Mantas are found in temperate, subtropical and tropical waters. Both species are pelagic; M. birostris migrates across open oceans, singly or in groups, while M. alfredi tends to be resident and coastal. They are filter feeders and eat large quantities of zooplankton, which they swallow with their open mouths as they swim. Gestation lasts over a year, producing live pups. Mantas may visit cleaning stations for the removal of parasites. Like whales, they breach, for unknown reasons.
Both species are listed as vulnerable by the International Union for Conservation of Nature. Anthropogenic threats include pollution, entanglement in fishing nets, and direct harvesting for their gill rakers for use in Chinese medicine. Their slow reproductive rate exacerbates these threats. They are protected in international waters by the Convention on Migratory Species of Wild Animals, but are more vulnerable closer to shore. Areas where mantas congregate are popular with tourists. Only a few aquariums are large enough to house them. In general, these large fish are seldom seen and difficult to study.
Appearance and Anatomy Edit
Manta rays are known for their large size, broad heads, triangular pectoral fins, and the horn-shaped cephalic fins located on either side of their mouths. They have horizontally flattened bodies with eyes on the sides of their heads (behind the cephalic fins) and gill slits on their ventral surfaces. Their tails lack skeletal support and are shorter than their disc-like bodies. The dorsal fins are small and at the base of the tail. The largest mantas can reach 1,350 kg (2,980 lb). In both species the width is approximately 2.2 times the length of the body; M. birostris reaches at least 7 m (23 ft) in width while M. alfredi reaches about 5.5 m (18 ft). Dorsally, mantas are typically black or dark in color with pale markings on their "shoulders". Ventrally, they are usually white or pale with distinctive dark markings by which individual mantas can be recognized. All black color morphs are known to exist. The skin is covered in mucus which protects it from infection.
The two species of manta differ in color patterns, dermal denticles, and dentition. M. birostris has more angular shoulder markings, larger ventral dark spots on the abdominal region, charcoal-colored ventral outlines on the pectoral fins and a dark colored mouth. The shoulder markings of M. alfredi are more rounded, while its ventral spots are located near the posterior end and between the gill slits, and the mouth is white or pale colored. The denticles have multiple cusps and overlap in M. birostris, while those of M. alfredi are evenly spaced and lack cusps. Both species have small square shaped teeth on the lower jaw but M. birostris also has enlarged teeth on the upper jaw. Unlike M. alfredi, M. birostris has a caudal spine near its dorsal fin.
Mantas move through the water by the wing-like movements of their pectoral fins, which drive water backwards. Their large mouths are rectangular, and face forward as opposed to other ray and skate species with downward-facing mouths. The spiracles typical of rays are vestigial, and mantas must swim continuously to keep oxygenated water passing over their gills. The cephalic fins are usually spiralled, but flatten during foraging. The fish's gill archeshave pallets of pinkish-brown spongy tissue that collect food particles. Mantas track down prey using visual and olfactory senses. They have one of the highest brain-to-body mass ratios of all fish. Their brains have retia mirabilia which may serve to keep them warm. M. alfredi has been shown to dive to depths of over 400 m, while their relative Mobula tarapacana, which has a similar structure, dives to nearly 2000 m; the retia mirabilia probably serve to prevent their brains from being chilled during such dives into colder subsurface waters.
Behavior and Ecology Edit
Swimming behavior in mantas differs across habitats: when travelling over deep water, they swim at a constant rate in a straight line, while further inshore they usually bask or swim idly around. Mantas may travel alone or in groups of up to 50. They may associate with other fish species as well as sea birdsand marine mammals. Mantas sometimes breach, leaping partially or entirely out of the water. Individuals in a group may make aerial jumps one after the other. These leaps come in three forms: forward leaps where the fish lands head first, similar jumps with a tail first re-entry or somersaults. The reason for breaching is not known; possible explanations include mating rituals, birthing, communication, or the removal of parasites and commensal remoras(suckerfish).
As filter feeders, manta rays consume large quantities of zooplankton in the form of shrimp, krill and planktoniccrabs. An individual manta eats about 13% of its body weight each week. When foraging, it slowly swims around its prey, herding it into a tight "ball" and then speeds through the bunched organisms with a wide-open mouth. If a ball is particularly dense, a manta may somersault through it. While feeding, mantas flatten their cephalic fins to channel food into their mouths and the small particles are collected by the tissue between the gill arches. As many as fifty individual fish may gather at a single, plankton-rich feeding site. Mantas are themselves preyed upon by large sharks and by killer whales. They may also be bitten by cookiecutter sharks, and harbor parasitic copepods.
Mantas visit cleaning stations on coral reefs for the removal of external parasites. The ray adopts a near-stationary position close to the coral surface for several minutes while the cleaner fish consume the attached organisms. Such visits most frequently occur when the tide is high. In Hawaii, wrassesprovide the cleaning; some species feed around the manta's mouth and gill slits while others address the rest of the body surface. In Mozambique, sergeant major fish clean the mouth while butterflyfishes concentrate on bite wounds. M. alfredi visits cleaning stations more often than M. birostris. Individual mantas may revisit the same cleaning station or feeding area repeatedly and appear to have cognitive maps of their environment.
Distribution and Habitat Edit
Mantas are found in tropical and subtropical waters in all the world's major oceans and also venture into temperate seas. The furthest from the equator they have been recorded is North Carolina in the United States (31ºN) to the north, and the North Island of New Zealand (36ºS) to the south. They prefer water temperatures above 68 °F (20 °C) and M. alfredi is predominantly found in tropical areas. Both species are pelagic. M. birostris lives mostly in the open ocean, travelling with the currents and migrating to areas where upwellings of nutrient-rich water increase prey concentrations.
Fish that have been fitted with radio transmitters have travelled as far as 1,000 km (620 mi) from where they were caught and descended to depths of at least 1,000 m (3,300 ft). M. alfredi is a more resident and coastal species. Seasonal migrations do occur, but they are shorter than those of M. birostris. Mantas are common around coasts from spring to fall, but travel further offshore during the winter. They keep close to the surface and in shallow water in daytime, while at night they swim at greater depths.