Organism diagram main organs structures and physiological adaptation

Physiological Evolution of Fish

The anatomy of a typical fish adapted from Moyle, P.B., & Cech, J.J. (1982).

How fish has evolved physiologically to become suited to its environment

Fishes have been around for utmost 500 million years during which time they have managed to adapt to any type of aquatic habitat. Physiologic features the fish posses that make them more adapted to the environment they live in are not universal to all the species of fish. Some fishes may have elongate bodies where as others may have greatly shortened bodies. In others, the bodies may be flattened while others may have laterally compressed bodies. Other fish species may have elaborately extended fins that form intricate shapes while others may have reduced or even lost fins. The positions of the eye, the mouth, the nostril, and even the gills also differ in different fish species. Some fishes are cryptically colored. Their colors match their environments. Some fish have light producing organs. Some are capable of living in shallow thermal springs and others beneath the ocean surface. This paper seeks to explain how fish has evolved physiologically to become suited to its environment (aquatic or marine).

Almost all fish species have streamlined bodies that enable them to swim freely in open waters. Fishes that inhabit both marine and fresh water habitats are capable of swimming at the surface of the water. Their mouths are adapted to feeding best when they are at the surface of the water. they have long and slender bodies that enable them to dart at surface insects or other surface insects. They in turn dart away from predators. Oceanic flying fishes are capable of escaping their predators by gathering speed above water surface. The lower lobes of their tails provide thrust in the water. Such fishes are capable of gliding hundreds of yards on enlarged, wing-like pectoral and pelvic fins. Tunas and trout are adapted for strong and fast swimming hence their ability to catch a prey speedily in ocean waters. Fishes living in quiet waters have flattened sides. This enables them to make short, quick bursts of speed to escape their predators. Bottom living fishes move by undulating their bodies.

Methods of reproduction among fishes are varied. However, most fishes lay a large number of small eggs, fertilized and scattered outside the body. The fish eggs are usually suspended in open waters. Freshwater shore fishes normally lay eggs at the bottom or among plants. Some of these fiches are capable of laying adhesive eggs. The likelihood of death of small fishes is very high. This is also true for the eggs. Out of the numerous small fishes, very few grow to maturity. This means that a larger proportion of the eggs laid sometimes go into waste. Fishes lay many eggs to counter the issue of mortality. Pelvic fins in bony fishes are modified to help in transmitting the milt to the eggs to the female's vent. Successful reproduction and defense of the eggs is guaranteed by elaborate courtship and parental behavior. Some species of fish hollow out depressions in the sand bottom, build nests with plant materials and sticky threads that come from their kidney. At times the fishes blow a cluster of mucus-covered bubbles on water surface to keep organisms that predate on their eggs at bay. Some are also capable of incubating eggs in their mouths. Fishes like salmon can undergo long migrations from the ocean into large rivers to spawn in gravel beds. Some can migrate from lakes into streams where they spawn. All they want is habitats that they don't normally occupy.

Gills are located just beneath the operculum. They are bright red in color. They lie behind and to the side of the mouth cavity. Gills have fleshy filaments that are supported by the gill arches. The gills have an array of blood vessels. Water that is taken in by the fish through the mouth passes backward through the gill bars and over the gill filaments. Exchange of gases takes place over the gill filament. Gill cover protects the gill filaments.