Since it was only very seldom that krill corpses or decayed bodies were found in the net catches of the expedition, the subject of how fast dead krill will sink to the depths was investigated. These measurements also included krill that were alive but not stroking with the pleopods. The energy needed by the krill to avoid sinking was estimated by two independent methods.
The sinking speed was determined with freshly killed (by 0 -lack), undamaged krill sinking along a measuring screen in the aquarium (4 degree Celsius, salinity 35.5) with time passing the markers recorded. Two experiments in situ confirmed the sinking rates: the underwater TV camera, at a depth of 15 m, registered the passing of dead krill thrown overboard (animal lengths between 40 and 50 mm, wave state 3-4).
The measurement of the sinking speed of the living krill was performed with the imaging device described in chapter 1.
The respiration measurement for the krill are described in KILS (1979). The particular type of behavior of Crangon crangon, which lives on the bottom as well as hovers free in the water at, could be used to make conclusions about the difference of the energy consumption between these two lifestyles. This detour had to be made, since Euphausia superba as a purely pelagic animal was never found at the bottom. For these investigations with Crangon crangon, a new type of a micro respiration measurement apparatus was constructed: conventional flow through measuring methods had too big a time factor, i.e., sudden changes in the respiration rate register fully only after several minutes; but in a respiration chamber the animal will not maintain the hovering condition for that long, so that the actual respiration increase during the hovering phase would only be partially recorded. In conventional closed-chamber-methods, the 02-saturation available to the animal is altered during the experiment (the calculation is only made possible by using this change), whereby the respiration to be measured is effected.
The new measuring system overcomes these shortcommings (fig.49), it has a time factor of only 20 seconds and operates at a static preset oxygen concentration.
The respiration chamber R has a 340 ml water space, in which the animal can hover freely, and a sterile substratum of quartz sand. In a closed water circuit driven by the hose pump P, a pO2 electrode is scanning the saturation. The signal of the electrode is filtered and feeds a threshold selector L, which in the schematic of fig 49 is set at 75 % 02-saturation. During the initial phase of each experiment the oxygen content in the chamber is lowered by the respiration of the animal; if it drops below 75 % saturation, the threshold selector turns on the precision hose pump P , (which starts and stops with very little lag). The pump draws water from the chamber, which for continuity reasons is replaced by water out of the aerated supply container A. This water is kept at exactly 100 % saturation and lifts the 02 -saturation in the chamber above 75 and the threshold selector stops the pump P - then the whole process begins over again, producing a frequency which is proportional to the respiration. The running times of the pump P are registered, also the 02 -saturation as a control, which remains almost constant at 75 + 0.35 %. A sudden increase in respiration (indicated by the arrow) is detected by this method through a direct frequency change (f1 to f2).
For quantification: the running time of the pump P in a certain time interval is proportional to the amount of water exchanged, out of which the animal has breathed 25 % of the oxygen saturation (100 - 75 %):
(100 - d%) * CTSP * F * 36
Q = --------------------------------------------------------
t * W * 1000
Q = oxygen consumption of the animal (mg 02 g-1 h-1)
d% = 02 -saturation of the water running out (% 02)
CTSP = O2 - content of 100 % saturated water at experimental conditions T, S and P (mg 02 l-1)
t = time of the measuring interval (s)
F = flow of water in the measuring interval (ml )
W = weight of the experimental animal (g)
The measuring apparatus was held at 20 degree Celsius (+- 0.03).
Dead krill sink in a horizontal position smothly and without wobbling. The back points downwards, the pleopods and filtering basket are drawn after. Fig.50 shows the measured sinking speeds for dead krill and the correlation to the size of the animals (for background information see chapter 4.4). Dead adult krill sink at 5 cm per second; a relatively high value: in an hour the animal descent 190 m, even at a water depth of 3000 m the bottom is already reached within 16 hours. Therefore, it is not surprising, that krill corpses are seldom found in the pelagic nets; but they should be found in large numbers at the bottom.
