Manta Rays

Manta birostris - (Hahalua)

Maximum Width: 30 feet (9.1 m)
Maximum Weight: 3,100 lbs (1,400 kg)
Lifespan: Unknown (over 20 years)

Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Chondrichthyes (cartilaginous fish)
Subclass: Elasmobranchii (sharks, skates and rays)
Superorder: Batoidea (skates and rays)
Order: Myliobatiformes (rays)
Family: Mobulidae (devil rays – cephalic fins)
Subfamily: Myliobatidae (manta and 9 devil ray species)
Genus: Manta
Species: birostris

Description

The manta ray is one of the largest and least known of the fishes. Like the sharks, skates and other rays, mantas do not possess a hard bony skeleton, and are part of a group known as elasmobranches. Although they do not possess a stinger, they are closely related to the stingray family (Dasyatidae). Manta rays, together with nine species of devil rays, (family Mobulidae) make up a subfamily known as Myliobatidae. The Myliobatidae contain five genera feeding on hard prey (mollusks and crustaceans), and two genera feeding on plankton [1]. All devil rays have common cephalic fins (extensions of the pectoral fins) to help funnel food into their mouths. When these fins are furled, they resemble a devil's horns. Manta rays can be further classified into the genus Manta and species birostris. Manta rays worldwide were once thought to comprise of several species based on differences in size, coloration, and location [2-4]. However, more recent consensus is that the genus Manta rays worldwide are a single species [5, 6].

Most rays are bottom feeders. These bottom feeders have their mouth located on their ventral side, and a pair of spiracles on the top of their head, from which they take in water and pump it pass their gills to breathe. Manta rays are unique in that they evolved to take advantage of large abundances of zooplankton that inhabit the open water. Their large, rectangular mouth projects forward instead of downward. The spiracles, although still present, are no longer used. Instead, water enters the manta ray's mouth while they swim, passes over their gills, and provides oxygen to the blood.

A manta ray's internal gill arches can be seen, when the mouth is wide open. Nine to twelve rows of tiny grinding teeth exist in the bottom jaw. These teeth barely penetrate the skin covering and are simply remnants of an evolutionary era when mantas hunted larger prey. There aren't any teeth in the upper jaw.

Mantas have an average wingspan of about 22 feet (6.7 meters), making them one of the largest animals in the ocean. They are 2.2 -2.3 times wider than they are long.
 

The word 'Manta' is Spanish for cloak, referring to their large, blanket shaped bodies. In Hawaiian, they are called hahalua.
 

Manta rays are generally dark brown, grayish blue, or black on their topside; they have pale edges and a white “belly.” Pale shoulder patches and varying color patterns on top as well as dark blotches underneath can vary between individuals. These pattern differences are what researchers use to identify different individuals (see manta research). For example, in Hawaii, the manta rays are either mostly black with white shoulder bars on their back; or, they are mostly white, with patterns of black spots on their belly. “Black mantas,” which are also found in Hawaii, have black bellies containing white spots. Although white mantas in Hawaii appear very different than black mantas, genetically they are more closely related to each other than white mantas from other areas. Black mantas in Hawaii are usually found in deeper waters and tend to be larger than their coastal cousins.

Manta ray pectoral fins have evolved into large triangular wings. These wings are used by the manta ray to propel through the water. Their skin is covered with dermal denticals (small tooth-like structures) just like their shark cousins. A mucus coating covers their skin, creating an important defense against infection. Their long and narrow tail lacks a spine or a stinger, is slightly flattened, and shorter than the width of the animal. A small dorsal fin is located at the base of the tail. The eyes are located laterally just behind their cephalic fins, giving them the ability to see forward and downward very easily. Their ability to see upward and behind their body, however, is poor. The oil content in their liver is high, providing them with buoyancy since they do not have a swim bladder.

The brain to body ratio for manta rays is the largest for all elasmobranches and similar in ratio to that of marsupials and birds. Mantas possess a rete system, a network of blood vessels that surround their braincase, presumed to keep this organ warmer than the surrounding tissue [7]. Some sharks also possess rete systems.
 

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Range and Habitat

Manta rays are found in tropical and subtropical oceans worldwide between 35 degrees north and 35 degrees south latitude. This is equivalent to waters between South Carolina and Brazil in the Atlantic. They are encountered offshore over deep water in areas such as the Caribbean, Gulf Stream of Cuba, Gulf of Mexico, and Bermuda [3], but they often occupy near-shore waters by coral and rocky reef environments. In their tropical range they appear to be year-round residents, but are seasonal, only appearing during the warm season in their subtropical range.

