Fish anatomy

Fish anatomy is the scientific study of the form and morphology of fish. Morphology refers to the physical structure and shape of an organism. This study is often contrasted with fish physiology. Physiology is the study of how the different parts of a fish function together in a living animal. While anatomy looks at how organs are put together, physiology looks at how they work. Scientists often use microscopes or dissecting tables to observe these structures. The physical environment of water heavily influences how fish are built. Water is much denser than air. It also absorbs more light and holds much less dissolved oxygen than air does.

A fish body is generally divided into three main regions: the head, the trunk, and the tail. These divisions are not always easy to see from the outside. The internal support structure is called an endoskeleton. This endoskeleton consists of an axial skeleton and an appendicular skeleton. The axial skeleton includes the skull and the vertebral column. The vertebral column is a series of articulating vertebrae that are strong yet lightweight. These vertebrae protect the spinal cord. The appendicular skeleton supports the fins. Most fins are made of bony or soft spines called rays. Except for the caudal fin, these rays have no direct connection to the spine. Instead, muscles in the trunk support them.

There are two primary types of skeletons in fish. Cartilaginous fish, such as sharks and rays, have skeletons made of cartilage. Bony fish have skeletons made of bone. Bone is a dense connective tissue that is rigid and hard. It serves many biological functions. Bones support the body and move the animal. They also protect internal organs and store minerals. Bone tissue can also produce red and white blood cells. In bony fish, the vertebrae are often fused into a solid piece of bone. In contrast, the vertebrae of cartilaginous fish consist of two cartilaginous tubes. The upper tube encloses the spinal cord in a continuous sheath.

Fish anatomy varies greatly between different lineages. Sharks and rays are considered basal fish. This means they possess many primitive anatomical features similar to ancient fish. Their bodies are often dorso-ventrally flattened, which means they are thin from top to bottom. They typically have five pairs of gill slits and a mouth located on the underside of the head. Their skin is covered in separate dermal placoid scales. These fish also have a cloaca, where the urinary and genital passages open. However, they do not possess a swim bladder. They often produce a small number of large, yolky eggs. Some are oviparous, meaning they lay eggs that develop externally. Others are ovoviviparous, where the young develop inside the parent.

The bony fish lineage shows more derived anatomical traits. These are traits that have changed significantly from ancient fish. Bony fish are generally laterally compressed, meaning they are thin from side to side. They have five pairs of gills protected by a bony cover called an operculum. Their mouths are usually located at or near the tip of the snout. Bony fish have overlapping scales on their dermis. A key feature is the swim bladder. This gas-filled organ helps the fish maintain a constant depth in the water. Unlike sharks, bony fish do not have a cloaca. Most bony fish spawn large numbers of small eggs with little yolk into the water.

Internal systems are highly specialized for life underwater. The fish heart has two chambers. It pumps blood through a single circulatory loop. The blood travels to the respiratory surfaces of the gills to collect oxygen. It then moves around the rest of the body. To sense their surroundings, fish use a lateral line system. This is a series of sense organs running along the sides of the body. The lateral line detects nearby movements and changes in water pressure. For vision, fish have eyes adapted for underwater use, though they often only have local vision. They also possess an inner ear to help with balance and sensing, though they lack an external or middle ear.

Fish share a basic body plan with all vertebrates. This plan includes a notochord, which is a stiff rod, and a well-defined head and tail. All vertebrates also have a hollow tube of nervous tissue called the spinal cord. While fish anatomy is very different from mammalian anatomy, these core connections remain. The study of fish anatomy helps us understand how life has evolved to master the dense and challenging medium of water.