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Flying Fish.jpg
Flying Fish print (detail). Marcus Elieser Bloch
Fantail Filefish Fishes of Hawaii
Brazilian Wrasse.jpg
"Orange Brazilian Wrasse" print (detail). Marcus Elieser Bloch
Longnose Butterflyfish print. Fishes of Hawaii
Peacock Flounder print (detail). Marcus Elieser Bloch

Fishes: A Definition

A fish is a cold-blooded, finned aquatic vertebrate. Most fishes are scaled and respire by passing water over their gills. A relatively small number of fishes are scaleless and some are able to breathe air directly from the surface of the water. A few highly unusual species can survive for varying periods out of water.



Fish Facts

Great White Shark.

The Fishes: A Snapshot

Bloch's Ocean Sunfish.
  • Of all living vertebrates, the fishes make up more than half of the approximately 55,000 known species.
    • Species of Fishes (including subspecies): 30,121
    • Primarily Marine: 14,021 (47%)
    • Primarily Freshwater: 13,228 (44%)
    • Brackish or Diadromous: 2,871 (10%)
  • Fish Anatomy


Ichthyologist Marcus Elieser Bloch.
  • Ichthyologist: One who studies fishes.
  • Ichthyology derives from the Greek: ikhthu (fish); and logos (knowledge); it is a branch of zoology devoted to the study of fishes.
  • Fish: a single specimen or a group that includes only members of the same species.
  • Fishes: more than one species of fish.

Classes of Fishes

Modern fishes are divided into three classes:

  • AGNATHA: primitive jawless fishes, such as lampreys and hagfishes.
  • CHONDRICHTHYES: jawed fishes with cartilaginous skeletons, such as the sharks and rays.
  • OSTEICHTHYES: fishes with bony skeletons, by far the largest group, with everything from guppies to moray eels, piranhas, and giant groupers.

Need help? Read the Species Profiles instructions for Fishes.

Reference: Reef Fishes Volume 1
Image credit: SWM
Text credit: AQG