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Behavior

Although manta rays are generally solitary, they can be found in large groups in areas with rich food sources and cleaning stations. They swim by flapping their large pectoral fins and can reach speeds of 15 mph (24 km/hr). Sometimes, manta rays hover motionless while being “cleaned” by cleaner wrasses. Cleaner wrasses remove parasites from the manta ray's skin and mouth. Several Mantas can be observed “lining up” to wait for their grooming turn. Manta rays have been known to dive greater than 720 feet (219 m).

Manta rays are occasionally observed leaping completely out of the water, sometimes one after the other. The purpose of this behavior is not well understood, and may be related to mating displays, giving birth, or may also be an attempt to get rid of parasites, such as Remoras (Remorina albescens), that “hitchhike” on their back.
 

When manta rays are swimming, their cephalic fins are rolled up like spirals. This gives them the appearance of having horns, like other members of the family Mobulidae, which came to be known as the “devil ray” family. Manta rays can be quite curious, especially when they are around divers. When threatened, they will curve one pectoral fin toward the back and the other toward the belly forming an s-shape cross section, while exposing their dorsal surface towards the threat.
 

Although there are historical reports of manta rays making sounds while being killed [8, 9] , elasmobranches lack the traditional structures used by teleosts (bony fish) to generate sounds (i.e. swim bladder and bony skeletal parts) [10]. Except for the cownose ray [11], sound production has not been recorded from elasmobranches [12].
 

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Reproduction

Manta rays are ovoviviparous, which means they produce live young, which are hatched from an egg. The pups, which are wrapped in a thin membranous egg case, will hatch inside the mother's oviduct. The pup feeds on the mother's uterine milk until it is fully developed and ready to be born. Newborns are between 3.61 and 4.27 ft in width (1.10 - 1.30 m) [13], and weigh about 24 pounds (11 kg) [14]. The pectoral fins tend to be folded over onto themselves prior to birth. Once the pup is born, the wings open and measure about three feet (0.9 m) across. Gestation is believed to be about 13 months. Adult females give birth to a single pup, every one to three years. The gestation period is unknown but is somewhere between 1 and 3 years [13, 14]. The manta rays give birth in shallow water, where the young will remain for several years before expanding their range offshore. Pups will double in size during their first year of life [14].

Male manta rays possess a set of claspers on the inner margins of the pelvic fins. Males will reach sexual maturity at a DW greater than 12.47 feet (3.80 m) [15], at which time their claspers have calcified and extended beyond their pelvic fins. Females will reach sexual maturity at DW greater than of 13.54 feet (4.13 m) [3, 15] when they are about five to six years of age [13, 14]. How the age was determined is unclear.
Mating trains have been observed in which one or more males rapidly chase a female for 20-30 minutes [16]. The male nips the tip of the female's

pectoral fin, impairing her swimming, and allowing him to move to her ventral surface, belly to belly, and insert one of his claspers into her cloaca for one to two minutes. He may hold on to the female's pectoral fin for several minutes after removing his clasper, before separating from the female. Mating trains can contain as many as 25 or more males following a single female.

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Food and Foraging

Manta rays are filter feeders, preying on microscopic organisms such as copepods, mysids (small shrimp-like creatures), and the larvae of fish, lobster, and octopus. When they are feeding their cephalic lobes are unfurled, directing passing water and plankton into their mouth. They sometimes swim in repeated summersaults, remaining in a dense patch of plankton. The excess water exits through five pairs of gill slits on their ventral surface and is termed ram-jet feeding. Finger-like projections on the gill arches, known as gill rakers, strain and capture the food. Manta rays can be found in large aggregations in areas with limited nutrient-rich waters found in the tropics. An adult manta ray may feed on 60 pounds (27 kg) of microscopic plankton, fish larvae, copepods, and zooplankton in a single day.

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Status and Conservation

Natural predators for manta rays are mainly large sharks. In Papua New Guinea, killer whales have been observed feeding on them [17]. Manta rays are able to use their size and maneuverability to escape predation.
Although rare, manta rays on occasion entangle themselves in anchor [3], and mooring lines (Hawaii, 2007).

