Physiology Assignment week 7
Debbie Hobbs
SCI 230
January 11, 2013

Fish have a very unique anatomy and physiology so that they can thrive in underwater environments. Fish have a closed circulatory system, in a fish blood flows from the heart, to the gills, to the body and back. Gills are comparable to the lungs of humans but the way they take in oxygen is very different. Water flows over the gills and the gills absorb dissolved oxygen. Fish sense their environment in a number of different ways that are unlike humans. Some fish can smell their environment through their skin in addition to using their nares. Additionally fish can sense electric fields and their environment using a structure called a lateral line. These structures are unique to fish and allow them to perceive their environment and live successfully. Fish maintain their buoyancy through an anatomical structure called a swim bladder. This is like a built in float that can inflate and deflate to help achieve neutral buoyancy at different depths. Some fish have swim bladders; while others that dwell on the bottom of the sea floor, which do not move up and down the water, do not have a swim bladder. The swim bladder also helps the fish to produce sounds. These sounds can be warning and mating signals to other fish. To help fish swim they have a number of fins (dorsal, ventral, pelvic, anal and caudal) and these fins could be rayed or composed of adipose tissue. Some fish have fins that can retract to help reduce drag while swimming and some fish only have a couple types of fins, which suit their body form and life style.
Fishes are considered to be the most primitive living vertebrates. (This means that fishes were the first vertebrates to evolve and that they have many characteristics that are thought to have existed in their earliest ancestors. It does not mean that they are somehow inferior to other types of vertebrates.) Fishes did not evolve from such organisms as living lancelets or tunicates. But similarities in structure and embryological development show that fishes and modern invertebrate chordates probably did evolve from common invertebrate ancestors that lived many millions of years ago. The first fishes--which are also the first vertebrates--were odd-looking jawless creatures whose bodies were covered with bony plates. They lived in the oceans of the late Cambrian Period, about 540 million years ago. For over 100 million years, fishes retained the basic armored jawless body plan. Then during the Ordovician and Silurian periods, fishes underwent a major adaptive radiation. Some of the groups that emerged from this adaptive radiation were jawless fishes that had very little armor--the ancestors of modern lampreys and hagfish. Others were armored jawless fishes in a variety of new forms. These fishes were ultimately evolutionary dead ends that became extinct around the end of the Devonian Period, about 350 million years ago. Still others were armored fishes that possessed a feeding adaptation that would revolutionize vertebrate, evolution: These fishes had jaws. Jaws are extremely important evolutionary innovations. Jawless fishes are limited to eating small particles of food that they can filter out of the water or suck up like a vacuum cleaner. Jaws made it possible for vertebrates to nibble on plants, munch on other animals, and defend themselves by biting. Another evolutionary innovation seen in the early jawed fishes were paired pectoral (anterior) and pelvic (posterior) fins that were attached to girdles of cartilage or bone.