Why Are Fish Shaped The Way They Are?

What is a fish? A fish is a vertebrate (an animal with a backbone) that lives in water. There are more fish than any other kind of vertebrate – not surprising since our planet is mostly water.

Now to our question – why ARE fish shaped the way they are? To answer that, it is important to understand the environment in which fish live. This affects their shape and how they survive.

First of all, fish are cold-blooded animals. Mammals, including humans, are ‘warm-blooded’- their bodies can control their own temperature. Mammals can grow a layer of fat or fur to protect them from the cold, or they can take shelter in a cave. If they get hot, they can sweat or hide in the shade. Fish cannot do this. Their body temperature is the same as the environment around them – cool-water or warm-water. When the water temperature goes up to high or down to low, they must move somewhere else if they are to survive.

Fish can live in amazingly different environments. Some have been found in the deepest of the oceans at around 11,000 metres (that is deeper than Mt. Everest is high!). Others can live in mountain lakes at 4,500 metres above sea level. That’s a difference of 15,500 metres. Humans can survive in only a portion of that range. So what makes fish so good at living in different locations? Adaptation! Evolving over millions of years, fish have adapted themselves to their environment.

The Flounder or Flatfish Family of fish are all flat, with both eyes on top of the head and the mouth underneath. Many species of flounder or sole live off the west coast of BC. They can feed along the ocean bottom and look out for enemies above at the same time. Their skin is speckled on top to look like sand.

The Deep-Sea Angler Fish lives in extremely deep water in the Pacific Ocean off California where it is under tremendous pressure from the water above and where hardly any light reaches. The anglerfish resists the pressure by being very round in shape, with soft, thin bones and jelly-like flesh. In fact, it is almost all water. As the anglerfish can’t use its eyes to hunt for food, it has developed a thin organ dangling from the top of its head, like a fishing line. Tiny bacteria at the tip produce bioluminescence (light), which attracts prey. Thus, drawn by the light, the food comes to the fish!

The Northern Pike is a hunting fish that lives in fresh water. It is long and slender, with fins placed far back on its body. With a quick burst of speed, the pike can race out of hiding and capture its prey. Pike are common in the Peace region of northern BC.

The Tuna is a large fish that can swim very fast, up to 77 kilometres per hour. It has a wide body that narrows to be very thin by the caudal (tail) fin. This aerodynamic shape and fast side-to side movement of the fin is why the tuna is able to swim so fast. This helps it escape from predators. Many species of tuna live off BC’s coast.

The American Eel doesn’t even look like a typical fish. It is a snake-shaped fish with extremely small scales, which give its body a smooth appearance. Unlike salmon, eels migrate from fresh to salt water to spawn. American Eels are very strong swimmers. They swim all the way from Lake Ontario out to the Sargasso Sea, an area way off in the North Atlantic Ocean to lay their eggs – a distance of about 6,000 kilometres!

Littlest but not least important as an example of adaptation is a population of Lake Chub, which live in the wetlands draining the hot springs in Liard River Hot Springs Provincial Park in northern British Columbia. They are small minnows that have adapted to these warm waters and we mean really warm (for fish) at temperatures from 15 to 26°C, during summer. As far as we know, this is the only place where chub survive in water that is this warm.

Author information: adapted for NatureWILD vol 18, issue 4 from an article by Nick Baccante, retired fisheries biologist who lives in Fort St. John and worked for the Ministry of Environment. (NatureWILD Volume 8, Issue 3).

Image credit: http://www.montereybayaquarium.org

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