Skates
Skates
Skates are members of the class Chondrichthyes, the cartilaginous fish, the same class that contains sharks, rays, and chimeras. Skates, and their relatives the rays, comprise the order Rajiformes, which
contains 513 species in 63 genera and 20 families. The skate family (Rajidae) is the largest family, encompassing about 250 species.
The many species of skate vary greatly in size. The largest species, the big skate (Raja binoculata ), is found off the Pacific coast of North America, and can grow to 8 ft (2.4 m) in length and weigh more than 200 lb (90 kg) The smallest species, the little skate (R. erinacea ), grows to about 20 in (51 cm) and weighs less than 1 lb (0.4 kg). Also called the hedgehog skate, it is the most common skate off the Atlantic coast of North America.
Skates and rays are unusual among fish because of their flattened shape. The pectoral fins of skates are much larger than those of other fish, and are attached the length of the body, from the head to the posterior. These fins are particularly large in the skates, creating a shelf like effect because they encompass the head. Skates also have an elongated snout.
Skates are common in both tropical and temperate oceans, where they are found at depths ranging from 100-7,000 ft (30-2,135 m) with young animals usually found in shallower water. Curiously, skates are not found in the waters around Hawaii, Polynesia, Micronesia, and northeastern South America.
Skates are primarily bottom dwellers, often burying themselves in bottom sand or mud to deceive potential prey and to avoid predators. In order to breathe while lying on the bottom, skates have two openings on their back called spiracles, immediately behind their eyes. Skates draw water in through the spiracles, which then passes out though the gill slits on their undersides. When skates swim, they undulate their pectoral “wings,” setting up a ripple effect that drives them forward through the water in a graceful manner.
The tail of a skate is shorter than that of its relatives the rays, and is studded with strong, sharp spines. These spines are effective in defense. Spines are also found on the back, and they can create a painful injury if stepped on by an unwary wader. Some species also have an electrical organ in their tail, which is not nearly as powerful as that found in the electric rays. These four-volt organs are thought to play a part in courtship.
Like their relatives the sharks, skates have well-developed lower jaws; the upper jaw is separate from the skull. In many species, the teeth have fused into bony plates that are strong enough to crush the shells of the clams and other shelled mollusks on which the skates feed. Skates also eat fish, octopus, crab, and lobster.
Studies have shown that skates have an excellent electromagnetic sense. They pick up weak electrical signals by means of the ampullae of Lorenzini, delicate organs in the snout. Researchers have noted transient slowdowns in the heartrate when skates have detected voltages as low as 0.01 microvolt—this is the highest electrical sensitivity known among any animals. A small fish, such as a flounder, naturally produces an electrical field greater than 0.01 microvolt, so no matter how well a flounder may be hidden by burial in sand, a skate can detect it.
Skates lay eggs, which are released into the environment in a protective egg case. The rectangular case is leathery and has a long tendril streaming from each corner; the tendrils anchor the case to seaweed or rocks. Sometimes called a mermaid’s purse, the egg case protects the young skates during the six to nine months it takes for them to hatch. Empty cases often wash up on beaches.
Skates are edible, although they are generally considered “trash fish” by American commercial fishers, who usually throw them back. Some fishers prefer to use the flesh from the pectoral wings as bait for lobster traps.
The European, or gray skate (Raja batis ) is an important food species in Europe. Many tons of this 100-lb (45.5-kg) skate are taken each year. Most of the “meat” is cut from the fleshy pectoral fins. The barndoor skate (Raja laevis ) of the northwest Atlantic has become threatened through excessive bycatch in commercial fisheries directed to other species, such as cod and haddock.
Resources
BOOKS
Hamlett, William C., ed. Sharks, Skates and Rays: The Biology of Elasmobranch Fishes. Baltimore: Johns Hopkins University Press, 1999.
Michael, Scott W. Aquarium Sharks and Rays: An Essential Guide to Their Selection, Keeping and Natural History. Neptune, NJ: TFH Publications, 2003.
Michael, Scott W. Reef Sharks and Rays of the World: A Guide to Their Identification, Behavior, and Ecology. Monterey, CA: Sea Challengers, 1993.
Nelson, Joseph S. Fishes of the World. 4th ed. New York: John Wiley & Sons, 2006.
F. C. Nicholson