Glossary

A mature male smooth skate (Malacoraja senta) with prominent malar spines

A mature male smooth skate (Malacoraja senta) with prominent malar spines

Aggressive mimicry: a form of similarity in which a predator gains an advantage over its prey by its resemblance to a third party, which can be either the prey species itself or a species that the prey does not regard as threatening.

Alar spine: a rearward and downward projecting spine. In male skates, a number of rows of very sharp alar spines develop near the lateral margins of the pectoral fins, which serve to anchor the male firmly against the body and pelvic fin of the female, and are therefore a secondary sexual characteristic. The number and/or state of alar thorns has been used in some studies along with other reproductive organ characteristics to indicate sexual maturity.

Albumen: a clear, jelly-like liquid contained within the egg capsule along with the ovum. Its primary purpose is thought to be protection of the fragile ovum during the initial stages of development within the egg capsule. It may also provide additional nutrition for growth of the embryo, as it is rich in proteins.

Ampullae of Lorenzini: a complex of small, subcutaneous, jelly-filled vesicles innervated by nerve fibers located around the head of elasmobranchs and visible on the surface of the skin as small pores. The ampullae of Lorenzini allow sharks, skates, and rays to detect weak electic fields emitted by fish in distress, and may also respond to mechanical stimuli.

Batoid: a superorder of cartilaginous fishes (superorder: Batoidea) containing more than 500 described species in approximately thirteen families. Batoids include stingrays, skates, electric rays, guitarfishes and sawfishes.

Bucklers: thorns (or large dermal denticles) with a heavy base.

Cephalophoil: the lateral extensions of the head in hammerhead sharks. Hammerheads have been observed preying on rays, using their cephalophoils to both deliver powerful blows to rays and to restrain them on the substrate prior to ingesting them.

Clasper: a posterior extension of the pelvic fins of male elasmobranchs that is inserted into the female during copulation and aids in the transmission of sperm.

Concave: curving in (or hollowed inward).

Convex: curving out (or bulging outward).

Crown: the part of the tooth that is visible above the connective tissue of the jaw. In thorns (or dermal denticles), the crown is the part of the denticle that is covered with enamel, the tip of which is called the cusp.

Crus: a functionally distinct leg-like appendage that is part of the highly-modified anterior lobe of the pelvic fin of some species of skate (such as the little skate). The crus consists of three flexible joints that allow the skate to push (or "punt") off the substrate. Asynchronous movement of the crura also allows the skate to turn sharply and quickly to capture prey, a movement that is not possible with pectoral-fin locomotion.

Cusp: a pointed structure on a tooth or thorn.

Denticle: a small tooth or tooth-like projection.

Elasmobranch: a type of fish that differs from body fish due to its cartilaginous skeleton and the presence of five or more gill slits on each side of the head. In contrast, bony fish (or teleosts) have bony skeletons and a single gill cover called an operculum. Elasmobranchs include sharks, sawfishes, rays and skates.

Epifauna: benthic fauna (or organisms) that live on a surface, such as the sea floor.

Extended oviparity: a mode of egg-laying (or oviparity) where embryonic development occurs almost exclusively outside of the mother, and where development within the egg case is an extremely slow process lasting for periods of up to 15 months.

Follicle: the basic unit of female reproductive biology found in the ovary consisting of a roughly spherical aggregation of cells and containing a single oocyte (aka ovum or egg).

Heterocercal: a type of caudal fin characterized by unequal lobes in which the vertebral column turns upward into the larger lobe. Many sharks have heterocercal (or nonlunate) caudal fins, while others such as mackerel and basking sharks have homocercal (or lunate fins). Skates and rays have neither heterocercal nor homocercal caudal fins. Instead, their caudal fins are either reduced in size or absent.

Infauna: aquatic fauna (or organisms) that live in the substrate of a body of water, especially in the soft sea bottom.

Keratinoid: formed of keratin, a tough, fibrous, insoluble protein.

Malar spine: a buccal spine (aka pertaining to the cheek)

Myxopterygia: see clasper.

Ontogenetic diet: a diet that changes or shifts with the life stage of an organism. Many species of skate have ontogenetic diets, whereby smaller, younger skates feed on different organisms than larger, older, mature skates.

Oogenesis: the process of ovum formation and development in female animals.

Oscillatory propulsion: when batoids swim by flapping their pectoral fins up and down like the wings of a bird. Examples of oscillatory appendage propulsors include eagle and manta rays.

Ostium (ostia): a funnel-shaped opening posterior to the ovary down which the released ovum travels to reach the anterior oviduct. More specifically, ova released into the peritoneal cavity from the ovaries are collected by the ostia and transported to the oviducts to the oviducal (or shell) gland.

Oviduct: a tube through which the ovum passes from the ovary to the uterus. In skates and rays, the oviducts are differentiated into an anterior and a posterior oviduct.

Oviparity: a reproductive mode involving the production of large, leathery egg cases containing sufficient yolk to nourish embryos through development. Egg cases are deposited on the bottom without further contact from the mother, and after gestation, young emerge from the egg capsules as mini versions of their parents. Animals that exhibit this reproductive strategy are called oviparous.