Euphausids: vd = 0.0701 L 1.07 r = .933
Crangonids: vd = 0.3930 L .701 r = .986
(units: centimeter per second, length in millimeter)
Alive krill almost always beat with the pleopods. It was only very occasionally observed that the pleopod rhythm was stopped. Such animal sank immediately, but took on by reflex a characteristic, stiff position (fig.51).
The abdomen with spread-out telson is bent upwards, the filtering basket is opened and the pleopods are widely spread out. The V-shaped bending of the body prevents the animal from flipping over due to the dorsal oriented center of mass. The areas formed by the telson, filtering basket and pleopods obviously function like a parachute to reduce sinking velocity. This assumption is supported by the plot of sinking speeds found for living animals in the described body posture in fig.52: here compared to the regression lines of dead animals from fig.50: relative to dead krill the sinking speed is reduced by 13.8 % (s = 8.00); i.e., the krill actively tends to minimize sinking by a characteristic posture reflex. But even with the use of this "parachuting", an adult animal would lose 3800 m in the run of one day, a circumstance which is not exactly beneficial to the pelagic way of life of this species.
Fig.52. Reduction of sinking speed of living animals with posture reflex.
No significant difference between % Euphaasia superba and % Meganyctiphanes norvegica: t = 1.28 t5% = 2.13 The curves are the regression lines from fig.50
A method for energy calculations used in airplane construction has as its basis, that the energy necessary for hovering without loss of height within a certain time interval at least corresponds to the energy that would be necessary to lift the object that distance which it would have sunk within that time (without propulsion) - lim dt approaching zero leads to the hovering condition.
If this approach is applied to the krill, the energy need for hovering, that is, for the pelagic way of life, can be calculated. It is to be considered that the energy carried over to the water is calculated, which is naturally smaller than the energy which actually has to be produced by the animal through respiration. The passing on of this energy on the way from the ATP over muscles and pleopods to the resulting propulsion force creates losses. From the determined underwater weights (fig.38), the sinking speeds (fig.52) and the gravitational force of the earth, the following function can be deduced:
KrillEnergy P = m * g * h * t -1
Wuw (kg) s. fig.38 g (m * s-2) h (m) s. fig.50 and 52
--------------------------------------------------------
1.25 * 10-8 L3.76 * 9.81 * 0.862 * 7.01 * 10-2 L1.07 * 10-2
P = --------------------------------------------------------------------------------------
1 (60-1 * 60-1 * 24-1)
---------------------------
t (s)
P = 6.54 * 10-9 L 4.83
(units: energy Joule per day per individuum, length of individuum in millimeter)
For a 60 mm euphausiid, for example, the energy production necessary for hovering is 2.5 Joules per day. The energy per gram of body weight (and also per cm cube of body volume) depending on the body length is represented in fig.53.
Euphausids: P/Ww = 0.00175 L 1.66
Crangonids: P/Ww = 0.03210 L 1.12
(units: Joule per day per gram wet weight, Joule per day per centimeter cube, length in millimeter)
It is remarkable that the hovering-energy to be produced per body unit increases with the animal length. This development runs contrary to the total respiration, which decreases with the size of the animal. According to BERTALANFFY et al., 1953, the respiration per body unit for crustaceans decreases approximately with the inverse value of the length. This scissors-effect grows wider apart, the bigger the animals become: although a 60 mm long krill has a lesser total metabolism per body unit, its hovering metabolism is 61 times higher compared to a 5 mm krill (the bigger krill has to produce 165,000 times more energy with only 2700 times the body.
From a physiological point of view, therefore, an increase in body size is certainly no advantage to a pelagic way of life, but rather the deteriorating energy balance should work as a limiting factor on the size of pelagic crustaceans.
Which conditions make it so hard for the adult krill to hold itself in the water, and has it developed adaptations for its pelagic way of life? In answer to these questions some of the results presented in the chapter "Basic Biometric Data" are to be analyzed, especially in comparison between the pelagic euphausiids and the primarily benthic living Crangon crangon.
