Plants aren't the only organisms that are drawn to light. Nocturnal moths and flies dash themselves against camping lanterns and fly into illuminated windows. ‘Many animals have evolved mechanisms to navigate towards regions of preferred luminance’, says Alex Chen from Harvard University, USA. But it wasn't clear why vulnerable zebrafish larvae prefer to gravitate toward light. ‘People have hypothesized that being in the light helps them see potential predators and prey’, Chen says. Most intriguingly, how do the larvae adjust their manoeuvres to take account of natural light fluctuations?

‘We sought to understand how the direction of phototaxis [movement directed by light] depends on past light levels’, says Chen, who held zebrafish larvae in the dark, or shone lights ranging from the levels experienced on a very overcast day (158 lx) to full daylight (2874 lx) on the youngsters for 10 s. Then, he switched the lights, dividing the youngster's world in two, so the light falling on one of the larva's eyes was relatively dark, while the light on the other was bright, recording which direction the youngster turned. However, after months of patiently tracking the larvae's responses, Chen and PI Florian Engert were surprised; instead of consistently turning toward the brightest light, the youngsters seemed to be selecting the light levels most similar to the conditions they experienced just before their world was divided in two; if the larvae had been kept in the dark, they turned away from the light side. ‘This was unexpected, because previous studies had reported mostly positive phototaxis [moving towards the light]’, Chen explains. In fact, the youngsters’ behaviour suggested that they prefer to maintain the status quo, no matter how bright or dark that was. The question was, how?

This time, the team adapted postdoc Armin Bahl's larva tracking system to keep one of the larvae's eyes in the dark while the other was in brighter light, before suddenly dimming the bright side or brightening the dark side. Keeping track of the larvae's manoeuvres, the team realised that the youngsters turned to the side where the light was most similar to the previous average brightness across their two eyes. But how long did the fish need to experience a particular light level before it became seared in their minds as their preferred light environment?

To answer this, the team exposed the fish to 16 s of darkness, before presenting them with increasingly long (3–12 s) flashes of light and testing whether they turned toward the light or the dark. After repeating the experiment – exchanging the darkness for light – the team realised that the brightness preferred by the larvae varied as the light in their environment altered over a matter of seconds, allowing them to keep track of the brightest patches of light even when a cloud momentarily passes across the sun.

Collating Chen's observations, Diptodip Deb from the Janelia Research Campus, USA, designed a computer simulation – based on the larvae's ability to adapt to changing light conditions – to see whether the simulated fish could seek out the same light environments as the genuine larvae. Impressively, the simulated larvae crowded into the light half of the tank in just the same way as the flesh and blood youngsters, even on the most overcast days. Zebrafish larvae don't just swim blindly toward the light, they adjust their preference depending on the prevailing conditions and select a comfortable intensity that is the average of the recent brightness across both eyes.

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Algorithms underlying flexible phototaxis in larval zebrafish
J. Exp. Biol.