Spotted eagle ray, Aetobatus narinari
In zoology, ray is the common name for cartilaginous fish comprising the order Rajiformes (or Batoidea), characterized by enlarged and flat pectoral fins continuous with the head, no anal fin, eyes on the dorsal surface, and a narrow tail. The skates comprise a family, Rajidae, within the Rajiformes, and thus can be considered a type of ray. For those taxonomic systems recognizing Batoidea as a superorder, ray may also be used as a term for members of this superorder.
Rays provide various ecological, culinary, and aesthetic values. Ecologically, rays are important in food chains, consuming mollusks (snails, clams, oysters), crustaceans, small fish, and even plankton (in the case of manta rays), while being consumed by sharks, among other predators. Some rays are eaten by people, with the large pectoral fins being edible. Aesthetically, rays with their unique form and undulating movements add to the wonder of nature and are popular attractions in public aquariums.
However, there is concern that certain ray species are vulnerable to overfishing as they are sought for food, particularly given that they tend to be characterized by slow growth, low reproductive rates, and high age at maturity (MCS 2007). The Marine Conservation Society recommends that consumers avoid eating rays unless certain they are one of the smaller ray species (spotted, cuckoo, or starry rays) whose populations are relatively stable and that individuals not be eaten below the size at which the species matures (MCS 2007). On the other hand, there also is concern that overfishing of sharks in some areas has led to an increase in particular ray populations, such as the cownose rays, and as a consequence this might be endangering the scallop populations, which are being eaten by the rays (Schmid 2007).
The Chondrichthyes or "cartilaginous fishes" are jawed fish with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of cartilage rather than bone. They are divided into two subclasses: Elasmobranchii (sharks, rays, and skates) and Holocephali (chimaera, sometimes called ghost sharks).
The Elasmobranchii are sometimes divided into two superorders, Selachimorpha (sharks) and Batoidea or Batidoidimorpha (rays, skates, sawfish). Nelson (1994) notes that there is growing acceptance of the view that sharks and rays together form a monophyletic group (superorder Euselachii), and sharks without rays are a paraphyletic group.
There are diverse taxonomies, and Batoidea is variously considered a superorder (eg., Taxonomicon 2007; Frisk et al. 2005; Systema Naturae 2000 2007; Rocco et al. 2007) or an order (ITIS 2007; Froese and Pauly 2005; Myers et al. 2006). As a superorder, Batoidea typically includes three orders: Rajiformes (true rays), Torpediniformes (electric rays), and Pristiformes (sawfishes). Some recognize additional orders such as Myliobatiformes (eagle rays) and Rhinobatiformes (guitarfishes). On the other hand, Nelson (1994) recognizes the sawfishes, electric rays, and guitarfishes as families under the order Rajiformes.
True rays, including skates, are placed in the order Rajiformes. Members of Rajiformes are characterized by the anterior edge of the pectoral fin, which is greatly enlarged, being attached to the side of the head anterior to the gill openings (Nelson 1994). They also have ventral gill openings, and the eyes and spiracles are on the dorsal surface (Nelson 1994). The body is normally strongly depressed and in most water for breathing is taken in through the spiracle rather than the mouth. In addition, members of Rajiformes lack an anal fin and lack a nictitating membrane with the cornea attached directly to the skin around the eyes (Nelson 1994).
Like sharks, rays are cartilaginous marine fish, which means their skeleton is characterized by rubbery cartilage, which is very light and flexible, rather than bone, as in the bony fishes (class Osteichthyes (such as cod or salmon)). As with other chondrichthyans, rays have jaws, paired fins, paired nostrils, scales, and two-chambered hearts.
Rays also are like sharks in having slot-like body openings called gill slits that lead from the gills. Batoid gill slits lie under the pectoral fins on the underside, whereas a shark's are on the sides of the head. Most rays have a flat, disk-like body, with the exception of the guitarfishes and sawfishes, while most sharks have a streamlined body.