The underwater weight presented in fig.38 increases with length faster 3.76 (L ) than the wet weight (L 3.17, fig.30). Therefore, the percentile portion of the underwater weight from the wet weight is more important for large animals, so that an adult krill even under water still retains 3 % of its weight above water (fig.41). The reason for this is that the volume and, therefore, also the water displacement which determines the upward force (fig.45) do not grow as quickly as the wet weight (fig.28). Thus, large animals show a higher density. How this phenomenon of density change with body length is to be explained must be shown by further chemical analyses of the krill. One indication is given by KRYUCHKOVA et al.,(1969) who found a higher fat content in juvenile Krill.
The density of the adult krill (fig.47) is at 1.070 g*cm extraordinarily high for a pelagic living animal. According to ALEYEV ,1977, most pelagic animals have densities similar to that of the sea water; of 67 investigated species from different animal groups, all values were under 1.055 g per cm cube Where the density of the body tissue exceeds this value, the animals possessed other diverse buoyancy aids: gas-filled spaces in cephalopods and fish or large fat deposits (20 - 30 % of the wet weight), often in combination with a decalcification of skeletal elements, high ammonium content, low sodium content and higher water content, so that the density of the whole body again lies under 1.055 g*cm . Euphausiids with a body length of more than 30 mm clearly deviate from this picture.
Although the krill has not succeeded in adapting its density to the normal values for a pelagic way of life, it does show some adaptations which make hovering easier for it. The comparison with the primarily benthic living Crangon crangon gives some hints. This animal lends itself to comparison, since body build, size and habits are comparable and the species are systematically quite close together (both Eucarida). During the night, Crangon crangon swims for several hours with the help of its pleopods, in a way similar to that of krill; most of the time it -lives on the bottom, where it moves itself forward with the thoracopods.
In all the regressions of figures 39, 41, 43, 45, 47 and 50, Crangon crangon clearly lies above the euphausiids. To make the comparison of equally heavy individuals easier, such are joined by an arrow in the figures (equally heavy animals of both species are not equally long). Thus the euphausiids have increased their water content in comparison to Crangon crangon (fig.39). Weight saving is certainly also contributed to by the fact that the krill has reduced the otherwise typical (heavy) calcium deposits ( typical for crustaceans of its size): its cuticula consists of a very thin, transparent (fig.26) and relatively soft chitin layer. The average fat content of 3 % (related to wet weight, GRANTHAN, 1977, summary of 19 authors) also contributes slightly to bouyancy.
In conclusion it can be discussed that the krill takes a middle position between benthic and pelagic animals with respect to its density. Fish need a swim bladder for hovering, the volume of which normally makes up 5 - 10 X of the animals total volume (ALEXANDER, 1959 a, b); if adult krill wanted to hover without energy expenditure, it would have to have a swim bladder of at least 3 X of its body volume. In small euphausiids these conditions are more favorable and come closer to the normal values for pelagic animals. Indications in support of this are also found in swimming behavior. As shown in fig.54, the relatively small Meganyctiphanes norvegica eove euch more easily in space than Euphausia superba (fig.24, for theory see chapter 2. "Relations to gravity"); they are in comparison to the large Euphausia superba more independent of the earth’s gravitational force. It should be remarked here that the average angle again comes close to the 55' hovering angle.
Fig.54. Frequency distribution of observed swim angles of 319 Meganyctiphanes norvegica
A further factor which makes it harder for the large euphausiids to hold themselves in the water, is the following: while weights (in this case the underwater weight) increase to the third power of length (L3), surfaces only increase to the second power (L2). But propulsion is produced by surface (exo- and endopodites of the pleopods), so that with increasing body size an ever-higher weight presses on each surface unit. Also, during sinking the resistance in the water is determined by the cross-section area (L2) of the animal, while the underwater weight increases by L3, so that a linear relation is left over - as it was actually measured (fig.50).
Another method for arriving at conclusions about the energy management is the direct measurement of the 0~-consumption of the animals. The oxygen consumption of Emphausia superba hovering freely in the water is at 1'C on the average 1.28 mg 0~ g< h (KILS, 1979). This value lies above the measurements of other authors.
