The remarkable process of evolution gives rise to a myriad of different body forms, which are suited to every different habitat imaginable. Evolution occurs due to "survival of the fittest", or survival of the best adapted... Over many years a species evolves to become better adapted to its environment, so much so, that a subset of that species might branch off into a whole new species... One species might diverge into two, and into two again and again, until animals which shared a "common ancestor" in the past have adapted so extensively that they look startling different from each other. It might surprise you to learn that whales are more closely related to horses, camels and dogs, than they are to similarly-shaped animals like sharks. Whereas, the relatives of sharks look... well... absolutely nothing like sharks! Allow me to introduce you to the extended and bizarre shark family...
We Are Family
Sharks are part of a group of fish called the "Chondrichthyans" aka the cartilaginous fishes, which includes around 1,200 different species of chimaeras, skates, rays and sharks. It is thought that the chimeras (Subclass Holocephali) diverged around 410 million years ago, and that the sharks (Superorder Selachimorpha) and rays (Superorder Batoidea) subsequently diverged from each other approximately 300 million years ago (Heinicke et al, 2009; Stein et al, 2018). To learn more, you can check out On the Origin of Shark Species.
You are probably wondering how it is possible that scientists know which species are related when we are talking about millions of years of evolution... Well, it is possible because we can look at their genetic material. By sequencing their genome, we can compare between animals and extrapolate that species with similar DNA are more closely related to each other. This allows us to build what is known as a "phylogenetic tree" - a diagram which shows which species evolved along the same lineages (Stein et al, 2018).
When we look at the phylogenetic tree for the Chondrichthyans, the sharks are divided into several different groups known as "orders" (scientists disagree somewhat whether this should be 8 or 9 orders. To learn more you can check out The Magnificent 8. Despite being closely related, their cousins the batoids, look very different! Their bodies are very flattened and their elongated pectoral fins, fused to their head, are often used for swimming, giving them the appearance of flapping wings. Also, their mouth and gills are both situated on the underside of their body (Compagno, 1977; Compagno, 1984).
Flat Sharks
Yet, despite these similarities, even just the within batoids themselves, evolution has given rise to a remarkable amount of diversity! There are at more than 600 different species of batoids, divided amongst four orders known as the "Torpidiniformes" (electric rays), "Rajiformes" (skates), "Rhinopristiformes" (sawfishes, wedgefishes and guitarfishes), and the "Myliobatiformes" (stingrays, butterfly rays, eagle rays and devil rays) (Gaitán-Espitia et al, 2016).
Skates (Order Rajiformes) are different from rays, as their long tail is relatively fleshy and broad. Their pelvic fins are two-lobed, where rays only have one lobe and they often have spines along their backs (Compagno, 1977; Compagno, 1984).
Comparatively, the stingrays (Order Myliobatiformes) have a very slender tail with a stinging barb at the end. Yet even the stingrays' close relatives within the same order are completely different still! For example, the mobula rays (aka devil rays or manta rays) have horn-shaped fins projecting out of their mouths! The species in this group range from having quite rounded to very triangular shaped wings (Compagno, 1977; Compagno, 1984).
On the other hand, the torpedo rays (Torpediniformes) have very rounded bodies, with distinct fins projecting down their broad tails. They are uniquely capable of producing an electrical current to stun their prey (Compagno, 1977; Compagno, 1984).
Shark or Ray?
It can be very challenging to tell the different types of skates and rays apart, and where it gets even more interesting is when we also consider the batoid species which look more like sharks than rays! We find these in the order Rhinopristiformes or Pristiformes (to learn more about this group, you can take a look at The Rhino-Saws) (Gaitán-Espitia et al, 2016; Stein et al, 2018).
For instance, the group of species known commonly as sawfish (Pristis and Anoxypristis species), look remarkably similar to their cousins the saw sharks (Order "Pristiophoriformes"). They have a slender body, indistinguishable from a shark and an extended rostrum, with protruding rostral teeth, very similar to that of the sawsharks. However, they are more closely related to rays than to the sharks (Compagno, 1977; Compagno, 1984). To learn more about how to tell the difference between these species, you can read Sawfish or Sawshark?.
Another example is the Rhinobatoids (aka guitarfish or wedgefish). These species have muscular, long tails, with dorsal fins, like sharks do, whilst also having a flattened head with extended pectoral fins, like the batoids ... so they look like a cross between the two groups. Yet, again, these guys are more closely related to the rays than to sharks (Compagno, 1984).
Just to really confuse you, there are also several different types of sharks which, at a glance, you might think were rays. For instance, the angel sharks (Order "Sqatiniformes") and the wobbegongs (Family "Orectolobidae") have a flattened body, like a ray, but with a strong, muscular shark tail, very similar to the wedgefishes'. However, both these groups are more closely related to other sharks than to the batoids (Compagno, 1977; Compagno, 1984; Stein et al, 2018). To learn more about wobbegongs, you can read A What-a-gong!?.
The myriad of different species and contrasting body forms within the Chondrichthyans is just dizzying and, in my opinion, fantastic! Over millions of years, evolution has put on a truly spectacular show! I do not have the words sufficient to decribe the beauty and wonder of these animals... So I won't try and I'll just plagiarise someone far more eloquent than I ever hope to be; "There are more things in heaven and earth... than are dreamt of in your philosophy". Evolution is amazing!
References
Compagno LJV (1977). Phyletic relationships of living sharks and rays. American Zoologist, 17:303-322. Access online.
Compagno LJV (1984). Sharks of the World: An annotated and illustrated catalogue of Shark species known to date. FAO, Rome. Access online.
Gaitán-Espitia JD, Solano-Iguaran JJ, Tejada Martinez D & Quintero-Galvis J (2016). Mitogenomics of electric rays: Evolutionary considerations within Torpediniformes (Batoidea; Chondrichthyes). Zoological Journal of the Linnean Society, 178. Access online.
Heinicke MP, Naylor GJP & Hedges SB (2009). Cartilaginous fishes (Chondrichthyes). In Hedges SB & Kumar S (Eds). The Timetree of Life. Oxford University Press, 320–327. 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
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