Gymnuridae - butterfly rays

Rays, as well as sharks, are known as "cartilaginous" fishes, as opposed to true "bony" fishes because their skeleton is made out of cartilage, a soft somewhat elastic tissue, rather than bone. Cartilaginous fishes also differ by having five to seven gill slits whereas bony fishes have only one gill opening usually covered by a bony gill plate.


Rays are dorsoventrally flattened fishes with a mouth and five gill slits on their ventral surface and eyes and spiracles (openings) on the dorsal surface. Their pectoral fins are broad and fused to the sides of their head forming a circular, ovate or rhomboidal disk for a trunk. The tail is distinct from the rest of the body. Caudal and dorsal fins are small or absent. Anal fins are also absent. Rays are usually grouped under the term "batoid" fishes, which include all rays, skates, sawfishes and guitarfishes. Sawfishes and guitar fishes resemble sharks somewhat.


Batoid fishes are primarily marine but may enter brackish and freshwater at times. Only one family is strictly limited to freshwater.


Dasyatidae, the stingrays, are rhomboidal, ovate or nearly circular in disc shape. They have blunt or slightly pointed snouts. No dorsal or caudal fins are present. Pelvic fins have a single lobe. The tail is very slender, tapering with one or more serrated spines at its base. The tail is longer than the disc.


Rhinopteridae, the cownose rays, are large thick rays, rhomboid in shape with a disk broader than long. The head projects in front of the pectoral fins. The eyes and spiracles are on the sides of the head. They have a small dorsal fin at the base of the tail, but no caudal fin. Pelvic fins have a single lobe. Some have a spine at the base of the tail. The tail is slender, whip-like, and longer than the disk.


Rajidae, the skates, are small rays with rhomboidal shaped discs. There are two small dorsal fins and one small caudal fin at the end of the tail. The tail is thick, not whip-like. The pelvic fin is bi-lobed with a deep notch separating the lobes. The tail is slender with skin folds on each side. It is usually shorter than the disc length. There is no large spine at the base of the tail.


Gymnuridae, the butterfly rays, have a rhomboid-shaped disc that is distinctly broader than long. They have no dorsal or caudal fins. The tail is slender and short (shorter than disc) and may a have a serrated spine at the base.


Narcinidae, one of the families of electric rays, have rounded or oval disks, two prominent dorsal fins and a well developed tail fin. They have a pair of kidney-shaped electric organs on their head that are usually not visible from the exterior. These organs can produce a moderate shock (8-37 volts) when the fish is disturbed and contact is made with the organs. They are collectively known as "numbfishes".