Tarpons are large fish of the genus Megalops; one species is native to the Atlantic, and the other to the Indo-Pacific Oceans. They are the only members of the family Megalopidae.
The two species of tarpons are Megalops atlanticus (Atlantic tarpon) and the Megalops cyprinoides (Indo-Pacific tarpon). M. atlanticus is found on the western Atlantic coast from Virginia to Brazil, throughout the coast of the Gulf of Mexico, and throughout the Caribbean. Tarpons are also found along the eastern Atlantic coast from Senegal to South Angola. M. cyprinoides is found along the eastern African coast, throughout southeast Asia, Japan, Tahiti, and Australia. Both species are found in both saltwater and freshwater habitats, usually ascending rivers to access freshwater marshes. They are able to survive in brackish water, waters of varying pH, and habitats with low dissolved oxygen content due to their swim bladders, which they use primarily to breathe. They are also able to rise to the surface and take gulps of air, which gives them a short burst of energy. The habitats of tarpons vary greatly with their developmental stages. Stage-one larvae are usually found in clear, warm, oceanic waters, relatively close to the surface. Stage-two and -three larvae are found in salt marshes, tidal pools, creeks, and rivers. The habitats are characteristically warm, shallow, dark bodies of water with sandy mud bottoms. Tarpons commonly ascend rivers into freshwater. As they progress from the juvenile stage to adulthood, they move back to the open waters of the ocean, though many remain in freshwater habitats.
One of the unique features of Megalops is the swim bladder, which functions as a respiratory pseudo-organ. These gas structures can be used for buoyancy, as an accessory respiratory organ, or both. In Megalops, this unpaired air-holding structure arises dorsally from the posterior pharynx. Megalops uses the swim bladder as a respiratory organ and the respiratory surface is coated with blood capillaries with a thin epithelium over the top. This is the basis of the alveolar tissue found in the swim bladder, and is believed to be one of the primary methods by which Megalops“breathes”. These fish are obligate air breathers, and if they are not allowed to access the surface, they will die. The exchange of gas occurs at the surface through a rolling motion that is commonly associated with Megalops sightings. This “breathing” is believed to be mediated by visual cues, and the frequency of breathing is inversely correlated to the dissolved O2 content of the water in which they live.