Taxonomy is a branch of science which classifies different organisms and studies their relationships to each other. This allows us to look back across evolutionary time to figure out when different species of animals had a common ancestor, and when and how they have adapted. Sharks have been evolving for 420 million years and thousands of species have arisen and gone extinct in that time, but what types of sharks are alive today and how are they related?
Ch Ch Changes
Evolution is driven by random mutations (changes in the genetic code), which confer differences in morphology. If the mutation makes the organism more well adapted to its environment, it is likely that the change will prevail. However, if the mutation makes the animal poorly adapted, it will likely die and the genetic change will die with it.
Family Tree
When taxonomists build "phylogenetic trees" (a diagram which depicts the lines of evolutionary descent of different species) the goal is to divide species (or groups of species) into increasingly small groups based upon similarities in morphology or genetics. The idea is to create a "parsimonious" tree, whereby the minimal amount of divisions are made. This is because we must assume the most simple explanation is probably right. If we build a phylogenetic tree which requires 1,000 different mutations to be true, but we also build another which only requires 100 mutations, it is more likely that the simpler tree, with fewer changes is more close to the truth. What this means is, when you look at a phylogenetic tree, species positioned on arms nearer to each other are more closely related and probably have more similarities (Stein et al, 2018).
Sharks and Skates an Rays, Oh My!
Sharks and their close relatives are classified as: Domain Eukarya (complex, multicellular organisms), Kingdom Animalia (animals), Phylum Chordata (animals with a backbone), Class Chondrichthians (fishes with a cartilagenous skeleton), Sub-Class Elasmobranchii (sharks, skates and rays) (Stein et al, 2018).
Today, extant sharks are then divided into nine different orders:
ground sharks (Carcharhiniformes),
bramble sharks (Echinorhiniformes),
bullhead sharks (Heterodontiformes),
frilled and cow sharks (Hexanchiformes),
mackerel sharks (Lamniformes),
carpet sharks (Orectolobiformes),
dogfish (Squaliformes),
angel sharks (Squatiniformes),
sawsharks (Pristiophoriformes).
The angel sharks, dogfish and sawsharks are thought to be more closely related to each other compared to the ground sharks, mackerel sharks, and frilled and cow sharks, as they do not have an anal fin (Stein et al, 2018).
Order Echinorhiniformes
In the past, scientists organised sharks into only 8 orders. However, more recent DNA analysis has revealed that the bramble shark (Echinorhinus brucus) and prickly shark (Echinorhinus cookei) should be seperated into an order all of their own. We now think they might actually be more cloesly related to the angel sharks and the sawsharks, than to the dogfish, where they were oiginally placed (Stein et al, 2018; Friedlander et al, 2019).
Order Squaliformes
The dogfish sharks have no anal fin, but they do have two dorsal fins which often have spines on them. They have a "nictating membrane" over the eye and five to seven slits. The Squaliformes are very diverse in terms of their size; ranging from 1 metre dogfish like (Squalus acanthias) to the 7 metre long Greenland shark (Somniosus microcephalus) (Compagno, 2002; Stein et al, 2018).
This order includes many species of deep-sea sharks, some of which have evolved to glow in the dark. To learn more, you can check out Twinkle, Twinkle, Little Shark.
This order is made up of six families and includes the viper dogfish (Trigonognathus kabeyai), lantern sharks (Family Etmopteridae) and kitefin sharks (Family Dalatidae) such as the pigfaced shark (Oxynotus centrina) (Compagno, 2002).
Order Squatiniformes
Angel sharks are particularly distinctive as their dorso-ventrally flattened body makes them look a bit like a ray. Yet, angel sharks are indeed true sharks. They have a broadened, muscular tail like other sharks and have no anal fin. They have five gill slits, two "dorsal fins", and large "pelvic" and "pectoral fins". Their eyes and "spiracles" are positioned on the underside of the body. Whilst many sharks have an asymmetrical "caudal fin", the angel sharks are unique because the lower lobe of their tail is larger than the upper. This gives them extra lift to launch off the seafloor when they ambush their prey (Compagno, 2002; (Stein et al, 2018).
These are amongst the most endangered sharks alive today! Of the 23 different species known (all in the family Squatinidae), 50% are thought to be at risk of extinction (IUCN). To learn more, head over to Fallen Angels.
Order Pristiophoriformes
The saw sharks can be easily distinguished from all other sharks, thanks to their elongated snout (aka "rostrum"), with alternating large and small "rostral teeth". They have two dorsal fins, fiive to sixgill slits and sensory "barbels" hanging down from their snout, which allow them to detect prey. This is a small order, including only eight species grouped into two genera: Pliotrema and Pristiophorus (Compagno, 2002).
These sharks are commonly mistaken for sawfish, which are quite similar. Yet sawfish, whilst looking more like a true shark than sawsharks do, are actually a type of ray. To learn how to tell them apart through Sawfish or Sawshark?
