Evolution is fuelled by variation. Random mutations in the genetic code, lead to variations in morphology, which will either be advantageous, neutral or disastrous. When these changes reduce the “fitness” of an individual, it will be less likely to reproduce and pass on these disadvantageous mutations to its offspring. However, sometimes these changes allow an animal to be more well adapted for its environment; increasing its fitness. Over millions of years, this is how animals change to be so perfectly adapted for their “niche” that it almost seems like they were designed to be there! Sometimes this process of adaptation gives rise to absolutely bizarre morphologies. At a glance some critters may appear completely alien and the sharks are no exception. So what bizarre adaptations can we find in sharks and why have they evolved to be this way?
Alopias pelagicus - the Pelagic Thresher Shark
This shark lives in the open ocean (the “pelagic” environment”) and also occasionally hangs out at seamounts. They are wide-ranging in the Indo-Pacific, but also sporadically in the Sea of Cortez, the Red Sea and Arabian Sea, and off the coasts of South Africa and the Galapagos. “Stomach contents analysis” has shown they feed on “mesopelagic” species - bony fishes and squid which live in the twilight zone between 200 to 1000 m depth. This species is “endothermic” and therefore capable of remarkable bursts of energy; swimming at high speeds and “breaching” out the surface of the water, often repeatedly (Compagno, 1984; Henneman, 2007).
These sharks are commonly around 3 m in total length (from tip of tail to tip of snout)… and look at that absolutely spectacular tail! The upper caudal lobe is extended to encompass around 50% of the sharks total length, tapering to a slender, elegant point. So what is the function of this tail? Pelagic thresher shark tails have evolved to be this long, as they are used during hunting. The whip-like tail can be used to stun prey, which makes finding their dinner much easier and more efficient (Compagno, 1984; Henneman, 2007). Quite a niche adaptation!
Etmopterus granulosus - southern lanternshark
The southern lantern shark is pretty bizarre to look at! These are relatively small sharks; only reaching around 30 cm in total length and their disproportionately large head, with blade-like teeth are used to feed on bony fish, squid and crustaceans. Whilst rarely studied, it is thought these sharks can be found globally on continental shelves and slopes (Compagno, 1984; Henneman, 2007).
What is truly fascinating about these guys, is that they are capable of “bioluminescence”, meaning they produce pigments in their skin which glow in the dark. This is an advantageous adaptation in their deep-sea environment (up to 1,500 m down”) where light does not reach, as it can ward off predators and communicate with “conspecifics” (other lanternsharks). (Compagno, 1984; Henneman, 2007).
Chlamydoselachus anguineus - Frilled Shark
This ludicrous creature can be found globally in temperate seas; in The Atlantic and off the coasts of South Africa, Australia and Japan. The frilled shark can reach a maximum of 2 m total length. Generally, sharks have “revolver dentition”, meaning that many rows of teeth are present in the jaw at once. When teeth at the front are shed, the tooth behind on the conveyor belt will pop up to replace it. This is bizarre in and of itself, but the “homodont dentition” (meaning all the teeth in both upper and lower jaw are the same) of the frilled shark are especially unique amongst sharks. Rather than one row, multiple rows of teeth are active at any one time. In combination with the hook-like shape of the teeth, this makes these sharks perfectly adapted to hold onto the strong, slippery fish that they hunt (Compagno, 1984; Henneman, 2007).
Isistius brasiliensis - Cookiecutter Shark
The cookie cutter shark is very unique, as it lack of fins means it looks more like an eel than a shark. This also makes them quite poor swimmers. These sharks can be found in oceanic tropical environments and they have evolved to look this way because of their hunting strategies… Rather than using strength and speed to take down large prey, the cookiecutter only takes pieces of its prey. It will wait for a tuna, whale, dolphin or other shark to pass, at which point it will lunge and sucker itself against the skin. The circular gape and triangular teeth are perfectly adapted to cut out a crater-shaped piece of flesh, which is how these sharks got their name! (Compagno, 1984; Henneman, 2007)
Orectolobus ornatus - Ornate Wobbegong
Wobbegongs are spectacularly strange and wonderful, and none more so than the ornate wobbegong. They are so named because of the projections of flaps of skin and “nasal barbels” which form a fringe, and their remarkable pigmentation patterns. Wobbegongs have evolved to the “benthic” environment, meaning they are commonly found on the ocean floor where they hunt crustaceans, fish, and octopi (Compagno, 1984; Henneman, 2007).
