Megalops

Life-forms

Superregnum: Eukaryota
Cladus: Unikonta
Cladus: Opisthokonta
Cladus: Holozoa
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Superclassis: Osteichthyes
Classis: Actinopterygii
Subclassis: Neopterygii
Infraclassis: Teleostei
Superordo: Elopomorpha
Ordo: Elopiformes
Familia: Megalopidae
Genus: Megalops
Species: Megalops atlanticus - Megalops cyprinoides

Vernacular name

There are two species of Megalops, commonly known as the tarpon, one native to the Atlantic, and the other to the Indo-Pacific oceans. They are the only members of the family Megalopidae and genus Megalops.

Species and habitats

There are two species of Megalops, the Megalops atlanticus and the Megalops cyprinoides. The Megalops atlanticus is found on the western Atlantic coast from Virginia to Brazil, throughout the coast of the Gulf of Mexico, throughout the Caribbean. It is also found along the eastern Atlantic coast from Senegal to Angola.[2] Megalops cyprinoides is found along the eastern African coast, throughout southeast Asia, Japan, Tahiti, and Australia. Both species are found in both salt and freshwater habitats usually ascending rivers to access freshwater marshes.[3] They are able to survive in brackish water, waters of varying pH, as well as habitats with low dissolved O2 content due to their swim bladders which they use primarily to breathe with.The habitat of the Megalops varies greatly with the developmental stage they are in. 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. It is quite common for Megalops to 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.[4][5]

Physical characteristics

Megalops grow to about 5–8 ft. long and weigh 80-280 lbs. They have dorsal and anal soft rays and have a bluish or greenish back. They possess distinctive lateral lines and have shiny silvery scales that cover most of the organism except for the head. They possess large eyes with adipose eyelids and a broad mouth with a prominent lower jaw that juts out farther than the rest of the face.[2][3][4]

Biology and behavior

Reproduction and life cycle

Tarpons breed offshore in warm isolated areas. Females have a high fecundity and can lay up to 12 million eggs at once. They reach sexual maturity once they are about 75–125 cm in length. Spawning usually occurs in late spring to early summer.[4] There are three distinct levels of development that usually occur in varying habitats. The first stage is the Leptocephali stage, or stage one, which is completed after 20–30 days. It takes place in clear, warm oceanic waters usually within 10-20m of the surface. The Leptocephali shrinks as it develops into a larva with the most shrunken larva, stage two, developing by day 70. This is due to a negative growth phase followed by a sluggish growth phase. By day 70 the Juvenile growth phase, stage three, begins and the organism begins to rapidly grow until it reaches sexual maturity.[6][2]

Diet

Stage one developing Megalops do not forage for food but instead absorb nutrients from sea water using integumentary absorption. Stage two and three juveniles feed primarily on zooplankton but also feed on insects and small fish. As they progress in juvenile development, especially those developing in freshwater environments, their consumption of insects, fish, crabs, and grass shrimp increases. Adults are strictly carnivorous and feed on mid-water prey; they swallow their food whole and hunt nocturnally.[4][5]

Predation

The main predators of Megalops during stage one and early stage two development are other fish and some zooplankton depending on the size of the Megalops larvae. Juveniles are subject to predation by other juvenile Megalops and piscivorous birds. Adults occasionally fall prey to sharks, porpoises, alligators, and piscivorous birds. They are especially vulnerable to birds when they come to the surface for air due to the rolling manner in which they move to take in air as well as the silver scales that line the sides of the Megalops.[7]

Swim bladder

One of the unique features of Megalops is the function of the swim bladder as a pseudo-respiratory organ. These gas structures can be used for buoyancy, as an accessory respiratory organ, or both. In Megalops, it is an unpaired air holding structure that 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 overtop. This is the basis of the alveolar tissue that is found in the swim bladder and is believed to be one of the primary methods by which Megalops “breathe”. Megalops are obligate air breathers, and if they are not allowed to access the surface they will die. The exchange of gas that occurs is done at the surface through a rolling motion that is commonly associated with Megalops sightings. It is believed that this “breathing” is mediated by visual cues and that the frequency of breathing is inversely correlated to the dissolved O2 content of the water in which they live.[8][4]

Megalops and humans

Megalops are considered one of the great saltwater game fishes. They are prized not only because of their great size but also because of the fight that they put up and their spectacular leaping ability. Megalops are bony fish and their meat is not desirable so most Megalops are released after they are caught. There are numerous tournaments around the year that are focused on catching Megalops.[9]

Geographical Distribution and Migration

Since Tarpons are not commercially valuable as a food fish very little has been documented concerning their geographical distribution and migrations. Tarpons inhabit both sides of the Atlantic Ocean. Their range in the Eastern Atlantic has been reliably established from Senegal to the Congo. Tarpons inhabiting the western Atlantic are principally found to populate warmer coastal waters focused in the Gulf of Mexico, Florida, and the West Indies. Nonetheless, Tarpons are regularly caught by anglers at Cape Hatteras and as far as Nova Scotia, Bermuda, and south to Argentina. There have been scientific studies principally “The Panama Canal as a Passageway for Fishes, with Lists and Remarks on the Fishes and Invertebrates Observed” by Samuel F. Hildebrand (1939) which indicate that schools of Tarpons have routinely migrated through the Panama Canal from the Atlantic to the Pacific and back for over 70 years. However it has not been scientifically established that they breed in the Pacific Ocean. Nevertheless anecdotal evidence by tarpon fishing guides and anglers would tend to validate this notion as over the last 60 years many small juvenile tarpon as well as mature giants have been caught and documented principally on the Pacific side of Panama at the Bayano River as well as in the Gulf of San Miguel and its tributaries but also as far as Coiba Island in the Gulf of Chiriquí plus at Piñas Bay in the Gulf of Panama. It would seem that since Tarpons tolerate wide ranges in salinity throughout their lives and will eat almost anything dead or alive, that their migrations are only limited by water temperatures. Tarpons prefer water temperatures of 72 to 82 F, below 60 degrees they become inactive and temperatures under 40F can be lethal.

Notes

1. ^ Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology 364: p.560. http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=611&rank=class. Retrieved 2008-01-08.
2. ^ a b c ” Megalops atlanticus”, www.fishbase.org, 11 February 2010.
3. ^ a b ” Megalops cyprinoides” , www.fishbase.org, 11 February 2010.
4. ^ a b c d e Zale, Alexander and Merrifield, Susan G. “ Life History and Environmental Requirements of Coastal Fishes and Invertebrates.” U.S. Fish and Wildlife Services. 1989.
5. ^ a b Wade, Richard Archer. “Ecology of Juvenile Tarpon and Effects of Dieldrin on Two Associated Species.” Bureau of Sport Fisheries and Wildlife. 1969.
6. ^ Tsukamoto Y., Okiyama, M. “Metamorphosis of the Pacific Tarpon, Megalops Cyprinoides (Elopiformes, Megalopidae) with Remarks on Development Patterns in the Elopomorpha.” Bulletin of Marine Science, 1997.
7. ^ Rickards, William L. “Ecology and Growth of Juvenile Tarpon, Megalops atlanticus, in a Georgia Salt Marsh.” Institute of Marine Sciences, University of Miami. 1968.
8. ^ Daniels, C., et. al. “The Origin and Evolution of the Surfactant System in Fish: Insights into the Evolution of Lungs and Swim Bladders.” Physiological and Biochemical Zoology. 2004.
9. ^ “Tarpon (Megalops atlanticus).” www.tpwd.state.tx.us. 2 June 2009.