page_title: Fish fin
section_title: Bony fishes
hierarchical_section_title: Fish fin / Bony fishes
caption_attribution_description: The skeleton of a Lingcod (Ophiodon elongatus) at the Monterey Bay Aquarium.
context_page_description: Fins are usually the most distinctive anatomical features of a fish. They are composed of bony spines or rays protruding from the body with skin covering them and joining them together, either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as seen in sharks. Apart from the tail or caudal fin, fish fins have no direct connection with the spine and are supported only by muscles. Their principal function is to help the fish swim.
  Fins located in different places on the fish serve different purposes such as moving forward, turning, keeping an upright position or stopping. Most fish use fins when swimming, flying fish use pectoral fins for gliding, and frogfish use them for crawling. Fins can also be used for other purposes; male sharks and mosquitofish use a modified fin to deliver sperm, thresher sharks use their caudal fin to stun prey, reef stonefish have spines in their dorsal fins that inject venom, anglerfish use the first spine of their dorsal fin like a fishing rod to lure prey, and triggerfish avoid predators by squeezing into coral crevices and using spines in their fins to lock themselves in place.
context_section_description: Bony fishes form a taxonomic group called Osteichthyes. They have skeletons made of bone, and can be contrasted with cartilaginous fishes which have skeletons made of cartilage. Bony fishes are divided into ray-finned and lobe-finned fish. Most fish are ray-finned, an extremely diverse and abundant group consisting of over 30,000 species. It is  the largest class of vertebrates in existence today. In the distant past, lobe-finned fish were abundant. Nowadays they are mainly extinct, with only eight living species. Bony fish have fin spines and rays called lepidotrichia. They typically have swim bladders, which allows the fish to create a neutral balance between sinking and floating without having to use its fins. However, swim bladders are absent in many fish, most notably in Lungfishes, which are the only fish to have retained the primitive lung present in the common ancestor of bony fish from which swim bladders evolved. Bony fishes also have an operculum, which helps them breathe without having to use fins to swim.
caption: Skeleton of a ray-finned fish
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