The number of manta rays that exist worldwide is unknown. They used to be hunted commercially for their liver oil and skin. The hunters would kill them by harpooning them from small boats in eastern Australia and the Sea of Cortez (Gulf of California). Today, they continue to be hunted in Mexico, Indonesia, and the Philippines, where their meat is considered a delicacy. Heavy fishing in Mexico has caused manta populations to collapse [13]. Additionally, fishing efforts in the Philippines have been growing in the past decade enabling local markets to expand to national and international market places [18]. There is an increasing concern about the sustainability of such a fishery. Hunters who are economically dependent on the fishery refute any conservation measures to limit the fishery.

A new market has emerged in Asia, creating a demand for dried manta gill rakers to be used in traditional medicines and in the treatment of cancer [19]. This demand has lead to an exponential increase in the Indonesian fishery in just a few years. This fishery is a great threat to the manta ray populations [20]. Also, as a result of over fishing, fishermen have turned to hunting manta rays as an alternative source of income. This has led to a ten-fold increase in manta ray harvesting. An estimated 1,500 manta rays have been taken over a period of six months in Lamakera [21]. There is an increased fear that this harvesting could spread to the Western Pacific. The life history characteristics of manta rays and most elasmobranches (slow growth, delayed ages of sexual maturity, low fecundity, long gestation periods) make them particularly vulnerable to overfishing [22]. Local populations of manta rays in Indonesia and the Philippines have the potential for crashing.

Manta rays have not yet gotten global recognition as a species of concern. The World Conservation Union categorizes them as “Data Deficient” (see details). Being data deficient means that there is an inadequate amount of information to make a direct, or indirect, assessment of their risk of extinction, based on their distribution and/or population status. Manta rays are listed as vulnerable in the Gulf of California, west coast of Mexico, and South China Seas.

Some commercial dive operations have exploited areas where the manta rays can be seen on a regular basis. On these dives, the Mantas can approach within inches of the divers, who can't refrain from touching them. When touched, the protective mucus layer over the skin is removed, and may cause pinkish skin lesions that can remain for months.

Increased data collection efforts worldwide are producing a valuable global database, providing information on manta ray populations, movements, habitat use, and behavior. Further information on the migration patterns, levels of abundance among regions and habitat preferences are necessary for global management and conservation strategies. A better picture of worldwide populations is necessary for conservation efforts both within local communities and on an international level.

While whales, dolphins, and sharks have been the focus of scientific study, little research is being conducted on Mantas. The Pacific Manta Research Group has been using direct observation and photo-identification as the main research tools to monitor Manta populations in the Sea of Cortez, and the Revillagigedo Islands south of Baja. The Manta Sight Project in Australia is collecting sighting records of Mantas for the Great Barrier Reef. The Nature Conservancy and Pfleger Institute of Environmental Research (Pier) are doing a three-year study in Komodo Marine Park, Indonesia. Tourism revenues for the tiny island of Yap in Micronesia have initiated identification efforts by dive operators in this region.  Keller and Wendy Laros, founders of the Manta Pacific Research Foundation in Kona, Hawaii, are working closely with Tim Clark of the University of Hawaii to monitor the movements of a local population of about 140 manta rays. Andrea Marshall (Manta & Whale Shark Research Centre) is studying a population of over 400 manta rays residing along the coast of Mozambique, Africa.  Her doctoral research is uncovering valuable information about manta ray reproduction, movements, and population structure.  
 

Hawaii

Sighting data along the Kona coast have identified about 140 mantas over the last 12 years of research. About 40 of these are re-sighted on a yearly basis. Tim Clark, a researcher with the University of Hawaii, has tagged and tracked many of these animals. It appears that the population within Kona is residential and does not migrate away from the coast. Similar populations seem to exist within Maui's nearshore waters (see Maui's manta rays ).

Manta ray night dive operations off Kona produce over 2.5 million in revenues annually through eco-tourism. In contrast, a single dead manta ray in Indonesia sells for approximately $160 [20], or about $545 in Indonesia [15].

Fortunately, manta rays are not hunted commercially in Hawaii. But as manta populations in Indonesia, the Philippines, and Mexico, continue to be overfished, Hawaii could become the next target for a fishery. Currently, Hawaii has no laws protecting manta rays.

Due to their small populations and very localized movements, a Manta fishery in Hawaii could never be sustained. Hawaii State legislature is considering enacting a law that would protect manta rays and make it illegal to kill, capture, or intentionally injure manta rays in Hawaiian waters. Without proper protection, Hawaii's manta ray populations are at risk of being wiped out by the emergence of a Hawaii fishery.
 