Oviposition: the process of laying eggs by oviparous animals. In skates, oviposition involves extrusion of the egg capsule and deposition of the capsule on the bottom substrate.

Ovoviviparity (also called aplacental viviparity): the most common mode of reproduction among elasmobranchs , in which eggs hatch in the uterus before the embryos are fully developed, and the embryos continue to grow in the uterus without a placental connection. Nourishment is provided by the yolk sac, which is attached to the embryo via a yolk stalk, both of which are fully absorbed by the embryo prior to birth. Animals that exhibit this reproductive strategy are called ovoviviparous.

Ovum (ova): the female reproductive cell(s).

Placoid: plate-like. In sharks, tiny, overlapping placoid scales are imbedded in the skin and cover most of the body, giving it a sandpaper-like feel. In skates and rays, placoid scales are usually found only as a row or rows of predominant denticles (or thorns) along the midline of the back. The teeth of sharks, skates and rays are also modified placoid scales.

Pricklers (or prickles): small dermal denticles that can cover the entire body of some skates.

Punting: a novel mode of pelvic fin locomotion used by some species of skate (such as the little skate) which is very similar to walking in tetrapods. Punting involves a thrust component, which is carried out by a modification of the posterior pelvic fin (termed a crus), followed by a glide component.

Ray(s): kite- or rhomboid-shaped cartilaginous fishes occurring in subtropical and tropical marine waters throughout the world, with some species occurring in estuarine waters. Most rays are benthic, while some species are pelagic. In contrast to skates - which have fleshy tails - rays usually have slender to whip-like tails, sometimes with one or two stinging spines. Rays are also live-bearing (viviparous), giving rise to a relatively small number of large young.

Root: the part of the tooth that is firmly implanted in the connective tissue of the jaw.

Sexual heterodonty: a type of dentition where the morphology of the teeth differs between the sexes, and often becomes apparent only after sexual maturation. In some species of skates and rays, mature males have sharper, more cusped teeth which are thought to aid in grasping the female during copulation. Females - on the other hand - have blunt, rounded teeth that do not change with sexual maturation.

Sexual homodonty: a type of dentition where the morphology of the teeth is the same between males and females, even after sexual maturation.

Shell gland (also called nidamental gland): a specialized organ that is part of the anterior oviduct of elasmobranchs where 1) fertilization of the ovum occurs and 2) the fertilized ovum is encapsulated in an egg case. Egg capsules range from thin and membranous (in the case of rays) to structurally complex and extremely durable (in the case of skates). In some species of batoid, the shell gland is also where sperm storage occurs.

Skate(s): bottom-dwelling, diamond-shaped, cartilaginous fishes found throughout the world in temperate and polar regions, as well as in deep waters of the tropics. Unlike rays, skates are oviparous, depositing tough, leathery egg cases on the sea floor from which their young hatch. More than 280 species of skate have been discovered, making this family (Family Rajidae) the largest of all the batoids.

Spiracle: a small respiratory opening located behind the eye of elasmobranch fishes that is used in skates and rays in place of the mouth to actively pump water over the gills. In sharks, the spiracle is sometimes used to provide oxygenated blood directly to the eye and brain through a separate blood vessel; however, it is often absent or reduced, especially in fast-swimming sharks.

Tapetum lucidum: a mirror-like reflecting layer located behind the retina in the eye of most elasmobranchs that consists of parallel, platelike cells filled with crystals. The crystals of the tapetum lucidum make the elasmobranch eye extremely efficient in low-light conditions.

Thorn: a large dermal denticle. Although skates lack the stinging barbs characteristic of many rays, they do possess thorns that act as predator deterrents.

Undulatory propulsion: when batoids swim by passing undulatory waves down their pectoral fins from anterior to posterior. Most batoids are undulatory appendage propulsors, including all species of skate.

Urogenital sinus: a hollow, chamber-like organ of the reproductive tracts of both male and female skates and rays. In males, the two sperm sacs unite posteriorly to form the urogenital sinus that ends as the urogenital papilla emptying into the cloaca. In females, the two uteri unite to form the urogenital sinus, which enlarges concomitant with sexual maturation, suggesting it to be a secondary sexual characteristic.

Uterus: a hollow, muscular organ located in the pelvic cavity of the female. In skates and rays, the uterus is an enlargement of the posterior oviduct, and is located posterior to each shell gland. In rays, development of the embryo occurs in the uterus. In skates, egg capsule hardening - or sclerotization - occurs in the uterus lumen.

Vitellogenesis: the formation of the yolk of an egg. Eggs that contain yolk are said to be vitellogenic (or yolked).

Viviparity (also called placental viviparity): the most advanced mode of elasmobranch reproduction, whereby developing embryos in the mother are initially dependent on stored yolk but are later nourished by a direct transfer of nutrients from the mother via a yolk sac placenta. Specifically, tissues of the embryo (or the yolk stalk) come into intimate contact with the uterine lining of the mother, creating the yolk sac placenta. Animals that exhibit this reproductive strategy are called viviparous.