Elasmobranchs constitute the subclass Elasmobranchii, comprising about 1,100 species, including sharks, rays, skates, sawfish, and others. Together with the subclass Holocephali (which includes chimaeras – ratfish, and elephantfish), they form the class Chondrichthyes, a group consisting of all cartilaginous fish.
These fishes are distinguished from chimaeras by five to seven pairs of gill clefts, rigid dorsal fins, and tiny placoid scales on their bodies. They lack swim bladders and instead depend on large livers rich in oil for buoyancy.
Unlike the chimaeras, elasmobranchs are characterized by five to seven pairs of gill clefts that open individually to the exterior. They also have rigid dorsal fins that are often equipped with spines, and their rough skin is covered with hard, tooth-like placoid scales, also known as dermal denticles. These scales bear spines and look like curved teeth with a sandpaper-like texture. The spines on the scales point towards the tail and help reduce drag, thereby ensuring efficient swimming underwater.
Unlike in most animals, both the upper and lower jaws of elasmobranchs move, and the upper jaw is not fused to the skull. They exhibit a range of different jaw suspensions, like amphistyly, orbitostyly, hyostyly, and euhyostyly.
They have polyphyodont teeth that are continually replaced throughout their lives and are arranged in a series of parallel rows. Different elasmobranchs have their teeth modified based on the type of diet they consume.
Sharks may have as many as 20,000 teeth in their lifetime. Some of them, like the great white shark (Carcharodon carcharias), have wide, wedge-shaped teeth with jagged edges for tearing prey. Others, like the porbeagle (Lamna nasus), have sharp, knife-like teeth meant to catch slippery fish, while Basking sharks (Cetorhinus maximus) are plankton feeders and thus have greatly reduced teeth.
The term Elasmobranchii was coined by Charles Lucien Bonaparte in 1838. It stems from the Ancient Greek words elasmo, meaning plate, and bránchia, meaning gill, referring to the broad and flattened gills unique to these fish.
His initial definition of elasmobranch was also based on gill architecture, which included the chimaeras. However, during the 20th century, scientists decided that chimaeras should be excluded from Elasmobranchii and moved to a separate subclass, Holocephali.
Currently, more than 1,000 species of elasmobranchs are classified under the broad division Neoselachii, which is further divided into two subdivisions: Selachii or Selachimorpha (modern sharks) and Batoidea (rays, skates, sawfish, and others). Neoselachii belongs to the larger group Euselachii, which also includes the extinct order Hybodontiformes and other extinct families.
However, recent molecular studies suggest that members of Batoidea form a monophyletic superorder within Elasmobranchii and share a common ancestor with selachians or modern sharks.
Phoebodus from the Middle Devonian Period (around 383 million years ago) represents the oldest undisputed crown-group elasmobranch. By the Late Devonian, groups like ctenacanths (order Ctenacanthiformes) and hybodonts (order Hybodontiformes) had already emerged and were thriving well.
During the Carboniferous Period (around 359 to 299 million years ago), some ctenacanths grew up to be as large as 23 ft (7 m) in length, comparable to many modern-day sharks. It was around this time, as well as in the Permian Period, that xenacanths (order Xenacantha) became abundant in both freshwater and marine habitats and continued their diversification into the Triassic Period, albeit with reduced diversity. However, many forms went extinct in the Permian-Triassic Extinction Event.
During the Permian, the hybodonts, too, diversified in both marine and freshwater environments and reached their peak diversity in the Triassic and Early Jurassic Periods. Modern sharks had already appeared by the Triassic but started to diversify only from the Early Jurassic. It was during the Jurassic Period that the skates and rays first appeared.
With time, the hybodonts began to decline and finally perished in the Cretaceous Period.
During mating season, mature sharks typically separate by sex. Males grip females with their teeth and insert their clasper into the female’s cloaca for internal fertilization. Pregnant females then separate from other similarly sized females. Although most sharks are ovoviviparous (eggs hatch within the parent’s body), some, like bull and hammerhead sharks, are viviparous and give birth to live young. When inside the eggs, the young are generally nourished by the yolk sac or even the egg capsule.
In skates, the males seize and bite the pectoral fins of the females, forcing their claspers into the cloaca. They hold the females using the clawlike spines on the dorsal side of their own pectoral fins. After fertilization, the females lay eggs in series (oviparous), usually two eggs at a time.
The young continue to grow, attain sexual maturity and may live for up to 50 to 100 years.
Although they reproduce sexually, asexual parthenogenesis has also been observed in some sharks, like in zebra shark females when the males were removed from their habitat.
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