 Literature Cited

1. Summers, A.P., Stiffening the stingray skeleton - an investigation of durophagy in Myliobatid stingrays (Chondrichthyes, Batoidea, Myliobatidae). Journal of Morphology, 2000. 243(2): p. 113-126.

2. Beebe, W. and J. Tee-Van, Manta hamiltoni (Newman). Zoological Journal of the Linnean Society, 1941. 26: p. 274-278.

3. Bigelow, H. and W.C. Schroeder, Fishes of the western North Atlantic. Sawfishes, guitarfishes, skates, rays, and chimaeroids. Mem. Sears Found. Mar. Res., 1953. 1: p. 500-514.

4. Whitley, G.P., The Australian devil ray, Daemomanta alfredi (Krefft), with remarks on the superfamily Mobuloidea (Order Batoidei). Aust. Zool., 1936. 8: p. 164-184.

5. Clark, T.B., Population Structure of Manta birostris (Chondrichthyes: mobulidae) from the Pacific and Atlantic Oceans, in Wildlife and Fisheries Sciences. 2001, Texas A&M University: Galveston, Texas. p. 68.

6. Last, P.R., and J. D. Stevens, Sharks and rays of Australia. 1994, Melbourne, Australia: CSIRO.

7. Alexander, R.L., Evidence of brain-warming in the mobulid rays, Mobula tarapacana and Manta birostris (Chondrichthyes: Elasmobranchii: Batoidea: Myliobatiformes). Zoological Journal of the Linnean Society, 1996. 118(2): p. 151-164.

8. Coles, R.J., Observations on the habits and distribution of certain fishes taken on the coast of North Carolina. Bull. Amer. Mus. Nat. Hist., 1910. 28: p. 337-348.

9. Coles, R.J., Natural history notes on the devil fish, Manta birostris (Walbaum) and Mobula ofersi (Muller). Bull. Amer. Mus. Nat. Hist., 1916. 35: p. 649-657.

10. Marshall, N.B., The biology of sound-producing fishes. Symp. Zool. Soc. Lond., 1962. 7: p. 45-60.

11. Myrberg, A.A.J., Sound communication and interception in fishes., in Hearing and sound communication in fishes., W.N. Tavolgo, A.N. Popper, and R.N. Fay, Editors. 1981, Springer-Verlag: New York, Heidelberg, and Berlin. p. 397-425.

12. Backus, R.H., Hearing in elasmobranchs, in Sharks and survival, P.W. Gilbert, Editor. 1963, D. C. Heath & Co.: Lexington, MA. p. 243-254.

13. Homma, K., et al., Biology of the Manta ray, Manta birostris Walbaum, in the Indo-Pacific, ed. B. Seret and J.Y. Sire. 1999, Paris (France): Societe Francaise d'Ichtyologie.

14. Ginis, L., Manta Ray, in Australian Geographic. 2002. p. 40-64.

15. White, W.T., et al., Data on the bycatch fishery and reproductive biology of mobulid rays (Myliobatiformes) in Indonesia. Fisheries Research, 2006.

16. Yano, K., F. Sato, and T. Takahashi, Observations of mating behavior of the manta ray, Manta birostris, at the Ogasawara Islands, Japan. Ichthyological Research, 1999. 46(3): p. 289-296.

17. Visser, I.N. and F.J. Bonoccorso, New observations and a review of killer whale (Orcinus orca) sightings in Papua New Guinea waters. Aquatic Mammals, 2003. 29(1): p. 150-172.

18. Alava, M.N.R., Fishery and trade of whale sharks and manta rays in the Bohol Sea, Philippines, ed. S.L. Fowler, T.M. Reed, and F.A. Dipper. 2002, Gland (Switzerland): IUCN.

19. Shen, X., F. Jia, and J. Zhou, Anti-tumor effect of the preparation extracted from sea fish Manta birostris. Chinese journal of marine drugs/Zhongguo Haiyang Yaowu, 2001. 20(6): p. 35-39.

20. Dewar, H., Preliminary Report: Manta Harvest in Lamakera. 2002, Report from the Pfleger Institute of Environmental Research and the Nature Conservancy. p. 3.

21. Anon, Fisheries Conservation Crisis in Indonesia: massive destruction of marine mammals, sea turtles and fish reported from trap nets in pelagic migratory channel. 1997.

22. Holden, M.J., Problems in the rational exploitation of elasmobranch populations and some suggested solutions., in Sea fisheries research, F.R.H. Jones, Editor. 1974, Wiley and Sons: New york. p. 117-137.
 

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