The broad stingray has only been confirmed to occur around the Hawaiian Islands, where it is the most abundant stingray. A record of this species from off Taiwan has yet to be verified. Broad stingrays are common in coastal bays with mud or silt bottoms, including Ma'alaea Bay off the island of Maui and Kaneohe Bay off the island of Oahu, but may also be encountered in sandy areas or near coral reefs. It has been reported from depths of, though most are found deeper than.
Description
The broad stingray has a diamond-shaped pectoral fin disc a fourth wider than long, with nearly straight leading margins that converge at an obtuse angle, and curved trailing margins. The tip of the snout is rounded and protrudes past the disc. The mouth is arched and contains five or six papillae on the floor, two of which are in front of the others. The pelvic fins are short and rounded. The whip-like tail is twice or more the length of the disc, and bears a serrated stinging spine on the upper surface near the tail base. A long, narrow fin fold occurs beneath the tail, which eventually becomes a keel that runs all the way to the tail tip. Larger rays have three large, elongated tubercles in the middle of the back; the tail is roughened by small dermal denticles, along with an irregular row of conical tubercles on each side and several large, flattened tubercles in front of the spine. This species is plain olive to brown above and white below. Though rarely found so far west, the similar-looking diamond stingray is the only other nearshore stingray that occurs off Hawaii; it can be distinguished from this species by its tail, which is shorter and has both upper and lower fin folds. The broad stingray can reach across and in weight, though few exceed across.
Biology and ecology
During the day, the broad stingray is relatively inactive and spends much time lying half-buried on the bottom. A tracking study in Kaneohe Bay found that individuals rays roamed over an average area of at night, compared to an average diurnal activity space of, and did not rest consistently in any particular spot. Rays were most active 2 hours after sunset and before sunrise, and were more active in the higher water temperatures of summer than winter. The behavior of this species was not significantly influenced by tides, likely because they inhabit deeper water. The broad stingray feeds mainly on bottom-dwelling crustaceans, while also taking polychaete worms and small bony fishes. It excavates large pits to uncover buried prey, and is often followed by opportunists such as jacks. Foraging rays favor areas close to reef boundaries, where many parrotfish, wrasses, gobies, and other reef fishes shelter at night. Known parasites of this species include the tapeworms Acanthobothrium chengi, Rhinebothrium hawaiiensis, Pterobothrium hawaiiensis, Prochristianella micracantha, and Parachristianella monomegacantha. Like other stingrays, the broad stingray is aplacental viviparous. Kaneohe Bay appears to be a nursery area for this species, where juvenile scalloped hammerheads and it are the dominant predators.
