Compare also fig.21: Soft beat.)
A further support is provided by fig.23: on the abscissa are shown the swim angles of animals moving horizontally through the water (position sketched in the box), on the ordinate their speed (positive forward, negative backward). There is a clear correlation between swimming angle and cruising speed; only animals with an angle between 50' and 55' hover with no horizontal displacement - at smaller angles the krill moves forward, at angles above 55 degree backward.
Fig.24 Frequency distribution of observed swim angles of 1019 Euphausia superba
Fig.24 shows a frequency distribution of observed swim angles (n = 1019, random testing). The animals by no means drift around as if weightless in space, but rather there is a preferred average anglearround of 45.3 degree. The animals show a definite reaction to the gravitational field. This average swim angle is 10 degree lower than the 55 degree hovering angle i.e. the animals -on an average- have a forward motion.
The mean and standard deviation of the swimming angle of the krill in the experiments changed when the animals came under oxygen stress. Fig.25 shows for an oxygen-lack experiment (description, see KILS, 1979) the means of the swimming angles and their standard deviation over deteriorating 02 -conditions. At high oxygen saturations the animals swim around in a lively manner, the deviation from the 55 degree hovering angle is considerably high, with an average swim angle of ca. 35 degree, also the standard deviation is high at ca. 25 degree. At decreasing oxygen saturations the average angle approaches the 55' hovering angle more and more and the variance of the angles decreases, long before the L50 -value is reached (50% animals dead, plotted below). Below 75 % saturation the animals stop cruising and remain nearely stationary at a 55' angle and try to struggle against sinking. Below 70 % oxygen the animals are no longer able (even though the pleopods still work with a frequency of 2.4 b*s but at weakened intensity) to keep themselves hovering in the water: at 65 % nearly all animals lie on the bottom (on their sides!), but the pleopods keep on working, until the animal dies (indicator: heartbeat).
Fig.25 Changes of mean swimming angle and standard deviation at deteriorating oxygen saturations
The observed correlation between swimming speed and swimming angle could be utilized for future laboratory experiments as a new bio-test: the more the average angle approaches 55, and the less the standard deviation, the higher the stress factor is. The swim angle can be monitored relatively easy, continuously and with low intrusion to the animals by optical methods.
Krill is described in the literature as without statocyst; therefore, the question arose, how do the animals always manage to control their body position so that gravity is compensated for and sinking is prevented? In the daytime the krill seems to orient itself by the light, as the following experiments showed: when hovering krill were lighted from above (0.3 - 700 lux), they showed the normal 55' angle. When lighted from the side, the angle could not succeed hovering but tilted and slid diagonally downwards until they hit the wall of the aquarium. Lighting from below resulted in a swim position of 55 + 180 degree, in which, however, the animals immediately plunged downward and hit the bottom. When the animals were prevented from turning their bodies by holding them with a special apparatus, the eyes turned, nonetheless, and always kept the same position to the light, over an angle of 360' (Arthur Baker - personal observations). One of the functions of the large, well developed eyes is certainly the control of the position of the animals in space. Further experiments with light of different polarization planes gave no clear results.
During the Antarctic winter and under the ice, the light intensity is very low. Experiments in total darkness were conducted to test whether the krill loses its powers of orientation (missing a classical statocyst). The measured angle frequency distributions under no light condition were very similar to the one plotted in fig 24, with an average swimm angle of 437 degree and a variance of 25 degree, which indicates that the animals were able to sense the earth's gravity field. As possible receptors could be taken into consideration the antennae, they are heavier than water and they always are slightly bend downwards during hovering. They are well innovated with nerves at the base (fig.26).
Fig. 26 Front part of Euphausia superba. Notice the transparent cuticula and the antenna nerves