Manta ray facts for kids
Quick facts for kids Manta rayTemporal range: Lower Miocene to Recent
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| Manta ray at Hin Daeng, Thailand | |
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Manta
Bancroft, 1829
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M. birostris
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| Manta birostris (Walbaum, 1792)
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The manta ray (Manta birostris) is the largest species of the rays. The largest known specimen was more than 7.6 metres (25 ft) across, with a weight of about 1,300 kilograms (2,900 lb). It ranges throughout tropical waters of the world, typically around coral reefs. They have the largest brain-to-body ratio of the sharks, rays and skates (Elasmobranchii), a brain which is kept warm during lengthy dives to as deep as 500 metres (1,600 ft) in cold water.
Mantas may be at least two different species, the giant manta (Manta birostris), which migrates, and another smaller one called the reef manta (Manta alfredi), which does not. The genus may need revising.
Manta rays are probably at the top of the food chain. Some shark species, such as the tiger shark may hunt them.
Contents
Biology
Appearance and anatomy
Manta rays have broad heads, triangular pectoral fins, and 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 arches have 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 and the largest brain size 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.
Lifecycle
Mating takes place at different times of the year in different parts of the manta's range. The fertilized eggs develop within the female's oviduct. At first they are enclosed in an egg case while the developing embryos absorb the yolk. After hatching, the pups remain in the oviduct and receive additional nutrition from milky secretions. With no umbilical cord or placenta, the unborn pup relies on buccal pumping to obtain oxygen.
Brood size is usually one or occasionally two. The gestation period is thought to be twelve to thirteen months. When fully developed, the pup resembles a miniature adult and is expelled from the oviduct with no further parental care.
Manta rays may live for as long as 50 years.
Behavior and ecology
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 birds and 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 planktonic crabs. 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, wrasses provide 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
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.
Conservation issues
Threats
The greatest threat to manta rays is overfishing.
Both commercial and artisanal fisheries have targeted mantas for their meat and products. They are typically caught with nets, trawls and harpoons. Mantas were once captured by fisheries in California and Australia for their liver oil and skin; the latter were used as abrasives. Their flesh is edible and is consumed in some countries, but is unattractive compared to other fish. Demand for their gill rakers, the cartilaginous structures protecting the gills, has recently entered Chinese medicine.
To fill the growing demand in Asia for gill rakers, targeted fisheries have developed in Philippines, Indonesia, Mozambique, Madagascar, India, Pakistan, Sri Lanka, Brazil and Tanzania. Each year, thousands of manta rays, are caught and killed purely for their gill rakers. Targeted fisheries for manta rays in the Gulf of California, the west coast of Mexico, India, Sri Lanka, Indonesia, and the Philippines have reduced populations in these areas dramatically.
Manta rays are subject to other anthropogenic threats. Because mantas must swim constantly to flush oxygen-rich water over their gills, they are vulnerable to entanglement and subsequent suffocation. Mantas cannot swim backwards and, because of their protruding cephalic fins, are prone to entanglement in fishing lines, nets / ghost nets, and even loose mooring lines. When snared, mantas often attempt to free themselves by somersaulting, tangling themselves further. Loose, trailing line can wrap around and cut its way into its flesh, resulting in irreversible injury. Similarly, mantas become entangled in gill nets designed for smaller fish. Some mantas are injured by collision with boats, especially in areas where they congregate and are easily observed. Other threats or factors that may affect manta numbers are climate change, tourism, pollution from oil spills, and the ingestion of microplastics.
Status
In 2011, mantas became strictly protected in international waters because of their inclusion in the Convention on Migratory Species of Wild Animals.
In the same year, M. alfredi was also classified as 'Vulnerable' with local populations of fewer than 1000 individuals and little or no interchange between subpopulations. The Manta Trust is a UK-based charity dedicated to research and conservation efforts for manta rays. The organization's website is also an information resource for manta conservation and biology.
Besides these international initiatives, some countries are taking their own actions. New Zealand has banned the taking of manta rays since the introduction of the Wildlife Act in 1953. In June 1995, the Maldives banned the export of all ray species and their body parts, effectively putting a stop to manta fishing as there had not previously been a fishery for local consumption. The government reinforced this in 2009 with the introduction of two marine protected areas. In the Philippines, the taking of mantas was banned in 1998, but this was overturned in 1999 under pressure from local fishermen. Fish stocks were surveyed in 2002, and the ban was reintroduced. The taking or killing of mantas in Mexican waters was prohibited in 2007. This ban may not be strictly enforced, but laws are being more rigidly applied at Isla Holbox, an island off the Yucatán Peninsula, where manta rays are used to attract tourists.
In 2009, Hawaii became the first of the United States to introduce a ban on the killing or capturing of manta rays. Previously, there was no fishery for mantas in the state, but migratory fish that pass the islands are now protected. In 2010, Ecuador introduced a law prohibiting all fishing for manta and other rays, their retention as bycatch, and their sale.
Relation to humans
The ancient Peruvian Moche people worshipped the sea and its animals. Their art often depicts manta rays. Historically, mantas were feared for their size and power. Sailors believed that they ate fish and could sink boats by pulling on the anchors. This attitude changed around 1978 when divers around the Gulf of California found them to be placid and that they could interact with the animals. Several divers photographed themselves with mantas, including Jaws author Peter Benchley.
Aquariums
Due to their size, it is rare for mantas to be kept in captivity and few aquariums currently display them. One notable individual is "Nandi", a manta ray which was accidentally caught in shark nets off Durban, South Africa, in 2007. Rehabilitated and outgrowing her aquarium at uShaka Marine World, Nandi was moved to the larger Georgia Aquarium in August 2008, where she resides in its 23,848-m3 (6,300,000-US gal) "Ocean Voyager" exhibit. A second manta ray joined that aquarium's collection in September 2009, and a third was added in 2010.
The Atlantis resort on Paradise Island, Bahamas, hosted a manta named "Zeus" which was used as a research subject for three years until it was released in 2008. The Okinawa Churaumi Aquarium also houses manta rays in the "Kuroshio Sea" tank, one of the largest aquarium tanks in the world. The first manta ray birth in captivity took place there in 2007. Although this pup did not survive, the aquarium has since seen the birth of three more manta rays in 2008, 2009, and 2010.
Tourism
Sites at which manta rays congregate attract tourists, and manta viewing generates substantial annual revenue for local communities. Tourist sites exist in the Bahamas, the Cayman Islands, Spain, the Fiji Islands, Thailand, Indonesia, Hawaii, Western Australia and the Maldives. Mantas are popular because of their enormous size and because they are easily habituated to humans. Scuba divers may get a chance to watch mantas visiting cleaning stations and night dives enable viewers to see mantas feeding on plankton attracted by the lights.
Images for kids
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Dorsal view of M. birostris showing shoulder markings
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Manta alfredi with mouth closed, cephalic fins rolled and ventral surface showing distinctive markings
