Fish anatomy

Lore & Background

The body of a fish is divided into a head, trunk and tail, although the divisions between the three are not always externally visible. The skeleton, which forms the support structure inside the fish, is either made of cartilage (cartilaginous fish) or bone (bony fish). The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. The main external features, the fins, are composed of either bony or soft spines called rays which, with the exception of the caudal fins, have no direct connection with the spine; they are supported by the muscles that make up most of the trunk. Sharks and rays are basal fish with numerous primitive anatomical features similar to those of ancient fish, including skeletons composed of cartilage. Their bodies tend to be dorso-ventrally flattened, and they usually have five pairs of gill slits and a large mouth set on the underside of the head. The dermis is covered with separate dermal placoid scales. They have a cloaca into which the urinary and genital passages open, but not a swim bladder. The bony fish lineage shows more derived anatomical traits, including a bony skeleton, generally lateral flattening, five pairs of gills protected by an operculum, and a mouth at or near the tip of the snout. The dermis is covered with overlapping scales, and they have a swim bladder which helps them maintain a constant depth in the water column, but not a cloaca.

Reader's Guide

Fish anatomy is notable for its fundamental differences from mammalian anatomy, yet it shares the same basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail. The heart has two chambers and pumps blood through the respiratory surfaces of the gills and then around the body in a single circulatory loop. The eyes are adapted for seeing underwater and have only local vision. There is an inner ear but no external or middle ear; low-frequency vibrations are detected by the lateral line system of sense organs that run along the length of the sides of fish, responding to nearby movements and changes in water pressure. The vertebral column consists of a centrum, vertebral arches, and various processes; the centrum of a fish is usually concave at each end (amphicoelous), which limits motion. In cartilaginous fish, the vertebrae consist of two cartilaginous tubes, while in most ray-finned fishes, the vertebral structures are fused into a solid piece of bone. Lampreys have vertebral arches but no vertebral bodies, and hagfishes lack a true vertebral column but possess a cranium, leading to their historical exclusion from Vertebrata in some classifications, though molecular analyses since 1992 have shown them to be vertebrates in a phylogenetic sense.

Did You Know?

• Cartilaginous fish have a cloaca but not a swim bladder, while bony fish have a swim bladder but not a cloaca.
• The centrum of a fish vertebra is usually concave at each end (amphicoelous), which limits the motion of the fish.