Most rays give birth to live young (ovoviviparous), but the skates in family Rajidae are characterized by eggs that are encased in a horny capsule with four long tips (Nelson 1994).
Most species live on the sea floor, in a variety of geographical regions—many in coastal waters, few live in deep waters. Only a few species, like manta rays, live in the open sea, and only a few live in freshwater. Some rays can live in brackish bays and estuaries. Bottom-dwelling rays breathe by taking water in through the spiracles, rather than through the mouth as most fishes do, and pass it outward through the gills.
Members of Rajiformes tend to have pavementlike teeth (Nelson 1994). The heavy, rounded teeth are useful for crushing the shells of bottom-dwelling species such as snails, clams, oysters, crustaceans, and some fish, depending on the species. Manta rays feed on plankton.
Nelson (1994) recognizes four suborders, 12 families, 62 genera, and about 456 species in Rajiformes. He give the following classification for the order Rajiformes:
- Superorder Pristidae
- Family Pristidae (sawfishes)
- Superorder Torpedinoidei
- Family Torpedinidae (electric rays)
- Family Narcinidae
- Superorder Rajoidei
- Family Rhinidae
- Family Rhinobatidae (guitarfishes)
- Family Rajidae (skates)
- Superorder Myliobatoidei
- Family Plesiobatidae (deepwater stingray)
- Family Hexatrygonidae
- Family Dasyatidae (stingrays)
- Family Urolophidae (round stingrays)
- Family Gymnuridae (butterfly rays)
- Family Myliobatidae (eagle rays)
- Superorder Pristidae
In this classification by Nelson, he recognizes the four orders of Compagno (1973) as suborders. Compagno recognizes Batoidea as a superorder
- Compagno, L. J. V. 1973. Interrelationships of living elasmobranhs. In P. H. Greenwood, R. S. Miles, and C. Patterson, eds. Interrelationships of fishes. J. Linn. Soc. (Zool.) 53 (Suppl. 1): 15-61.
- Frisk, M. G., T. J. Miller, and N. K. Dulvy. 2005. Life histories and vulnerability to exploitation of elasmobranchs: Inferences from elasticity, perturbation and phylogenetic analyses J. Northw. Atl. Fish. Sci. 35: 27-45. Retrieved January 3, 2008.
- Froese, R. and D. Pauly, eds. 2005. Order Rajiformes FishBase'. Retrieved January 3, 2008.
- Integrated Taxonomic Information System (ITIS). 2007. Batoidea Taxonomic Serial No.: 563992 ITIS Report. Retrieved January 3, 2008.
- Marine Conservation Society (MCS). 2007. Fish to avoid: Skates and rays Marine Conservation Society. Retrieved January 3, 2008.
- McEachran, J. D., K. A. Dunn, and T. Miyake. 1996. Interrelationships of the batoid fishes (Chondrichthyes: Batoidea). In M. L. J. Stiassny, L. R. Parenti, and G. David Johnson, Interrelationships of Fishes. San Diego: Academic Press. ISBN 0126709505.
- Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. Rajiformes. The Animal Diversity Web (online). Retrieved December 18, 2007.
- Nelson, J. S. 1994. Fishes of the World. New York: John Wiley and Sons. ISBN 0471547131.
- Rocco, L, I. Liguori, D. Costagliola, et al. 2007. Molecular and karyological aspects of Batoidea (Chondrichthyes, Elasmobranchi) phylogeny Gene 389(1): 80-86. Retrieved January 3, 2008.
- Schmid, R. E. 2007. Shark deaths upset rest of food chain: Shark overfishing may be endangering scallop populations, say scientists Associated Press/ABCNews, May 29, 2007. Retrieved January 3, 2008.
- Systema Naturae 2000. 2007. Superorder Batoidea Systema Naturae 2000. Retrieved January 3, 2008.
- Taxonomicon. 2007. Superorder Batoidea Taxonomicon. Retrieved January 3, 2008.
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