Introduction
The yellow perch, Perca flavens, is an actinopterygian. Besides the sharks and their relatives, there are two groups of living fish-like vertebrates, the Actinoptery- gii, the ray-finned fishes, and the Sarcopterygii, which includes the lobe-finned fishes and their tetrapod relatives.
The latter is a relatively small group, at least in terms of their fish-like forms, and includes the coela- canths and lungfishes, barely a handful of species, which is a rather dismal record as far as fish go. Conversely, the actinopterygians are a huge success story, both in terms of diversity and numbers, with about 25,000 species known (although this number varies depending on author). The fins of actinopterygians are supported by slender, rod-like rays or lepidotrichia radiating from the base of the fin; the musculature controlling the fin are within the body wall.The actinopterygians are subdivided into two broad groups, the “palaeoniscids”, a small, paraphyletic grouping of some 40 species that includes basal forms such as bichirs (Polypteriformes) and sturgeons and paddlefishes (Acipenseriformes), and the Neopterygii, a clade that includes, clearly, the great majority of ray- finned fishes. In older classifications, the basal forms were considered chondrosteans, representative of a more primitive stage of ray-finned fishes. Neopterygians include several basal forms, such as gars and bowfins, that were grouped as “holosteans”, intermediate between the chondrosteans and the advanced ray-finned fishes, the Teleostei.
Teleosts have undergone extensive radiations to produce fishes that have invaded nearly every aquatic niche. There are far too many groups to discuss here, but we may mention the Elopomorpha (eels and tarpons), the commercially important Clupeomorpha (anchovies and herrings), and the Paracanthopterygii, another commercially important group that includes codfishes. The perch belongs to the Perciformes, a member of the Acan- thopterygii, the spiny-finned fishes. Another interesting group is the Ostariophysi, which includes catfishes, characins, and minnows. These fish share a unique sound detection system, the Weberian apparatus, which involves the swim bladder and modified elements of the first few trunk vertebrae. The vertebrae act as a conduction system between the swim bladder and the inner ear at the back of the skull. Vibrations of the swim bladder caused by sound waves are transmitted by the bony elements to a posterior extension of the ear’s membranous labyrinth, stimulating the auditory center of the brain.