Study shows zebrafish use visual cues to find food in turbid water
Delhi’s winter haze is infamous for disrupting the air, railway and road traffic. With visibility dropping to near zero on a few days, life-threatening accidents spike during this season. Animals, on the other hand, seem to have a trick up their sleeves—they use sounds or visual cues to help them ‘see’ through low-visibility conditions. But what about fish? Can they navigate through turbid waters? Indeed, says a study by researchers at two Indian Institutes of Science Education and Research (IISERs), Mohali and Kolkata. The researchers have shown, for the first time, how zebrafish find food in turbid waters.
Zebrafish (Danio rerio), a tropical freshwater fish found widely in India, are known to use their power of sight to find food and ward off predators. They live in a wide variety of water bodies—from clear streams to ponds and lakes close to human habitation—both prone to turbidity. Thus, they are an ideal candidate to study how they change their behaviour and survive in murky water.
“Episodes of extreme turbidity can be natural, such as flash floods causing surface run-off in streams or periodic algal blooms. In human-dominated landscape, the discharge of sewage water into lakes or soil erosion due to mining and agriculture results in turbidity,” says Dr. Manjari Jain, the corresponding author of the study. She is an assistant professor at IISER Mohali and has worked extensively on understanding how animals deal with altered sensory environments.
In the current study, published in the journal Biology Letters, the researchers have studied how zebrafish navigate and find food using visual clues in turbid water, thus displaying behavioural plasticity. Behavioural plasticity is the ability of animals to change their behaviour in response to changes in their surroundings. In nature, such flexibility is seen in dogs and monkeys. Those that live around humans tend to be bolder in approaching us than their wild-counterparts. However, these changes are short-term adjustments and not long-term adaptations.
The researchers collected 160 wild zebrafish from a stream in Kolkata and after a month, divided them into three groups and put them in tanks that contained clear water and water with high and low levels of turbidity. These fish remained in these three tanks for a month to get acclimated to muddy conditions. After that, the researchers started their experiments to test how well zebrafish could forage in the waters they were in.
“These experiments allowed us to examine how turbidity can impact foraging and if acclimatisation could mitigate the negative impacts of turbidity” explains Dr Jain.
In the second set of experiments, the researchers placed zebrafish in two tanks—one with clear water and other with highly turbid water and both contained visual landmarks, for a month. They placed coloured stones, akin to those seen in their natural habitats, below the feeder, to guide the fish towards food and act as visual cues. The fish were then tested for their foraging ability with and without these landmarks.
The study found that fish in clear water got to their food faster than in turbid water before they were acclimatised—a finding consistent with many previous studies. Interestingly, when they got used to living in murky water for just a month, they foraged much better in turbid water than in clear water. In the second set of experiments, the researchers found that zebrafish perform better with visual landmarks and used the coloured stones as a guide to find food, say the researchers. When their primary sensory information—vision—was disrupted, these fish used additional cues, like landmarks, to help them find food.
“The fact that the presence of visual landmarks did not make any difference to their foraging ability in clear water but greatly improved their ability to find food in turbid water shows that these fish are not only capable of using visual landmarks to aid navigation but they put it to use selectively under muddy conditions,” explains Dr. Jain, on their findings. “In clear water, they are perhaps foraging at maximum efficiency anyway and the presence of landmarks doesn’t seem to improve this any further,” she adds.
This study is the first one to show behavioural plasticity among individuals from the same fish population. The findings show how fish, which rely on their vision to forage, are adversely affected by turbidity.
“Sustained increase in turbidity can degrade water clarity and risk the survival of fish, especially the already-endangered ones,” warns Dr. Jain. “Management plans need to be formulated to check the degradation of water clarity, which can severely impact fish inhabiting such susceptible water bodies,” she suggests.
The study opens up new avenues of research on fish behaviour and ecology.
“Further studies in this direction can assess if fish that inhabit water bodies with extreme turbidity experience metabolic stress, and if so, how do they cope. Ecological studies that assess water clarity can help devise mitigation plans in areas which may be prone to sustained and unnatural change in turbidity,” signs off Dr. Jain.
This article has been run past the researchers, whose work is covered, to ensure accuracy.