Anatomy and ecology of bony fish

A-Level: Biology

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The Osteichthyes, or bony fish, are the most widespread living aquatic vertebrates. They have mostly bony skeletons and a body covered with bony scales originating from skin. The scales have the shape of large thick plates in more primitive groups, but appear as thin laminae superimposed as roofing tiles imbricate. In addition, more recent ones called Teleostei. The scales are permanent and become larger as the animal grows, forming typical ring. The age of the fish can be estimated from the number of these rings. There are one or two dorsal fins and an anal fin. In more primitive groups, the tail is heterocercal, but in most species the two lobes have a similar length, homocercal fin. During embryo development, the swim bladder is formed from an outpocketing of the pharynx. It can contain gas or function as a buoyancy organ. In addition, a fish secretes a layer of mucus that covers its entire body. Mucus helps protect a fish from infection.

Thousands of species of bony fishes are less than a few centimeters long as adults. Some species can reach tremendous sizes - much larger than a human. Bony fishes show great variety in body shape, but the "typical" fish body shape is roughly cylindrical and tapering at both ends. This characteristic fusiform shape is quite energy efficient for swimming. Various species of fishes deviate from the fusiform body shape in three ways: compression, depression, and elongation.
All fishes have fins. Bony fish families show various degrees of fin fusion and reduction. Fins help stabilize or propel a fish in the water. Except in the lungfishes and the coelacanth, fins lack bones. In Actinopterygians, fins are supported by structures called rays. Fishes have two kinds of fins: paired fins (pectoral and pelvic) and median fins (dorsal, caudal, and anal). Typically, the paired pectoral fins help a fish turn. In some fishes, pectoral fins are adapted for other functions. Paired pelvic fins add stability, and some fishes use them for slowing. The dorsal fin may be a single fin or separated into several fins. In most bony fishes, the dorsal fin is used for sudden direction changes and acts as a "keel", keeping the fish stable in the water. In some fishes, the dorsal fin is adapted for other functions. The caudal fin, or tail, is responsible for propulsion in most bony fishes. Caudal fins come in many shapes. Many continuously swimming fishes have forked caudal fins. Fishes with lunate caudal fins, such as tunas, tend to be fast swimmers that can maintain rapid speed for long durations. The anal fin adds stability. In some fishes, the anal fin is adapted for other functions. Some species of bony fishes have reduced or absent fins. For example, morays Family Muraenidae lack pectoral fins and pelvic fins. Several species lack an anal fin.
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