The world's largest ray can be told apart by a T on its back
The giant oceanic manta ray can span 7 metres across and weigh around 3,000 kilograms. Divers distinguish it from its smaller reef cousin by the pale patches on its shoulders: on the oceanic species the line between them forms a crisp T, while the reef manta's blurry markings form a Y.
Mobula birostris is the biggest ray on Earth, reaching up to 9 metres in length with a disc 7 metres wide, though a typical adult is about 4.5 metres. Until 2017 it sat in the genus Manta alongside the reef manta ray. DNA work then showed both mantas nested among the Mobula devil rays, so the two were moved into Mobula.
It ranges through tropical, subtropical and temperate seas worldwide, from southern California, New Jersey and northern Japan down to Peru, South Africa and New Zealand. Unlike the reef species, it spends most of its life in open water far from land, riding currents and heading for upwellings where nutrient-rich water fuels blooms of zooplankton. It often gathers around remote oceanic islands.
The body is flattened from top to bottom, with large triangular pectoral fins. At the front, a pair of cephalic fins can be rolled into spirals while swimming or unfurled to steer water into the broad, rectangular mouth when feeding. Its teeth sit in 18 rows confined to the middle of the lower jaw. The long, whiplike tail lacks the spine found on devil rays, bearing only a knob at its base. The skin is smooth, black to steely blue above with a pale edge and white below, and individual markings are distinct enough to identify single animals.
Telling the two mantas apart is not always easy, especially with young animals, since size overlaps. Adults of the oceanic species usually measure 4 to 5 metres against 3 to 3.5 for reef mantas. Colour is the surest guide: besides the T-shaped shoulder markings, the oceanic manta's belly spots cluster low on the abdomen, and its cephalic fins, mouth interior and gill slits are often black.
Source: Giant oceanic manta ray
