Few animals are born fully equipped to deal with the world. Most continue developing after birth, and for a zebrafish larva, one of the key structures that is thought to undergo massive changes during the earliest weeks of life is the lateral line system. ‘The lateral line is how fish sense water flow’, explains Matt McHenry, from the University of California Irvine, USA, who adds that the sensory system may allow peckish larvae to detect swirls in the water produced by juicy water fleas sweeping past. ‘If you have the ability to feed in the dark or in dim, turbid conditions… then you have a great advantage’, says McHenry. Wondering if the ability of zebrafish larvae to forage in the dark improved as their flow-sensitive lateral line system developed, McHenry and graduate student Andres Carrillo began testing how zebrafish youngsters coped with catching live meals when the lights went out.
Carrillo fed young larvae on live rotifers before upgrading them to a larval brine shrimp diet as they grew, and then filmed the tiny fish foraging for food in light and dark conditions at various stages of development. ‘The fish got much better at foraging as they grew, and they also had the ability to forage in the dark’, recalls McHenry. Then Carrillo temporarily knocked out the sensory cells in the lateral line using neomycin sulphate, and this time the zebrafish larvae's success rate plummeted when the lights were switched off. So the lateral line was essential for the larvae's success, but could changes in the lateral line sensory cells explain the larvae's improvements with age?
Anticipating that they would find dramatic changes in the structure of the lateral line's sensory cells (neuromasts), McHenry and Carrillo measured the diameter of each neuromast and the number of hair cells associated with each structure as the larvae grew, but they were in for a shock. ‘We had this big behavioural change; we knew it was mediated by this sensory system but we didn't see anything in the sensory system that reflected that change’, says McHenry. ‘We were pretty puzzled,’ he admits.
Carrillo presented the disturbing results to colleagues during a group meeting in Irvine and McHenry recalls the discussion, saying, ‘Tim Bradley suggested that maybe it's an effect of learning; that they basically have the same receptors at all ages, but they are just getting more attuned to using them as they grow’. Realising that they would have to raise fish that had never experienced the ‘whoosh’ feeling as lunch swam past in order to test Bradley's theory, Carrillo suggested temporarily disabling the fish larvae's lateral lines with neomycin sulphate every time that they were fed, so that they were unable to associate fluid movements with the presence of food.
Painstakingly bathing the youngsters in the chemical just before they were fed every day for a month, Carrillo then allowed the fish's lateral lines to recover before testing their responses to lively brine shrimp larvae at the age of one-month. And this time the zebrafish failed to grab a morsel. They had not learned to interpret the tell-tale swirls in the water associated with live food. And when Carrillo fed the developing fish on a diet of dead brine shrimp, these fish larvae were equally inept at catching a meal.
So, the zebrafish larvae with uncompromised lateral lines had learned to associate disturbances in the water with the presence of dinner and Carrillo and McHenry are now eager to learn more about the factors associated with fluid movements that trigger zebrafish larvae to snap at passing prey.
- © 2016. Published by The Company of Biologists Ltd