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Exploring the krilling fields

The larvae of the Antarctic krill (Euphausia superba) are skinny things, having very low fat reserves. Given that the free-swimming ocean-dwelling crustaceans spawn in late spring and do not develop into juveniles for the best part of a year, their young lives encapsulate a bit of a mystery.

A critical part of their development takes place during winter, when much of their habitat is covered by thick sea ice – and the water beneath it is dark and nutrient-poor.

This phase, note a team of scientists led by Bettina Meyer of Germany’s Helmholtz Centre for Polar and Marine Research at the Alfred Wegener Institute, represents a “critical bottleneck for larval krill development”.

Meyer and colleagues recently undertook a research trip through krill breeding and feeding grounds in the Weddell Sea, on board an icebreaker called RV Polarstern, in an attempt to better understand how the young survive the harsh conditions.

It was previously assumed that larval krill were somehow able to access the nutrients contained in the large amounts of algae trapped in the sea ice, but Meyer and her colleagues discovered this wasn’t the case.

In a paper published in the journal Nature Ecology and Evolution, the team reports that during winter days krill larvae do indeed congregate beneath the sea ice, which offers protection from predators and acts to collect and concentrate sparse amounts of food that well up from below on warm water currents.

At night, however, the crustaceans travel downwards – though no deeper than 20 metres – and move towards more marginal open-ice zones where food is more abundant.

These open zones, the scientists report, present “favourable feeding conditions due to light, nutrients and the grinding of the ice floes”.

The researchers also found that during the winter the krill larvae were highly omnivorous.

Better understanding the complex dynamics of krill feeding habits is extremely important, Meyer and her colleagues conclude. The Antarctic krill is a foundation species in the Southern Ocean, and its robust survival is essential to the maintenance of the ecosystem.

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