Darkness sheds light on krill reproduction

Antarctic krill
Antarctic krill (Photo: Stephen Brookes)
Krill in their experimental tanks.Figure showing the maturation cycle of krill under three different light treatments and the normal cycle of maturation of wild krillA cartoon by Lisa Roberts of the calyptopis I phase in the krill life cycle.

5 June 2009

When it comes to reproduction, Antarctic krill like the lights off.

Preliminary research at the Australian Antarctic Division’s krill aquarium has shown that the annual cycle of krill maturation and reproduction can be altered by a period of darkness immediately after spawning.

‘We’ve been able to reset the animals’ internal clocks, so that they become sexually mature three months earlier than if they were exposed to a natural Antarctic light cycle,’ principal researcher, Dr So Kawaguchi says.

The research promises to allow scientists to control when different groups of krill spawn, so that they have access to all phases of the life cycle (larvae, juveniles, sub-adults and adults) all year round. It will also spread the effort of collecting millions of krill eggs for the breeding program, over the year, rather than all at once.

In Antarctica, krill experience four months of darkness over winter, between June and September. During this time they actually shrink in size and lose their mature reproductive characteristics – a process known as ‘regression’. As spring approaches and the day length and food supply increases, they begin to grow and become sexually mature again, before mating begins in December and continues until about February.

Research at the Antarctic Division in the late 1980s showed that krill would steadily progress through their reproductive cycle regardless of whether they were exposed to constant darkness, constant light, or a natural Antarctic light cycle.

‘This research suggested that the cycle of regression, maturation and reproduction was controlled by an internal biorhythm rather than environmental factors,’ Dr Kawaguchi says.

Then in 2003, Japanese research showed that abrupt changes in the daily cycle of light and dark could trigger maturation and spawning.

Dr Kawaguchi’s research, however, shows that it is the timing of darkness, not light, that alters the reproductive cycle in krill – a paradigm shift in scientific thinking.

‘For many crustaceans and other animals, environmental cues like changes in light and temperature, influence when they reproduce, so previous research has only focused on the timings of changes in light, not darkness,’ he says.

In the recent study, three groups of krill were kept under different light/dark regimes. One group was exposed to four months of the natural Antarctic light cycle, followed by four months of darkness and four months of light (the ‘light 2’ treatment in the figure at right). A second group experienced four months of the Antarctic light cycle followed by eight months of darkness (the 'dark' treatment). These two groups followed the same cycle of regression, maturation and reproduction as wild krill.

The third group of krill experienced one month of the natural Antarctic light cycle, followed by two months of darkness, and nine months of light (the ‘light 1’ treatment). These krill reached sexual maturity three months earlier than the other groups – observed by changes in their exoskeleton (shell), which they shed as they grow.

‘This study has shown that the transition from light to dark to light is important in controlling the timing of spawning under laboratory conditions,’ Dr Kawaguchi says.

The research team are now working to reproduce the results of this experiment. In theory, if the 'reset' krill are again exposed to a period of darkness immediately after spawning, their reproductive cycle should advance another three months, putting them six months ahead of the krill running on ‘Antarctic time’.

‘We have more work to do to fine tune our methods and repeat our results. We also need to look at any physiological changes in krill that might result from this ‘resetting’ processes,’ Dr Kawaguchi says.

Krill life cycle animation

This krill life cycle animation was created by artist, Lisa Roberts, as part of her PhD in Fine Arts at the University of New South Wales. Lisa spent two days at the Australian Antarctic Division in January 2009, studying and drawing krill. On her return to her Sydney studio she consulted with Antarctic Division scientists, Dr Steve Nicol and Dr So Kawaguchi, on many aspects of krill development and the Antarctic environment. The music for this animation was an improvisation performed by 11 year old Sophie Celeste Green. To see a longer version of this Antarctic animation visit Lisa’s Antarctic Animation site.

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