Krill, a shrimp-like crustacean that is about 6 cm long, are the dominant prey item for many of the charismatic megafauna that inhabit Antarctic and sub-Antarctic waters.
The combined weight of just one species, Euphausia superba, exceeds that of the human race. Mega-swarms of this species can extend to several kilometres in extent.
These animals spend their whole lives swimming and feeding on phytoplankton that are found in enormous quantities close to the surface.
However, like all mid-water oceanic organisms spending too much time close to the surface in a well-lit environment leaves them vulnerable to predation.
Most species that inhabit open waters spend the day at depth, hiding from predators and only venture upwards under cover of darkness. This is a good strategy for species that live away from the poles where day and night are regular occurrences. Towards the poles however, “day” can be a very long time during summer.
Research by Dr Magnus Johnson from Hull University and Geraint Tarling from the British Antarctic Survey measuredswimming performance of krill on a pendulum while working on the RRS James Clarke Ross.
Previous work using this technique had been carried out in the stability of Kristineberg Marine Station in Sweden.
However, by comparing the movements of pendulums bearing live krill with one hosting a dead control, Magnus and Geraint found that krill with full stomachs slipped into a dozy state where they stopped swimming and in the wild would have gently drifted down through the water column until nature took its course and theydefecated at depth.
Krill with empty stomachs were found to be much more active than their sated colleagues and would start swimming again.
The implication is that krill are likely to move up and down through the water column several times a day, feeding at the surface before returning to depth.
Each time they do they are transporting carbon from the surface to depths beyond the mixed surface layers. It seems likely that they are an important component of the oceanic carbon pump –much more important than they would be if they just migrated once a day.
Unfortunately,the perfect storm of increasing atmospheric carbon levels, increasing temperatures leading to physiological challenges, reductions in available habitat under ice (which is essential for young krill) and increasing numbers of predators due to conservation efforts in the southern ocean may threaten the superabundance of krill.
This in turn may have significant implications for carbon cycling in the form of a negative feedback loop –fewer krill, less carbon sequestrated, higher temperatures, less ice, fewer krill.