The truly spectacular   array of body forms within this class can only be appreciated by familiarizing oneself with some of the more than 25,000 species of actinopterygians – the largest and most diverse of all vertebrate classes – that exist today. Consider the fact that actinopterygians may fly, walk, or remain immobile (in addition to 'swimming'), exist in virtually all types of habitats except constantly dry land (though some can walk over land), feed on nearly every type of organic matter, utilize several types of   sensory systems (including chemoreception, electroreception, magnetic reception and a “distance-touch” sensation – see Communication), and some even produce their own light or electricity. In addition, color diversity in ray-finned fishes is “essentially unlimited, ranging from uniformly dark black or red in many deepsea forms, to silvery in pelagic and water-column fishes, to countershaded in nearshore fishes of most littoral [near-shore] communities, to the strikingly contrasted colors of tropical freshwater and marine fishes” (Helfman et al. 1997:367). Of course, extravagant coloration is not helpful for fish at risk of being eaten, yet bright coloration is environment-specific (see Helfman et al. 1997:367) and bright colors at one depth are cryptic at others due to light attenuation (see Communication). Further, color change is common in brightly colored (as well as many other) fishes and occurs under a variety of circumstances. Pigments are responsible for a many types of color change, but there are also structural colors, resulting from light reflecting off of crystalline molecules housed in special chromatophores (cells located mainly in the outer layer of skin). The silvery sheen displayed by many pelagic fishes is an example of structural color. Numerous actinopterygians are also   sexually dimorphic (males and females look different), and body form changes drastically during development, so there is significant diversity within, as well as among, species.

Among the largest actinopterygians are the pirarucu (also known as giant arapaima , up to 2.5m in length) in freshwater and the black marlin (up to 900kg) in saltwater; the longest is the oarfish , Lampris immaculatus, which averages between 5 and 8m in length; and the smallest, a variety of diminutive gobies in saltwater and minnows , catfishes and characins in freshwater. At various points in this account, there is further discussion of physical characteristics as they relate to particular topics (i.e. Systematic/Taxonomic History, Communication, Food Habits and Predation), but for a technical description of actinopterygians, see below. (View an illustration of   external fish parts or a   fish skeleton).

Actinopterygians may have   ganoid, cycloid, or ctenoid scales, or no scales at all in many groups. With the exception of Polypteriformes, the pectoral radials are attached to the scapulo-coracoid, a region of the pectoral girdle skeleton. (The pectoral radials are one of a series of endochondral - growing or developing within cartilage - bones in the pectoral and pelvic girdle on which the fin rays insert). Most have an interopercle and branchiostegal rays and the nostrils are positioned relatively high on the head. Finally, the spiracle (respiratory opening between the eye and the first gill slit – connects with the gill cavity) and gular plate (behind the chin and between the sides of the lower jaw) are usually absent, and internal nostrils are absent.

Other Physical Features: heterothermic ; bilateral symmetry ; polymorphic ; poisonous ; venomous

Sexual Dimorphism: sexes alike; female larger; male larger; sexes colored or patterned differently; female more colorful; male more colorful; sexes shaped differently; ornamentation