Biology of the Manta Ray (Manta birostris)
Recognition: a gigantic devil ray with gracefully sculpted, falcate pectoral 'wings', paddle-like cephalic lobes projecting forward from the front of the head (actually extensions of the pectoral fins, supported by radial cartilages), and a very broad, rectangular mouth at the end of the head (a mouth type known as 'terminal'), pebble-like teeth in the lower jaw but not the upper.
Color: variable, ranging from black, grayish blue, to greenish or reddish brown above (black color morph sometimes with whitish, triangular shoulder patches), and nearly black, white with various degrees of grayish blotching (the precise pattern of blotching has been used to identify individuals), to almost pure white below; one albino specimen has been reported.
Size: disc width at birth about 4 feet (1.2 meters); maximum disc width 22 feet (6.7 meters), possibly larger (reported to widths of up to 30 feet or 9.1 meters), maximum recorded weight 3,100 pounds (1,400 kilograms).
Distribution: circumtropical (around the globe, generally between 35 degrees north and south latitude), including: off southeastern South Africa, Madagascar, Mozambique, Tanzania, Kenya, and Somalia; in the Gulf of Aden, Red Sea, Arabian Sea, and the Bay of Bengal, off Burma (Myanmar), Thailand, Malaysia, Indonesia, northern Australia, New Guinea, Solomon Islands, Philippines, Kampuchia, Viet Nam, China, Korea, and southern Japan; off Guam, Palau, Yap, New Caledonia, Fiji, Maui, Kona Hawaii; off southern California to northern Peru, North Carolina to southern Brazil, the Azores, Senegal to Liberia.
Habitat: occurs most often in near shore waters (mainly over continental and insular shelves, but occasionally over deep water), near coral and rocky reefs; swim by flapping their large pectoral fins and are usually encountered near the surface or mid-waters of lagoons or seaward reefs, particularly near surge channels; in Australian waters, mantas sometimes migrate into temperate waters; they are capable of rapid speed and juveniles sometimes leap well clear of the water, landing with a loud slap (adults drive their bodies only partway out of the water and then fall back into the sea, sometimes performing 2 or 3 of these jumps in succession); often host to one or more disc fishes (especially the remora, Remorina albescens) and sometimes observed in association with dolphins, seabirds, sharks, and other rays (notably the spotted eagle ray, Aetobatus narinari); these rays actively seek the services of cleaner wrasses (Labroides spp.), often hanging over or swimming near "cleaning stations", sometimes several lining up to wait their turn.
Feeding: a gigantic filter-feeder, preying on plank tonic crustaceans and small schooling bony fishes; filtering mechanism consists of transverse plates of pinkish-brown spongy tissue that bridge the gaps between successive gill bars (cartilaginous skeletal elements that support the gills); when feeding, the cephalic lobes are unfurled and spread (directing and sometimes apparently scooping plankton-bearing water into the mouth), the animal often swimming in slow somersaults (vertical loops) that are repeated over and over (this behavior may keep the rays within a patch of particularly rich feeding, and may also concentrate plank tonic prey to facilitate feeding); most often encountered as solitary individuals, but members of this species often aggregate in regions offering predictably rich feeding (such as off the Kona Surf Hotel at night -- with up to 50 individuals clustered in the same general area, but not forming true schools.
Reproduction: ovoviviparous, with each of the pups wrapped in a thin-shell that hatches inside the mother, later to be born alive; one specimen harpooned off North Carolina expelled a pup (with one pectoral fin wrapped above its body, and the other wrapped below) in mid-trajectory when it leaped, a behavior which may represent spontaneous abortion (ejected prematurely due to capture stress) or be part of normal birthing (If so, what a dramatic way to enter the world!); litter size 1 to 2 (usually 2) pups, with birthing apparently occur in relatively shallow water, where the young remain for several years before expanding their range farther offshore; mating behavior in this species has recently (July 1997) been filmed off the Ogasawara Islands, Japan, and featured one or more males rapidly chasing a female for 20--30 minutes, after which one male nips the tip of either of the female's pectoral fins (which severely impairs her swimming), moves to her ventral surface, and inserts one of his claspers (paired intermittent organs, developed along the inner margins of the pelvic fins) into her cloacae (vent), thereby mating belly-to-belly; the male may hold onto the (much larger) female's pectoral fin for several minutes after removing his claspers, eventually letting her go free.
Age & Growth: growth of newborns is apparently very rapid, virtually doubling the disc width at birth during the first year of life; males may mature at a disc width of about 13-15 feet (4.0-4.5 meters'), females at a disc width of about 16-18 feet (5.0-5.5 meters'), but this needs confirmation; no data on age at maturity for either sex
Danger to Humans: minimal unless attacked (especially harpooned) or otherwise startled, but the enormous size and power of this ray should invite respect.
Utilization: formerly hunted commercially by harpoon from small boats off eastern Australia and in the Sea of Cortez (Gulf of California), this species is now rarely hunted; places where these rays can be reliably encountered (off the Kona coast of Hawaii and off Yap, to name two of the better-known locations), have spawned substantial snorkel-tourism industries; seemingly inquisitive, individual mantas sometimes approach and even solicit attention from snorkelers, apparently enjoying the tactile stimulation provided by human contact (as well as the bubbles from scuba exhaust); in areas frequented by snorkelers, however, mantas often become very wary of divers and cease to approach them. When approached or grasped rapidly, some individuals roll onto their backs (perhaps to keep the offending snorkeler in sight?), sound (dive), and swim away rapidly, righting themselves only when some distance away; another behavioral response to approach by a diver from the side is curving one pectoral fin toward its back and the other toward its belly -- forming an S-shape in cross-section -- tilting its dorsal (top) surface toward the diver (possibly a defensive response?); in areas where snorkelers often touch mantas, the rays often develop pinkish skin lesions that may persist for months (apparently these lesions occur where human touch has removed the protective mucus that coats the rays' skin). When diving or snorkeling with these magnificent, supremely graceful rays, resist the temptation to touch them and avoid obstructing their looping glide path while they feed.