Modern osteichthyans are of every imaginable shape and size, and exploit most marine and freshwater habitats. Many of them have complex, highly evolved physiologies, organs, and behaviors adapted for dealing with all sorts of environments.
Actinopterygii are by far the most abundant fishes today. Flexible or hard rays support their fins and some have poison on their hard fin rays. Even if they are not poisonous, the rays defend the fish by making it hard to grasp and swallow.
The Actinopterygii include the Chondrostei. Present forms include the Polypteridae, sturgeons and paddlefishes. Sturgeons live mainly in coastal waters, but the adults often migrate to rivers and lakes to breed. They are in danger of extinction, as Sturgeon flesh has a high commercial value, and the eggs are consumed as caviar.
Living holosteans include the Lepisosteiformes (beak fish) and the Amiiformes (bowfins), which inhabit fresh waters in central North America and Cuba. There are around 21,000 species of teleosts adapted to various environments. Their success depends on several factors, such as food sources, improved swimming techniques, and increased reproductive activity.
There are five extant species of sarcopterygian lung fishes, all of which are virtually identical to their fossil ancestors of the Devonian. The living Australian Neoceratodus (left) in particular is a true "living fossil". Today the only living species are the lungfishes (Dipnoi) and the coelacanth Latimeria. Extant lungfish will attack almost anything that moves, including the hands of humans. The lungfish can survive in almost any water conditions, including those with a low supply of oxygen.