We share more than 99% of our DNA with chimpanzees. We all know this, and it is easy to imagine, given both humans and chimps are apes. We share almost three-quarters of our genes with a rather unlikely candidate: a diminutive fish that has horizontal stripes that run across the length of its body, called zebrafish. These fish are becoming an increasingly popular choice of study animals in laboratories, thanks to their ability to reproduce fast in large numbers.
The fish is endemic to the tropics of South Asia, and is widely distributed in India from Jammu and Kashmir to Kerala. These fish are found in rivers, paddy fields, small streams, agricultural canals and pools attached to larger water bodies. They have been widely considered to occur in stagnant or shallow slow-moving waterbodies. However, recently, they’ve also been found in rocky, fast flowing streams. Dr. Anuradha Bhat from the Indian Institute of Science Education and Research, Kolkata has been studying behavioural and morphological changes in these fish under different ecological conditions, with different teams of scientists.
Why was the zebrafish singled out for such studies? “Zebrafish being a model organism used in genetics, medical sciences, and developmental biology, there is already vast literature on the species. Despite this, there is little understanding of the natural history of wild populations, their behaviour and ecology. Given the range of habitats and climatic/ecological conditions they are known to occur in, the species is amenable to a variety of comparative studies that can provide answers to questions on the role of genes and environment leading to population and ultimately species level diversification of traits”, explains Dr. Bhat.
With Rohitashva Shukla, her PhD student, Dr. Bhat has compared variations in morphology, or the external appearance of the fish, across populations taken from different parts of the country. How do the differences between the different habitats affect the fish’s appearance? They found that factors like stream velocity, the amount of dissolved oxygen and presence of predators influenced the body shape and the size of the fish.
“Morphological adaptations of species to their natural habitats are the result of natural selection over several generations. Variations in seasonal data (rainfall, temperature) can play a major role in determining body shape differences in fish, as important functions like locomotion (swimming), metabolism can be affected by the shape of the body”, comments Dr. Bhat.
The caudal fin or the tail of the fish is attached to the rest of the body by a narrow region called the caudal peduncle. This part is usually compared to identify variations in morphology between fishes. Fish that were taken from slow moving waters had deeper caudal peduncles, deeper bodies and longer fins that those that were taken from stagnant waters. Foraging and feeding ecology can influence head length and shape of the jaws. The researchers also found that fish collected from waters that had high dissolved oxygen content had relatively smaller heads.
The reason for such variations could be that fishes found in different habitats adapt to their specific surroundings and resources. This in turn produces morphological variations that we can observe. Many such interacting environmental factors can contribute to the morphology of individuals, they note. Another important cause for variation between fish populations is genetics, wherein modified genes are passed down from generation to generation.
In another study that Dr. Bhat carried out with researchers from the Department of Biology and Center for the Integrative Study of Animal Behavior at Indiana University, they compared behavioural differences between wild and laboratory grown fish. They tested ‘plasticity’ or the ability of an organism to change its phenotype in response to changes in the environment. Natural environments of zebrafish often fluctuate, with the onset of monsoons, pollution levels and other factors. The researchers predicted that zebrafish taken from the wild would show more flexibility in behaviour than those that were bred in the lab or in hatchery environments.
In light of the experiments they conducted to ascertain this, feeding latency was measured as the time taken to approach food and consume it. The time the fish took to approach food, or latency to feed, was an important aspect of zebrafish feeding behaviour. Zebrafish live in shoals of about six individuals. They observed shoaling distance in the fish to see how cohesive the groups were. This, they found, did not depend on either environmental factors or populations. They found that behaviour of the fish does depend on differing levels of interactions between environmental and genetic factors, just as morphological traits do. For example, they found that aggressive behaviour varied with environmental factors, but latency to feed was more population-dependent.
Dr. Bhat added, “It is of interest to understand whether these traits (shoaling, aggression, anti-predatory response, feeding latencies) are fixed population level responses, or whether these responses are more flexible, whether individuals are capable of shifting from one response to another depending on immediate environmental conditions. Habitat factors such as vegetation (presence or absence) and water flow conditions can play an important role in these responses.”
A team comprising of researchers from both the Indian Institute of Science Education and Research, Kolkata and Indiana University also studied the effect of water flow on zebrafish group behaviour in particular. Water flow can influence the manner in which zebrafish seek out prey, or are found by their predators; it can decrease or increase the energy spent for movement, affect sensory systems and change the way in which the fish form shoals. Since zebrafish live in waters whose levels and flow rates vary year round, their ability to adapt immediately to their surroundings is an important coping mechanism. Dr. Bhat and others predicted that obstacles and flow would makes groups of zebrafish more aggressive and less cohesive than when they were in flowing water without obstruction. They observed groups of fishes under different conditions for four minutes and tracked movement and behaviour in the laboratory. They found that even weak flow of water had an immediate effect on shoaling behaviour – the fish tended to form less cohesive shoals. The fish also displayed higher activity and aggression levels in such cases.
Future environmental changes may also bring about newly evolved traits in zebrafish. Understanding the mechanisms that underlie such changes in the fishes offers an interesting field of study in the future. Studying the effects of rearing conditions in genetically similar fishes might also help to uncover the role of genetic and psychological mechanisms in the way these traits are shaped. Zebrafish can be a model for understanding how morphological changes come about in similar tropical fish species too.