For such large animals, manta rays are actually planktivores, mainly feeding on tiny organisms called zooplankton, which are often less than a few millimetres in length. They use their cephalic fins, the two rolls of skin on either side of their mouth, to funnel plankton into their mouth as they swim, feasting on tiny creatures such as copepods, mysid shrimps and arrow worms. Mantas have developed several strategies for feeding, depending on the location of their food source in the water column, and are often seen swimming in a coordinated formation that ensure each manta gets more food.
They are also known to make seasonal migrations to take advantage of moving food sources. In a very few special places around the world, mantas been seen in mass feeding event known as a manta cyclone, where tens of mantas gather together in plankton dense water and loop around to form a spiralling column involving as many as 150 individuals. In terms of how much they eat, captive mantas have been recorded eating roughly 12% of their own body weight each week, although this figure is likely to be quite different in wild populations.
Once manta rays have finished feeding, they have often been spotted heading to sites called cleaning stations, where they swoop in and hover over rocky outcrops or coral heads while cleaner fish remove excess food particles and detritus from inside their gills. The cleaner fish, usually a member of the wrasse family, also remove tiny ectoparasites which live and feed on the body of the manta ray, hiding inside mouths, spiracles and gills. Not only this, the cleaner wrasses are known to clean wounds, such as shark bites and propeller cuts, enabling the injuries to heal quicker. Cleaning stations are also common sites where mating behaviour is exhibited.
It is thought that manta rays reach sexual maturity when they are between 15 and 20 years of age, when they begin to exhibit courtship behaviour. The courtship between mantas can last days and is an elaborate process led by the female. Male mantas compete with each other to mate with a female, following her around the reef in a long line known as a mating train. Scientists think that the female tests the males with difficult manoeuvres, which only the fittest males will be able to perform. Once she has selected the strongest male, the two will mate as a pair, belly to belly in the water column; the male will bite down on the left pectoral fin (in almost all pairings) of the female to secure them together.
The gestation period of a manta is approximately 12 months. As mantas are ovoviviparous, the fertilised egg grows inside the female’s uterus until the fetus develop into a fully functioning manta pup, at which point she will give birth to a single, live pup, usually with a disc width of 1.5m. On occasion, pregnant manta rays have been found with two developing embryos inside the uterus (during autopsies), although one pup seems to be the norm. Current research suggests that each female manta will give birth to one pup every 2-5 years. With such a slow rate of reproduction, long gestation period, late maturity age and small litter size manta rays are particularly vulnerable to exploitation.