The largest and most well known species was D. The largest fish of the now-extinct class Placodermi was the giant predatory Dunkleosteus. The largest species is the sea lamprey ( Petromyzon marinus), which can grow to 1.2 m (3.9 ft) and weigh 2.5 kg (5.5 lb). They are predatory and often attach themselves to a fish or other small animal and gradually drain blood and organs. They have cartilaginous skeletons and have been evolving separately from any other group for over 400 million years. Sea lamprey feeding on a lake trout As with the hagfish, lampreys appear eel-like in shape. This species can range up to 1.28 m (4.2 ft) in length and weigh to 6.2 kg (14 lb). The largest form is the Goliath hagfish ( Eptatretus goliath). All of the 77 known species have elongated, eel-like bodies but can be distinguished by their downward-facing mouths.

There is only one order and family in this animal class. The hagfish, which are not taxonomically true fish, are among the most primitive extant vertebrates. As such, cross group comparisons on this page only serve a colloquial purpose.

Fish are a paraphyletic group that describes aquatic vertebrates while excluding tetrapods, and the bony fish that often represent the group are more closely related to cetaceans such as whales, than to the cartilaginous fish such as sharks and rays. The whale shark and basking shark exceed all other fish by a considerable margin in weight and length.

Biggest giant oceanic manta ray