The Magnificent 8*
Updated: Apr 26, 2021
(*Or 9... depending who you talk to)
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?
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.
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).
Today, extant sharks are divided into 8 (sometimes 9) orders:
Carcharhiniformes (ground sharks)
Heterodontiformes (bullhead sharks)
Hexanchiformes (frilled / cow sharks)
Lamniformes (mackerel sharks)
Orectolobiformes (carpet sharks)
Pristiophoriformes (saw sharks)
Squatiniformes (angel sharks)
Squaliformes (dogfish)
(Echinorhiniformes (bramble sharks))
Some scientists believe that the Echinorhinus sharks should be placed in an order separate from the Squaliformes, but others disagree... They can be an argumentative lot and it is very difficult to know who is right! (Stein et al, 2018).
The Squatiniformes, Squaliformes and Pristiophoriformes are thought to be more closely related to each other compared to the Hexanchiformes, Carcharhiniformes, Lamniformes, Orectolobiformes and Heterodontiformes, as they do not have an an anal fin. Within these orders, the Pristiophoriformes and Squaliformes are more closely related to each other than to the Squatinformes (Stein et al, 2018).
(Potential) Order Echinorhiniformes
DNA analysis has revealed that the bramble (Echinorhinus brucus) and prickly shark (Echinorhinus cookei) might actually be more cloesly related to the Squatiniformes and Pristiophoriformes, than to the Squaliformes where they were orginially placed. Therefore, some taxonomists think they should be placed in a separate order of their own (Stein et al, 2018).
Order Squaliformes
The dogfish sharks have no anal fin, and two dorsal fins which often have spines on them. They have a "nictating membrane" over the eye and 5-7 gill slits. The Squaliformes are very diverse in terms of their size; ranging from 1m dogfish like (Squalus acanthias) to the 7m Greenland shark (Somniosus microcephalus) (Compagno, 2002).
Order Squatiniformes
Angel sharks are particularly distinctive as their dorso-ventrally flattened body which makes them look similar to their ray relatives, whilst they also have a broadened, muscular tail like other sharks. They also have no anal fin. They have five gill slits, two "dorsal fins", large "pelvic" and "pectoral fins", and a longer upper lobe of the "caudal fin". Their eyes and "spiracles" are positioned on the dorsal side of the body (Compagno, 2002).
These are amongst the most endangered species of sharks! To learn more, head over to Fallen Angels.
Order Pristiophoriformes
The saw sharks can be distinguished from all other sharks, due to their elongated snout (aka "rostrum"), with alternating large and small "rostral teeth". They have two dorsal fins, 5-6 gill slits and sensory "barbels". This is a small order, including only eight species (Compagno, 2002).
These sharks are commonly mistaken for sawfish, which are quite similar. To learn how to tell them apart through Sawfish or Sawshark?
Order Hexanchiformes
Within the orders of sharks which do have an anal fin, the Hexanchiformes can be easily identified, as they have 6-7 gills slits and only one dorsal fin, which is positioned far along the back, towards the tail. These sharks also do not have a nictating membrane on the eyes. This is the most primitive order of sharks, which evolved earliest in evolutionary history compared to other extant sharks (Compagno, 2002).
To learn more you can check out The Evolution of Sharks.
Order Heterodontiformes
This very small order of sharks, only including nine species. 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 defence (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).
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. 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 they relatives, you can check out Kissing Cousins.
References
Compagno LJV (2002). Sharks of the World: An annotated and illustrated catalogue of Shark species known to date. FAO Species Catalogue, Rome. 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, DOI: 10.1038/s41559-017-0448-4.
By Sophie A. Maycock for SharkSpeak.