Order Hexanchiformes
The Hexanchiformes are the oldest lineage of extant sharks. They have an anal fin and only one dorsal fin, which is situated far back along their body, a large mouth and small spiracles, located behind their eyes. Yet their most distinctive feature is that they have more than five pairs of gill slits on each side of their body; some have six, others seven. It's thought this feature makes them extra efficient at oxygen absorption (Compagno, 2002).
This order only includes two families and four genera. The bluntnose sixgill (Hexanchus griseus), broadnose sevengill (Notorynchus cepedianus) and the frilled shark (Chlamydoselachus anguineus) are in the order Hexanchiformes (Compagno, 2002).
Order Heterodontiformes
This very small order of sharks, only including nine species, all part of one family. The bullhead sharks are so named for their broad, elaborate brows around bulbous eyes. They are separated from the other orders with an anal fin, by the presence of spines on their two dorsal fins. This is an example of "convergent evolution", as these sharks are not closely related to dogfish, but both orders have developed dorsal spines for defense (Compagno, 2002).
Examples include the Port Jackson shark (Heterodontus portusjacksoni), zebra bullhead (Heterodotus zebra) and horn shark (Heterodontus francisci) (Compagno, 2002).
Order Orectolobiformes
These sharks are commonly called the carpet sharks, due to the beautiful patterns of pigmentation on their skin. Sharks in this order have five gill slits, two spineless dorsal fins and a small mouth that does not extend past the eyes. Many species also have sensory barbels (Compagno, 2002).
This order includes the zebra shark (Stegostoma tigrinum), whale shark (Rhincodon typus), nurse sharks (Family Ginglymostomatidae) and wobbegongs (Family Orectolobidae) (Compagno, 2002).
Order Lamniformes
Whilst they do look quite similar, the mackerel sharks can be distinguished from the Carcharhiniformes, as they do not have nictating eyelids and they can be separated from the Orectolobiformes because their mouth extends behind the eyes (Compagno, 2002).
This order includes the great white (Carcarodon carcharias), megamouth (Megachasma pelagios) basking shark (Cetorhinus maximus), sandtiger (Carcharias taurus) and the goblin shark (Mitsukurina owstoni) (Compagno, 2002).
Sharks in this order have two dorsal fins and five gill slits. This is a relatively large order of sharks, including 12 families, and the species within are quite diverse in form. For example, the threshers sharks (Family Alopiidae) have evolved an exaggeratedly elongated tail (you can learn more at Watch Me Whip). They also vary in terms of physiology. For example, "endothermy" has developed in some families (you can learn more about this at Sharks Packing Heat) (Compagno, 2002).
Order Carcharhiniformes
The final order of sharks, the ground sharks, is the largest of extant orders, including in excess of 270 species! This order is distinguished from its close relative as these sharks have a nictating membrane which protects the eye. Like other closely-related orders, they have five gill slits, two spineless dorsal fins and an anal fin. You would almost certainly recognise some of these sharks, as this order includes the iconic hammerheads (family Sphyrnidae) and the requiem sharks (family Carcharinidae), such as the reef sharks, Carcharhinus perezi and C. amblyrhynchos... which you might have seen in James Bond films (Compagno, 2002).
See! Not all sharks are the same - like the archetypal great whites and reef sharks! In fact, they are remarkably diverse; evolved to fill every "ecological niche" available in our wonderful, enormous oceans!
To learn more about the remarkable diversity of sharks and their relatives, you can check out Kissing Cousins and my other articles about evolution.
References
Compagno LJV (2001). Sharks of the World: An annotated and illustrated catalogue of Shark species known to date. FAO Species Catalogue, Rome. Access online.
Friedlander AM, Giddens J, Ballesteros E, Blum S, Brown EK, Caselle JE, Henning B, Jost C, Salinas-de-León P & Sala E (2019). Marine biodiversity from zero to a thousand meters at Clipperton Atoll (Île de La Passion), Tropical Eastern Pacific. PeerJ 7:e7279. Access online.
Gruber DF, Loew ER, Deheyn DD, Akkaynak D, Gaffney JP, Smith WL, Davis MP, Stern JH, Pieribone VA & Sparks JS (2016). Biofluorescence in catsharks (Scyliorhinidae): Fundamental description and relevance for elasmobranch visual ecology. Scientific Reports, 6:24751. Access online.
Naylor GJ, Ryburn JA, Fedrigo O & Lopez JA (2005). Phylogenetic relationships among the major lineages of modern elasmobranchs. Reproductive biology and phylogeny, 3:1, 25. Access online.
Stein RW, Mull CG, Kuhn TS, Aschliman NC, Davidson LNK, Joy JB, Smith GJ, Dulvy NK & Mooers AO (2018). Global priorities for conserving the evolutionary history of sharks, rays and chimaeras, Nature Ecology & Evolution. Access online.
By Sophie A. Maycock for SharkSpeak
extremely interesting