MANTA RAYS

In association with the Hawaii Association for Marine Education and Research, Inc. For more information on this fine organization, visit their web site at www.hamerhawaii.org. Our thanks to Mark Deakos – President, Co-founder, Treasurer, and Chief Scientist.

The manta ray is one of the largest and least known of the fishes. The manta ray, like the sharks, skates and other rays, does not possess a hard bony skeleton or elasmobranches. Although they do not possess a stinger, they are closely related to the stingray family (Dasyatidae). The manta ray, together with nine species of devil rays, make up a subfamily known as Myliobatidae. 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 nine different species based on size, coloration, and location. However, recent genetic studies by Tim Clark, from the University of Hawaii, show that all mantas belong to a single species.

Most rays are bottom feeders. These bottom feeders have their mouth located on their ventral side. They also have 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. Small vestigial teeth exist in the lower jaw. These teeth barely penetrate the skin covering and they are another ancestral feature the manta rays no longer use. 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 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 top side; 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. 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. The unique pattern of black spots is used to identify individuals (see manta research). “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.

Range and Habitat

Manta rays have a circumtropical distribution between 35 degrees north and 35 degrees south latitude worldwide. This is equivalent to waters between South Carolina and Brazil in the Atlantic. Manta rays mostly occupy near-shore waters by coral and rocky reef environments, but occasionally are observed in deep water.

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. Additionally, this behavior may also be an attempt to get rid of parasites, such as Remoras, 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.

Reproduction

Manta rays are ovoviviparous, which means they produce live young 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 they are fully developed and ready to be born. The embryo can reach 50 inches in width and weigh 20 pounds (9 kg) or more.

Gestation is believed to be about 13 months. Adult females give birth to one or two pups, every one to three years. 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.

Mating trains have been observed in which one or more males rapidly chases a female for 20-30 minutes. The male nips the tip of either of the female’s pectoral fins, which impairs her swimming. He then moves to her ventral surface, belly to belly, and inserts one of his claspers into her cloaca. He may hold on to the female’s pectoral fin for several minutes after removing his clasper. Mating trains can contain as many as 20 males following a single female.

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