Part III: Antarctica and Academe - Scott Polar Research Institute

directly to gravity. Krill do not swim actively downwards, but allow themselves to
sink at a rate which increases with their size (because their density increases with
size). The need to counteract gravity by active swimming necessitates a
disproportionate amount of energy, compared with other zooplankton, so less is
available for growth and reproduction. The high energy needs involve high
respiration rates and so the krill have particularly well-developed gills for oxygen
transfer. Moribund krill are virtually absent from the water column because a slight
weakening results in an inability to maintain their vertical position and they can sink
passively 500 m in 3 hours. In most parts of their range, by the time they have sunk
to depths of 250m they are dead because abundant oxygen is essential for their highenergy
lifestyle and the oxygen saturation of the water, which is related to surface
turbulence, is halved by that depth as a result of upwelling of deeper oxygen-poor
water, such as the Warm Deep Water. This sensitivity to lowered oxygen tensions is
most unusual. Juvenile krill are less affected, because their energy needs are not so
disproportionate and their higher fat content means that they are less dense. Nearly
all adult krill are caught at depths above 250 m except where water conditions allow
higher oxygen content at depth, possibly even to the sea floor (for example in the
Bransfield Strait). Paradoxically, the more buoyant juveniles are caught at greater
depths and the eggs at even greater depths.
There are other paradoxes in their physiology. Because the density increases with
body size, the standard metabolism (one can hardly use the term resting metabolism
!) increases with body weight, which is the reverse of the usual situation in animals.
It has been calculated that an average-sized adult krill (60 mm length) would have to
filter 44-ll2 litres of water containing average concentrations of phytoplankton daily
to provide for its energy needs. Of this about 40% is needed for hovering activity
alone, and with the same energy expenditure it could sustain indefinitely a forward
speed of l2 cm/second, using the body as a hydrofoil; it would spend no more
energy than if it stayed still. (To put this in proportion, at a swimming speed of l2
cm/second a krill could cover ll km/day, 350 km/ month, 4,100 km/year, not
allowing for ocean currents). To filter 44-ll2 litres of water it would have to swim at a
speed of at least 2.2 - 5.8 cm/second. If the krill doesn't find food or if the oxygen in
the water decreases slightly it plunges under its own weight to the depths, so quickly
that it would reach the bottom after only a few hours. In this way it contributes to the
transfer of nutrients and energy from the surface layers of the ocean to the bottom.
The stomachs of Antarctic bottom-living fish often contain fresh krill and brittle stars
have been observed feeding on krill on the sea floor.
In winter the extent of the krill range not covered by ice is only about l million km 2 ,
but the southward contraction of the pack-ice zone in summer uncovers and enlarges
this to a maximum of about l3 million km 2 in late summer. Together with vertical
migration towards the surface waters and high summer production (from growth
and reproduction), the retreat of the pack ice in summer is associated with an
enormous increase in the amount of krill available to predators in ice-free surface
waters. However, it has recently been demonstrated that the pack-ice zone itself has
an ice-associated (cryopelagic) community of plants and animals including krill and
269