When a fish went for walking

When a fish went for walking Dr. T V Venkateswaran Once upon a time, long long ago − about 400 million years ago − a kind of a fish...

When a fish went for walking

Dr. T V Venkateswaran

Once upon a time, long long ago − about 400 million years ago − a kind of a fish decided to take a walk; they came to the land and evolved into tetrapods (amphibians, reptiles, birds, and mammals) that roam four corners of the world today. Although copious fossil records suggest this scenario, how they used fishy bodies and fins in a terrestrial environment is one of the unsolved mysteries of evolution. A recent study by team of researchers led by Emily Standen of the University of Ottawa, Ontario, Canada on a type of fish of the genus Polypterus perhaps may shed some light on what might have happened when fish first attempted to walk out of the water (Nature, 4 September 2014 | DOI:10.1038/ nature13708).

a fish out of water
When Polypterus senegalus or Senegal bichir, as it is commonly known, finds itself on the banks of the river, it is not ‘fish out of water’. P. Senegalus is a strange African fish, one of the rare species that has both gills and lungs. It has an elongated shape, about 7-8 cm long, and is found abundantly in African rivers. It can use its fins and tail to scurry around the banks of the river and survive on land. It can breathe air, ‘walk’ on land and closely resembles the extinct ancient marine organism Tiktaalik that later evolved into land creatures. Emily wondered what would happen if juvenile P. Senegalus are raised in terrestrial conditions without permitting them to swim. Would there be any change in the phenotype (observable characteristics or traits)?

A poet rued in dejection, ‘With him for a sire and her for a dam. What should I be but just what I am?’ But truly speaking, there is nothing to despair; if heredity deals the cards; environment plays the hand. Take for example the freshwater planktonic crustacean, Daphnia. Under laboratory conditions, if the water in the aquarium is laced with chemical traces of the predator then the pregnant Daphnia gives birth to offspring with a defensive ‘helmet’ that protects the juveniles against predators. On the other hand, when the aquarium is free of predator scent, then the offspring hardly have the ‘helmet’, which in predator- free environment is a liability for both survival and reproductive success.

More and more studies show that phenotypes are not necessarily set in stone, though some may be more responsive to change than others, depending on the age of exposure and degree of severity, to different environmental variable. Genetics and environment are inextricably intertwined; there is no organism without a genome; but there is also no such thing as an organism without an environment. Hence it is not always just nature; nurture is as important in the growth and development of an organism.

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Taking a cue from this ‘developmental plasticity’ perception, Emily wanted to examine the effect of shallow water on the developmental phenotype of Polypterus and find out if that could shed light on the enigma of marine creatures migrating to terra-firma. She constructed a special tank with an unusual floor. While the bottom of the tank could hold only millimeter-deep water, the aisles on all sides were dry patches covered with sand. Schools of juvenile Polypterus were placed in these tanks and were observed for eight months. Having too little water to swim, these animals used their fins and tails to scoot around, looking for food in the fish tank.

As these Polypterus matured, Emily studied their anatomy, physiology and behavior. She found that, like soldiers trained to crawl under barbed wire keep their arms in the front and plod forward, the Polypterus raised in a terrestrial condition walked more effectively by placing their fins closer to their bodies. Further the Polypterus raised on land could lift their heads higher and keep their fins from slipping. They used the fin almost like a crutch, which gave them a little extra height when their “shoulders” rose upward and forward. As this posture temporarily hoisted more of the fish’s body into the air, there was less the tissue to rub along the ground and be slowed by friction. On the other hand, those Polypterus which were raised in tanks filled with water were clumsy; when placed on land, they sort of moved around gawkily.

Secondly, young Polypterus forced to walk, not swim, developed a sturdier build and certain specific bones in their heads and shoulder regions began developing differently. The clavicle bone in their chests was more strongly attached to the bone in the shoulder area. While normal Polypterus raised in water used buoyancy to support itself, the conditions faced by the land-living Polypterus enabled it to have a slightly altered the skeleton that could bear weight on land.

The gill area in Polypterus raised on land also enlarged a little and at the back of the head, bone connections loosened slightly. This enabled the fish to have a slightly more flexible neck, helping it to feed better on land. In contrast, in water, stiff neck is sufficient as the fish could dart the body above or below or elsewhere to attack the food.

Emily Standen says, “Stressful environmental conditions can often reveal otherwise cryptic anatomical and behavioural variation, a form of developmental plasticity. We wanted to use this mechanism to see what new anatomies and behaviours we could trigger in these fish and see if they match what we know of the fossil record”. The changes observed in the land-reared Polypterus suggest how some prehistoric fish-like ancestors of tetrapods might have moved.

In evolutionary terms, the changes observed in eight months are indeed lightning speed. This suggests that quirky conditions early in life similarly might have given ancient fish a little head start in adapting to life out the of water. Strictly speaking, however, showing that a modern fish has the flexibility to cope with land does not prove that prehistoric fish also had it. Yet, the experiment sheds some light on the kinds of changes that enabled fins to become limbs, as many of the anatomical changes observed in Polypterus mirror the fossil record.
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