CHEKUNOVA et al., 1974: average 0.72 mg 02 gd-1h-1
McWINNIE et al ., 1964: . average 0.98 mg 02 gd-1h-1
RAKUSA-SUSZCZEMSKI et al., 1978: average 0.56 mg 02 gd-1h-1,
but these measurements were performed in very small respiration chambers.
Respiration is a measure of the total metabolism. In order to define the metabolism portion for hovering, one could perform measurements on non-hovering (i.e., not stroking with the pleopods) krill and establish the difference; for this the krill would have to be, for example, sitting quietly on the bottom. But such a condition is not to be attained with healthy krill because it avoids active bottom contact and cannot stand on the bottom for anatomical reasons. Since measurements with drugged animals are questionable, an attempt was made to obtain results in a round about way. The experiments were performed on Crangon crangon, which maintain both conditions without manipulation for iong enough so that they can be technically measured. In fig.55, the respiration values of animals quietly wandering around on the bottom are standardized to 100 %; the upper values designate the respiration values for the same animals when they were hovering in free water (v~ = 0). With the big animals, during the hovering- phase the total metabolism increases to more than 200 % of the cruising- metabolism.
Fig.55 Respiration measurements of Crangon crangon: the respiration of animals wandering on the bottom is standardized to 100 % and the respiration of animals hovering is represented relative to it:
%h = 90.8 + 3.26 L
(unit mm) r = 0. 933
The figure also shows that the portion of energy consumed for hovering fs much smaller in the small animals; this result is in agreement with tha above mentioned experiments. This tendency also was reflected in behavior of the animals during the measurements: while the small ones often and seemingly without effort climbed into the free water, the big ones left the bottom only occasionally and awkwardly.
Two values, gained by independent methods, are now available for the calculation of the energy needed for hovering, of which the one under- and the other over-estimates, thus bracketing the real status somewhere in between.
The energy which a 60 mm long krill must pass on to the water is 2.5 J * d -1 (see chapter 4.3). Since the efficiency of muscles is only 0.25 - 0.30, they must be supplied with 0.28 mMol ATP per day. During the passing on of muscular energy to the water (over the pleopods), more energy is lost, so that the actual energy need on the level of the ATP is certainly higher.
On the other hand the respiration measurements showed that for a Crangon crangon, whose underwater weight corresponds to that of a 60 mm long krill, the portion of metabolism for hovering comes to ca. 60 % (fig.55). If one assumes, to start with, similar circumstances for the krill, a respiration portion of 0.77 mg 02 gd-1h-1 results (KILS, 1979), i.e., a 60 mm long krill breathes 5.3 mg 02 per day in order to hover. That corresponds to an ATP-provision of 0.89 mMol * d -1 (BARTOLOMEW, 1968; BRETT, 1962; MITCHEL, 1946). Since on the one hand the krill certainly can pass the energy on somewhat more effectively to the water compared to Crangon crangon, and on the other hand the measurement method for Crangon crangon somewhat underestimates the actual relations, the value for the krill should be only slightly lower.
Thus a 60 mm long krill uses per day an amount of ATP somewhere between 0.3 and 0.8 mMol ATP * d-1 in order to live its pelagic way of life.
The ATP quantity per animal and per day is
PATP = 1.40 * 10-9 L4.83
(units: mMol ATP per day, mm) (+-45 %)
The ATP quantity per gram of wet weight and per day is
PATP * gw-1 = 3.78 * 10-4 L1.66
(mMol ATP per day per gram, mm) (+- 45 %)
Adult krill leads a purely pelagic life in the upper 200 m of a body of water often 3000 m deep, so that the estimated amount of energy must be continually produced; therefore, for the krill no real "resting metabolism" exists. At best we can speak of a "standard metabolism", which includes an extra, constant and not insignificant portion for hovering. Also, the occasionally noticed stopping of the pleopod stroking does not lead to any saving of total energy, since the animal then sinks immediately and loses potential energy (Epot), which it later must produce again, in order to reach the former water stratum.