Fish Anatomy: External and Internal Structure

A reference to the body plan of a typical bony fish, from fins, scales and the lateral line to the swim bladder, heart and kidney, and how each part serves the fish.

The bony fish, the group that includes almost all aquarium species, shares a remarkably consistent body plan in which external shape and internal organs are closely matched to life in water. Knowing this anatomy makes it far easier to read a fish's behaviour, spot illness and understand why fish keep the conditions they do.

External anatomy

The body is often fusiform, a streamlined plan found in fast-moving fish, and is propelled and steered by fins. The caudal (tail) fin is used for propulsion; dorsal fins prevent the fish from rolling and assist in sudden turns and stops; anal fins stabilize the fish while swimming; and pelvic fins assist in going up or down, turning sharply and stopping quickly. The paired pectoral fins help some species manoeuvre and can create a dynamic lifting force. The skin carries numerous mucus-secreting goblet cells that produce a slimy coat, which aids insulation and protection from bacterial infection.

Two external features matter especially for fishkeepers. The operculum, or gill cover, protects the gills in bony fish and is absent in sharks and jawless fish. The lateral line is a sense organ used to detect movement and vibration in the surrounding water, letting fish navigate, school and sense approaching threats even in darkness.

Internal anatomy

Inside, fish have what is often described as a two-chambered heart driving a single-circuit circulation, in which blood passes through the gills to be oxygenated before reaching the rest of the body. The swim bladder is an internal gas-filled organ in many bony fishes used to control buoyancy, letting a fish hold its depth without constant effort. The digestive tract in many species includes pyloric caeca, small pouch-like structures along its length that increase the overall surface area for absorption. Other internal organs, the kidney, liver, spleen and gonads, handle excretion and osmotic balance, metabolism, blood function and reproduction respectively, while the gills carry out respiration.

Bony versus cartilaginous fish

Cartilaginous fish, the sharks and rays, differ in two visible ways relevant here: they do not have swim bladders and they lack an operculum, instead having separate gill slits. Their skeleton is cartilage rather than bone. Most ornamental aquarium fish, however, are bony fish with the structures above.