Sharks, skates, and rays, which together form a group of about 900-1150 species(1,2) of ocean-dwelling and freshwater-dwelling fish(3,4) called elasmobranchs,(1) are some of the most fascinating creatures of the deep. While they come in many sizes and shapes—from the giant whale shark(5) and the huge manta ray (3) to the dwarf lanternshark(6) and the tiny short-nosed electric ray,(3) and the from the odd-looking hammerhead sharks(4) to the totally bizarre sawfish(3)—all living elasmobranchs share certain key features. First of all, their skeletons are made up of a strong, flexible, and light material called cartilage,(7) rather than bone, making them (along with another fish group called chimaeras(1)) fundamentally different from other fish.(1,3,8) Other important characteristics include their rows of replaceable teeth(2) and the 5-7 gill slits on each side of their body.(3) In addition, although these creatures are ancient—the first elasmobranchs evolved at least 400 million years ago!(1,5,8,9)—they have many highly-developed senses,(4,10) including the amazing ability to perceive tiny changes in electricity around them.(10,11) Sharks, as well as rays and skates (which you can tell apart from sharks by their generally flattened, diamond-shaped bodies(3)), often use this sense for finding prey, as well as for finding their way through the water.(10,11) In at least some elasmobranchs this sense may even be used in various social and mating behaviors.(11) In part thanks to this electric sense, many elasmobranchs are skillful hunters, often serving as the top predators in the food chain and keeping their environments in the proper balance.(9,12,13) These creatures also have an important relationship with humans. Some rays, such as stingrays and electric rays, can cause injury to people.(3,4) And even if you haven’t heard much about the dangers of those fish, you’ve definitely heard of shark attacks, which have given sharks a very dark reputation even though these attacks are actually rare.(4,12,14) In fact, although elasmobranchs can pose dangers to humans, humans pose a much greater danger to them.(13) For over 5000 years shark meat has been eaten by people,(8) and ray meat, skate meat, shark skin, and other elasmobranch products are also sometimes used by humans today.(3,8,13) Overfishing, accidental catching (called “bycatch”),(1,9,13,15,16) higher numbers of people living on the coast, and greater damage to coastal environments(13,15) are all threatening sharks, skates, and rays. And because elasmobranchs generally grow slowly, reproduce late in life, and have only a small number of children, they have trouble recovering from population decline caused by humans.(1,5,8,12,13,15) As a result, many elasmobranchs around the world are endangered.(13,15)
Elasmobranchs lack swim bladders, and maintain buoyancy with oil that they store in their livers. Some deep sea sharks are targeted by fisheries for this liver oil, including the school, gulper and basking sharks (pictured).[1] All three of these species have been assessed by the IUCN as vulnerable due to overfishing.[2][3][4]
From a practical point of view the life-history pattern of elasmobranchs makes this group of animals extremely susceptible to over fishing. It is no coincidence that the commercially exploited marine turtles and baleen whales, which have life-history patterns similar to the sharks, are also in trouble.[6]
Elasmobranchii (/ɪˌlæzməˈbræŋkiaɪ/[7]) is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins and small placoid scales on the skin. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The details of this jaw anatomy vary between species, and help distinguish the different elasmobranch clades. The pelvic fins in males are modified to create claspers for the transfer of sperm. There is no swim bladder; instead, these fish maintain buoyancy with large livers rich in oil.
The definition of the clade is unclear with respect to fossil chondrichthyans. It has been used by different authors as equivalent to Neoselachii (the clade including modern sharks and rays and their last common ancestor) or for all chondrichthyans more closely related to modern sharks and rays than to Holocephali (the clade containing chimaeras and their extinct relatives).[8]
The earliest elasmobranch fossils came from the Devonian and many surviving orders date back to the Cretaceous, or even earlier. Many species became extinct during the Permian and there was a burst of adaptive radiation during the Jurassic.
The name Elasmobranchii comes from the Ancient Greek words elasmo- ("plate") and bránchia ("gill"), referring to the broad, flattened gills which are characteristic of these fishes.
Elasmobranchii is one of the two subclasses of cartilaginous fish in the class Chondrichthyes, the other being Holocephali (chimaeras).
