Giant oceanic manta ray
|Giant oceanic manta ray
Temporal range: 23–0Ma Early Miocene to Present
|Range of the giant oceanic manta ray|
The giant oceanic manta ray (Manta birostris) is a species of ray in the family Mobulidae, and the largest type of ray in the world. They are circumglobal and are typically found in tropical and subtropical waters, but can also be found in temperate waters.
Oceanic mantas are large, horizontally flattened fish that can grow to a width of 9 m (30 ft). They feed on plankton which they scoop up with their large mouths. Little is known of their reproductive habits, but one (or occasionally two) live young are born after a gestation period of about a year. They reside in deep-water, pelagic zones, making periodic visits to cleaning stations at seamounts and coastal reefs. Minimal concrete information exists on oceanic manta movements, but they are generally believed to be more transient and migratory than the smaller reef manta ray (Manta alfredi), which tends to be resident in shallower coastal habitats.
The giant oceanic manta ray can grow to a disc size of up to 9 m (30 ft) with a weight of about 1,350 kg (2,980 lb). It can live for 20 years. It is dorsoventrally flattened and has large, triangular pectoral fins on either side of the disc. At the front, it has a pair of cephalic fins which are forward extensions of the pectoral fins. These can be rolled up in a spiral or can be flared out to channel water into the large, forward-pointing, rectangular mouth when the animal is feeding. The teeth are in a band of 18 rows and are restricted to the central part of the lower jaw. The eyes are on the side of the head behind the cephalic fins, and the gill slits are on the ventral (under) surface. It has a small dorsal fin and the tail is long and whip-like. The manta ray does not have a spiny tail as do the closely related devil rays (Mobula spp.). The skin is smooth with a scattering of conical and ridge-shaped tubercles. The colouring of the dorsal (upper) surface is black, dark brown, or steely blue, sometimes with a few pale spots and usually with a pale edge. The ventral surface is white, sometimes with dark spots and blotches. The markings can often be used to recognise individual fish. Manta birostris is similar in appearnce to Manta alfredi and the two species may be confused as their distribution overlaps. However there are distinguishing features.
The giant oceanic manta ray has a widespread distribution in tropical and temperate waters worldwide. In the Northern Hemisphere, it has been recorded as far north as southern California and New Jersey in the United States, Aomori Prefecture in Japan, the Sinai Peninsula in Egypt, and the Azores in the northern Atlantic. In the Southern Hemisphere, it occurs as far south as Peru, Uruguay, South Africa, and New Zealand. It is an ocean-going species and spends most of its life far from land, travelling with the currents and migrating to areas where upwellings of nutrient-rich water increase the availability of zooplankton.
When travelling in deep water, the giant oceanic manta ray swims steadily in a straight line, while further inshore it usually basks or swims idly around. Mantas may travel alone or in groups of up to 50 and sometimes associate with other fish species, as well as sea birds and marine mammals. They are filter feeders and consume large quantities of zooplankton in the form of shrimp, krill, and planktonic crabs. An individual manta may eat about 13% of its body weight each week. When foraging, it usually swims slowly around its prey, herding the planktonic creatures into a tight group before speeding through the bunched-up organisms with its mouth open wide. While feeding, the cephalic fins are spread to channel the prey into its mouth and the small particles are sifted from the water by the tissue between the gill arches. As many as 50 individual fish may gather at a single, plankton-rich feeding site.
The giant oceanic manta ray sometimes visits a cleaning station on a coral reef, where it adopts a near-stationary position for several minutes while cleaner fish consume bits of loose skin and external parasites. Such visits occur most frequently at high tide. It does not rest on the seabed as do many flat fish, as it needs to swim continuously to channel water over its gills for respiration.
Males become sexually mature when their disc width is about 4 m (13 ft), while females need to be about 5 m (16 ft) wide to breed. When a female is becoming receptive, one or several males may swim along behind her in a "train". During copulation, one of these males grips the female's pectoral fin with his teeth and they continue to swim with their ventral surfaces in contact. He inserts his claspers into her cloaca and these form a tube through which the sperm is pumped. The pair remains coupled together for several minutes before going their own ways.
The fertilized eggs develop within the female's oviduct. At first, they are enclosed in an egg case and the developing embryos feeds on the yolk. After the egg hatches, the pup remains in the oviduct and receives nourishment from a milky secretion. As it does not have a placental connection with its mother, the pup relies on buccal pumping to obtain oxygen. The brood size is usually one but occasionally two embryos develop simultaneously. The gestation period is thought to be 12-13 months. When fully developed, the pup is 1.4 m (4 ft 7 in) in disc width, weighs 9 kg (20 lb) and resembles an adult. It is expelled from the oviduct, usually near the coast, and it remains in a shallow-water environment for a few years while it grows.
The giant oceanic manta ray is considered to be Vulnerable by the IUCN in its Red List of Endangered Species. The main threats it faces are from fishing or from getting entangled in nets or injured by cables. The manta ray's flesh is also eaten, and in some areas, artisanal fishing may be occurring at unsustainable levels. Some fishing is targeted at this species, as their gill rakers are used in oriental medicines. Other threats may be marine pollution, climate change, ingestion of microplastics, and tourism practices.
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