Elasmobranchii is a subclass of Chondrichthyes, or cartilaginous fish, encompassing sharks, rays, skates, and sawfish. The term Elasmobranch is derived from the Ancient Greek words elasmo- (“plate”) and bránchia (“gill”), and makes reference to the fishes’ broad, flattened gills.
Members of the Elasmobranchii subclass have a skeleton made of cartilage, lack swim bladders and have five to seven pairs of gill clefts that open to the outside, rigid dorsal fins, and small placoid scales. The teeth are arranged in different series; the upper jaw is not fused to the cranium as it is with bony fish, and the lower jaw articulates with the upper.
They lack swim bladders and rely on their large, oil-filled livers to keep them buoyant. Because the livers of deep-sea sharks can make up to 20% of their total weight, these animals are frequently targeted for their oil.
Elasmobranchs are found primarily in marine environments, but several species live in freshwater, accounting for approximately 60 of the 1154 known elasmobranch species. Rather than having a bone skeleton, they have one made of calcified cartilage.
Taxonomy of Elasmobranchs
The subclass Elasmobranchii is classified further into two subdivisions: Selachii – modern sharks, and Batoidea – rays, skates, and sawfish, both of which have worldwide distribution.
They are also divided into two habitat groups: euryhaline elasmobranchs, which are marine species that can survive and reproduce in freshwater environments, and obligated freshwater elasmobranchs, which live entirely in freshwater. There is only one clade in this group: the Potamotrygoninae subfamily. This clade is exclusive to South America’s tropical and subtropical waters and wetlands.
Sharks
Sharks are easily distinguished from skates and rays. The gill slits in selachian sharks are on the sides above and ahead of the pectoral fins, whereas the gills in batoid skates and rays are on the belly – ventral- side, below the pectoral fins and behind where the front end of the pectorals attach to the body.
The subdivision Selachii comprises all modern sharks, as opposed to the many that have gone extinct. It has two superorders: Squalomorphi, and Galeomorphi.
Squalomorphii, also called squalea, or squalean sharks, has five orders. These are:
- Hexanchiformes — frill and cow sharks
- Squaliformes — dogfish, gulper, lantern, sleeper, and kitefin sharks
- Pristiophoriformes — saw sharks
- Squatiniformes — angel sharks
- Echinorhiniformes — bramble sharks
Galeomorphi, also called galea or galean sharks, has four orders. These are:
- Heterodontiformes — bullhead and horn sharks
- Orectolobiformes — carpet, bamboo, Zebra, nurse, and Whale sharks
- Lamniformes — Sand Tiger, Goblin, Megamouth, thresher, Basking, and mackerel sharks
- Carcharhiniformes — cat, hound, requiem, and hammerhead sharks
Skates and Rays
It is not too difficult to distinguish selachian sharks from batoid skates and rays, but distinguishing between rays and skates can be more difficult. Generally speaking, rays give birth to live young while skates typically lay eggs with a horny shell; these egg cases are often referred to as a “mermaid’s purse”.
In skates, the tail is thicker and has a caudal (tail) fin at the end in addition to two small dorsal fins. The long, whip-like tail of rays has barbs (stings) on the top, close to the base.
The Batoidea superorder (skates and rays) contains four orders:
- Myliobatiforme – stingrays, manta rays, eagle rays and butterfly rays
- Torpediniformes – electric rays
- Rhinopristiformes – guitarfish, also called shovelnose rays, wedgefishes, banjo rays, sawfish
- Rajiformes – skates
Other Elasmobranchs
Some elasmobranchs, such as the selachian wobbegongs, angel sharks, and saw sharks, as well as the batoid sawfishes and guitarfishes, are intermediate between the two groups.
Due to their shape, coloring, and propensity for bottom-dwelling, wobbegongs (Orectolobidae), also known as carpet sharks, are genuine selachian sharks. They are severely flattened from top to bottom. However, the pectorals are attached behind the head, the gills are on the sides, and the teeth resemble fangs, which are all typical shark characteristics.
Angel sharks and saw sharks are also dorsoventrally flattened. In academic works, angel sharks are sometimes referred to as ray-like. But an angel shark is a true shark because its large pectoral fins are not fused to its head, its tail is thick and continuous with its body, its teeth are long and pointy, and it bears live young, unlike any skate.
Saw sharks are true selachian sharks with the same basic body shape as sawfishes, which are actually batoid rays.
Elasmobranch Life-History Strategy
Elasmobranchs have been evolving separately for at least 450 million years, and by the Carboniferous period, 358.9 million years ago, they seem to have developed life-history strategies similar to that seen today.
The phrase “life-history strategy” refers to a research paradigm that integrates the investigation of genetics, growth, and reproduction in an ecological context to generate hypotheses about evolutionary changes. Here, “strategy” is viewed as a collection of traits developed by natural selection to address specific ecological challenges.
In the game of life, an animal stakes its offspring against a capricious and unreliable environment. If the animal’s offspring survive to play another round, the animal wins the game. The successful life-history strategy consists of the appropriate tactics (pattern) for winning the game.
This pattern (K-selected), which typically consists of slow growth, large adult size, late reproduction, and the production of a small number of well-formed young, differs greatly from that found in the other great class of fish-like vertebrates, the teleosts.
Teleosts have a general pattern of rapid growth, a relatively short life cycle, and many fragile offspring. Elasmobranchs have evolved a very similar strategy to that of marine reptiles and mammals.
The life cycle strategy of elasmobranchs makes skates, sharks and rays highly susceptible to overfishing. It is not a coincidence that marine turtles and baleen whales, whose life histories resemble those of sharks, are also in peril.
References:
- Benton, M. J. (2005). Vertebrate Palaeontology (3rd ed.). Blackwell.
- Ebert, David A.; Fowler, Sarah; Dando, Marc (2021). Sharks of the world: a complete guide. Princeton: Princeton University Press. ISBN 978-0-691-20599-1
- J. H. L. Pratt, S. H. Gruber and T. Taniuchi (eds.) Elasmobranchs as living resources: advances in the biology, ecology, systematics and the status of the fisheries, US Department of Commerce, (1990) NOAA technical report NMFS 90
- McEachran, J.D.; Aschliman, N. (2004). Phylogeny of batoidea. In Carrier, J.C.; Musick, J.A.; Heithaus, M.R. (eds.). Biology of sharks and their relatives. Boca Raton, Florida: CRC Press
- Nelson, J. S. 2006. Fishes of the World. 4th ed. Hoboken, NJ: John Wiley.
- Pough, F. Harvey; Janis, Christine M. (2018). Vertebrate Life, 10th Edition. Oxford University Press. ISBN 9781605357218
- Stearns, S. C. 1976. Life-history tactics: a review of the ideas. Quart. Rev. BioI. 51 :3-47
- Vanden Berghe, Edward (2013). Rajiformes. WoRMS. World Register of Marine Species. Accessed Feb. 24, 2023