What is a Ray?
Rays are a type of flattened fish and are closely related to sharks. Rays evolved from sharks. These social animals live in seas all over the world, and even in some estuaries. Rays often congregate in huge groups of up to thousands of individuals, but other rays live alone.
Unlike other fish, rays and sharks have no bones; their skeleton is made of cartilage, which is a tough, fibrous substance, not nearly as hard as bone.
Many rays have spines on their tail which can poison other animals when stung. Some rays have long, whip-like tails, other species have short tails. Some rays have a series of thorns on their body as a defense against predators. The color variation among rays is huge; color even varies from male to female in some species.
Size
Manta birostris is the largest ray. This graceful swimmer is up to 29.5 ft (9 m) wide. Mantas eat microscopic plankton, small fish, and tiny crustaceans. They funnel the food into their mouth while they swim, using two large, flap-like cephalic lobes which extend forward from the eyes.
Rays range in size from just a few inches to over 22 feet wide. The smallest ray is the Short-nose electric ray, which is the size of a pancake; it is only 4 inches (10 cm) across and weighs about 1 pound (0.5 kg). The biggest ray is the manta ray which is over 22 feet (6.7 m) wide and weighs many tons (thousands of pounds).
Most rays are in-between these two extremes. More than half of all ray species are over 20 inches (50 cm) long. In fact, rays are some of the largest fish in the sea!
Body Shapes
The Bluespotted Ribbontail Ray - Taeniura lymna is a very common, timid ray with an oval-shaped disc and two venomous tail spikes toward the tip of its tail. It grows to be 2.25 ft (0.7 m) long.
Rays have a flattened body shape and an elongated tail. The pectoral fins are large and connected to the body to form the ray’s “disc.” The shape of the disc differs from species to species and may be circular, oval, wedge-shaped or triangular. Some body shapes are adapted for living on the sea bed; others are adapted for almost constant swimming.
The ray’s distinctive tail also varies from species to species. It ranges from stubby (on the Shorttailed electric rays) to incredibly long (e.g., over 10 feet (3 m) long on the Whip-like sting rays).
Varieties of Rays
There are about 500 different living species of rays and skates, which are divided into 18 families. These different families of rays are very different in the way they look, live, and hunt. They have different shapes, sizes, color, fins, teeth, habitat, diet, personality, method of reproduction, and other attributes.
Skeleton
Rays have no bones; their
skeleton is made of cartilage.
Rays and sharks are a type of fish that have no bones, only cartilage. Some parts of their skeleton, like their vertebrae, are calcified. Cartilage, a strong fibrous substance, is softer than bone; our nose and ears are made of cartilage.
Even the ray’s skull is flattened.
Rays belong to the group of fishes called Elasmobranchii, which also includes the sharks, skates, and ratfish. The Elasmobranchii are all fish that have no bones, only cartilage.
Diet and Hunting
Rays vary in their diets, but they are all carnivores. They eat fish, crustaceans, mollusks, and worms. Rays mostly hunt on or near the bottom of the ocean.
- Some (like the manta ray, which is the largest ray) are filter feeders, sieving small prey (like microscopic plankton, small fish, and small crustaceans) as they swim almost continuously.
- Some rays are active hunters (of bottom-dwelling animals like mollusks and crustaceans). They find the prey on the ocean floor, dislodge it from the floor, grab it (with the rostral lobes), crush it with the teeth (which are mostly flattened plates), and eat it.
- Electric rays (numbfish) stun their prey with electricity.
Intelligence
Rays have a high ratio of brain weight to body weight; they are probably very intelligent, even smarter than sharks. They are known to be very curious animals, often approaching a diver and simply observing the intruder.
Habitat
Rays live in oceans and seas all over the world, and some even spend part of their life in estuaries. Rays live mostly on or near the sea bed. Different ray species are found in habitats ranging from close to shore to the extreme depths of the ocean (over 10,000 feet = 3,000 m deep).
Swimming
Rays swim very differently than other fish. They are propelled through he water with their powerful, wing-like pectoral fins which ripple and flap. Their large pectoral fins also let them glide trough the water. Some rays (like the Mangrove stingray) can even jump above the water.
Many species of rays are coated with a slimy mucous which reduces the surface tension and drag of the water and increases swimming speed.
Like sharks, rays lack a swim bladder and use their oily liver to maintain buoyancy (other fish use an air-filled bladder to help them float). When a ray stops swimming, it sinks down to the sea bed.
Defenses
Blind Electric Ray - Typhlonarke aysoni can give an electrical shock and has almost useless eyes. This poorly-known ray uses electroreceptors to “see.” This numbfish has one dorsal fin and lives at great depths (200-900 m).
Rays defend themselves from predators in many ways. Some use a whip-like tail to lacerate an enemy, some sting enemies with a poisonous stinging tail, electrical rays give electrical shocks (up to 200 volts), and some have hard, bony spines that puncture their victims. Teeth are not used very much by rays as a defense, although some can bite. Camouflage on the sea bed is probably among their best defenses.
Rays do not normally attack people. There are some rays who have a tail sting that can be deadly.
Reproduction
Some rays are oviparous (laying eggs) while others reproduce via aplacental viviparity (giving birth to live young that develop in the womb without a placenta). All skates are oviparous. Fertilization is always internal.
Rays and skates have a long gestation period and produces relatively few young (compared to other fish). The growth of ray populations, therefore, is slow.
Fossils
The earliest known rays (probably guitarfish) date from the Jurassic period, about 150 million years ago. Since rays have no bones (only cartilage), fossil rays are rare, but their teeth, composed of very hard enamel, fossilize well. Many fossilized Guitarfish teeth and some fossilized spines have been found.
Classification
- Kingdom Animalia
- Phylum Chordata
- Subphylum Vertebrata
- Class Chondrichthyes (cartilaginous fish)
- Subclass Elasmobranchii (upper jaw that is not fused to the braincase, no swim bladder, advanced electroreceptive system, a spiracle, skin with placoid scales, teeth modified placoid scales, and 5-7 separate slit-like gill openings on each side of the body: sharks, skates, and rays)
- Superorder Batoidea - Rays and skates (about 480 species)
- Orders - Rays and skates are divided into the following orders:
- Pristiformes - saw-like snout (e.g., the saw fish)
- Rhinobatiformes - 2 dorsal fins (the first closer to pelvis), no electric organ (e.g., the guitarfish, banjo shark)
- Torpediniformes - 2 dorsal fins (the first closer to pelvis), electric organ (e.g., the electric rays)
- Rajiformes - if 2 dorsal fins (the first closer to tail), pelvic fin divided into two lobes (e.g., skates)
- Myliobatiformes - if 2 dorsal fins (the first closer to tail), pelvic fin with one lobe
- head indistinguishable from disc (e.g., stingrays like the bat ray)
- head partially distinct from body (e.g., eagle and devil rays)