Members of the elasmobranchii subclass have no swim bladders, five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins, and small placoid scales. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper.
Extant elasmobranchs exhibit several archetypal jaw suspensions: amphistyly, orbitostyly, hyostyly, and euhyostyly. In amphistyly, the palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. In orbitostyly, the orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support.
In contrast, hyostyly involves an ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. Finally, in euhyostyly, also known as true hyostyly, the mandibular cartilages lack a ligamentous connection to the cranium. Instead, the hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid.[9][10][11] The eyes have a tapetum lucidum. The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm. These fish are widely distributed in tropical and temperate waters.[12]
Many fish maintain buoyancy with swim bladders. However elasmobranchs lack swim bladders, and maintain buoyancy instead with large livers that are full of oil.[13] This stored oil may also function as a nutrient when food is scarce.[6][14] Deep sea sharks are usually targeted for their oil, because the livers of these species can weigh up to 20% of their total weight.[1]
Fossilised shark teeth are known from the early Devonian, around 400 million years ago. During the following Carboniferous period, the sharks underwent a period of diversification, with many new forms evolving. Many of these became extinct during the Permian, but the remaining sharks underwent a second burst of adaptive radiation during the Jurassic, around which time the skates and rays first appeared. Many surviving orders of elasmobranch date back to the Cretaceous, or earlier.[15]
Elasmobranchs are mostly a marine taxon, but we know several species that live in freshwater environment (approximately 60 species which represent only 5% of the 1154 described species). They can be divided into two groups: The euryhaline elasmobranchs, which are marine species that may survive and reproduce in freshwater environments, and the obligated freshwater elasmobranchs, which only lives in freshwater environment their entire life. This group contains only one clade: the subfamily Potamotrygoninae. This clade is endemic (i.e. exclusive) to one specific region: tropical, subtropical water and wetland of South America.
Recent research in Paraná river[16] have shown that obligated freshwater elasmobranchs were more susceptible to anthropogenic threats as overfishing and destruction of habitats due to the very small areas they live in compared to the marine species.
New research has highlighted the importance of coastal wetlands, like mangroves and seagrasses, as habitats for many species of elasmobranch.[17]
Compagno's 2005 Sharks of the World arranges the class as follows:
Recent molecular studies suggest the Batoidea are not derived selachians as previously thought. Instead, skates and rays are a monophyletic superorder within Elasmobranchii that shares a common ancestor with the selachians.[20][21]
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Elasmobranchs lack swim bladders, and maintain buoyancy with oil that they store in their livers. Some deep sea sharks are targeted by fisheries for this liver oil, including the school, gulper and basking sharks (pictured). All three of these species have been assessed by the IUCN as vulnerable due to overfishing.
Radiation of elasmobranchs, based on Michael Benton, 2005 From a practical point of view the life-history pattern of elasmobranchs makes this group of animals extremely susceptible to over fishing. It is no coincidence that the commercially exploited marine turtles and baleen whales, which have life-history patterns similar to the sharks, are also in trouble.
Elasmobranchii (/ɪˌlæzməˈbræŋkiaɪ/) is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins and small placoid scales on the skin. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The details of this jaw anatomy vary between species, and help distinguish the different elasmobranch clades. The pelvic fins in males are modified to create claspers for the transfer of sperm. There is no swim bladder; instead, these fish maintain buoyancy with large livers rich in oil.
The definition of the clade is unclear with respect to fossil chondrichthyans. It has been used by different authors as equivalent to Neoselachii (the clade including modern sharks and rays and their last common ancestor) or for all chondrichthyans more closely related to modern sharks and rays than to Holocephali (the clade containing chimaeras and their extinct relatives).
The earliest elasmobranch fossils came from the Devonian and many surviving orders date back to the Cretaceous, or even earlier. Many species became extinct during the Permian and there was a burst of adaptive radiation during the Jurassic.
The name Elasmobranchii comes from the Ancient Greek words elasmo- ("plate") and bránchia ("gill"), referring to the broad, flattened gills which are characteristic of these fishes.
