Low to extremely turbid water conditions are quite common in areas that humans inhabit. Under such conditions, it is a challenge for fish to forage food as they rely on vision to navigate and find food. Now, in a study carried out using zebrafish, researchers at the Indian Institute of Science Education and Research (IISER) Mohali have found hard evidence to support how turbidity negatively affects foraging efficiency and how they learn to cope with it. The study was done in collaboration with IISER Kolkata.
The team led by Dr. Manjari Jain from the Department of Biological Sciences at IISER Mohali found that with acclimatisation to turbidity conditions zebrafish did overcome the challenges posed by reduced visibility. The study found that acclimatisation to the immediate visual environment does play an important role in determining success in foraging food. The results of the study were published in the journal Biology Letters.
In addition to acclimatisation, the presence of colourful landmarks that are visible even in extremely turbid conditions plays an important role in improving foraging success. The study could conclusively show that zebrafish use visual cues to navigate to foraging sites especially when turbidity is high and visibility is low.
“The cornerstone of the study is the visual learning capability of zebrafish. We found the wild freshwater zebrafish (Danio rerio) are capable of using familiar visual landmarks to find food locations even when vision is compromised,” says Dr. Jai
“This is the first study to show the ability of zebrafish to use any landmark for navigation,” she says. The study highlights the important role of behavioural plasticity and spatial learning to cope with altered sensory environments, which in this case is increased turbidity of the water they live in.
Effect of turbidity
“Here we demonstrate the detrimental effect of turbidity on zebrafish with respect to foraging. Turbidity is likely to increase the metabolic cost associated with foraging behaviour,” Dr. Jain stresses. “But what is unique is that within one month of acclimatisation they seem to deal with it. The foraging ability improves drastically. It highlights the importance of acclimatisation.”
If zebrafish do indeed learn to cope with turbidity and find food in a matter of one month, should the effect of turbidity on zebrafish matter at all? “Just because they are able to cope with turbidity in a month does not mean that chronic exposure to turbid water is not going to affect them. The resilience to survive in turbid water has not been studied,” she warns.
“If one of the sensory modalities is compromised in some species they are capable of switching over to another. But in the case of zebrafish we found they continue to use visual sensory to locate food,” Dr. Jain says.
For the study, the researchers collected 160 zebrafish from a clear stream near Kolkata. The fishes kept in tanks were fed fish pellets and Artemia (genus of aquatic crustaceans). They were trained for two weeks to navigate and locate food. After training fish to find food, 75 fishes were split into three groups and the different groups were acclimatised for a month in clear, low turbid and high turbid conditions. The low turbid condition mimicked coastal water and high turbidity mimicked an estuary mouth.
The researchers found that one month of acclimatisation was sufficient for fishes to forage food even when the turbidity was high.
Even during training, the researchers did find that zebrafish in turbid water did learn to use colourful stones as proxy for food locations. During the tests, even in highly turbid conditions, the ability to locate food was superior when there landmarks. Their ability to locate food was compromised when the landmarks were not present.