The ornate wobbegong's colouration and shape mean it is perfectly adapted to be almost invisible when resting on the “substrate” of sand or coral during the day. This is critical to their survival as wobbegongs are themselves prey to larger fish and marine mammals (Compagno, 1984; Henneman, 2007).
Mitsukurina owstoni - Goblin shark
This wonderfully strange animal is often called a ‘living fossil’, as its morphology has remained unchanged for millions of years. The aptly named goblin shark is a left-over from an ancient lineage of sharks (most of which have gone extinct), but their strange form is so well adapted to their environment that they have had no significant pressure to change (Compagno, 1984; Henneman, 2007).
Despite the unusual, flabby body-from, goblin sharks are actually remarkably well adapted to be efficient ambush predators. Unlike many deepsea species, they are capable of altering theie pupil size, meaning they can adapt their vision to different light levels; allowing them to hunt in benthic habitats in both deep and shallower waters. They have even been caught at the incredible depth of 1,300m! Their jaw morphology also makes them formidable. Their jaws are specialised to protrude entirely out of the front of the face, in order to snap prey back into the mouth at incredible speed. This makes them perfectly adapted for hunting their “teleost” fish prey (Compagno, 1984; Henneman, 2007).
Pristiophorus japonicus - Japanese sawshark
The saw sharks (not to be confused with saw fishes), have a very distinctive look. Reaching a maximum size of about 1.5 m, they live in benthic environments of temperate and sub-tropical waters. This unique group of sharks has an extended “rostrum” which is armed with alternating large and small lateral “rostral teeth”, which are grown, shed and replaced throughout life (much like the teeth of other sharks). This feature makes them perfectly adapted to hunting for small fish which hide in sandy substrates (Compagno, 1984; Henneman, 2007).
They locate prey using their sensory “barbels” and electrosense (found in the sensory organs called the “Ampullae of Lorenzini”) and then employ the toothy rostrum to dig or slash to disable their prey. This makes it possible for them to target prey which is both invisible and sheltered (Compagno, 1984; Henneman, 2007).
Sphyrna mokkaran - great hammerhead
The iconic hammerhead sharks are so-called due to their remarkable elongated "cephalofoil" at the front of their face. This feature has not just evolved to make them look super badass, but makes them perfectly evolved for efficient hunting. The great hammerhead can grow to be 6 metres in length and maintaining such an enormous size requires a lot of food. Great hammerheads have quite a generalist feeding strategy; eating other sharks, bony fishes, squid and benthic crustaceans (Compagno, 1984; Henneman, 2007).
They are perfectly adapted to be formidable hunters as they have an especially high concentration of Ampullae of lorenzini, spread across a relatively longer distance across the elongated cephalofoil. This makes hammerheads significantly more sensitive to electrical impulses, so they can detect tiny electrical impulses which are generated during muscle contraction. Basically they can sense their prey's heart beat! It also means that great hammerheads are able to track the Earth's geomagnetic field. Therefore, not only are they able to locate prey which is hidden, but are also capable of navigating effectively over very long-distance migrations (Compagno, 1984; Henneman, 2007).
Squatina australis - Australian angelshark
The angel sharks have a distinctively flattened body and broad pectoral fins, suggesting they are an intermediate step in the evolution between sharks and rays. Yet, this does not mean that angel sharks are not perfectly adapted to their niche. On the contrary, they are so well adapted that they experience little pressure to change (Compagno, 1984; Henneman, 2007).
The Australian angel shark is found in subtropical waters of southern Australia, usually on the continental shelf down to depths of 130 m. Their camouflage pigmentation and their body shape means they are able to submerge themselves in sediment, where they are excellently hidden and can ambush any prey that comes too close (Compagno, 1984; Henneman, 2007).
The posterior portion of their body (towards the tail) retains a muscular appearance more similar to other sharks, which makes angel sharks good swimmers. Angelsharks are also the only types of sharks with a "hypocercal tail"- where the bottom lobe of the tail larger than the upper. This creates lift. Therefore, they are able to take off and actively hunt their fish, crustacean and mollusk prey (Compagno, 1984; Henneman, 2007).
Oxynotus centrina - angular roughshark
Yes, you are seeing this correctly- that is a triangular shark! The unusual shape of the angular roughshark is due to their two large dorsal fins and hump-backed body. These sharks can be found only in the Atlantic (and the Mediterranean), in a strip of ocean running circumglobally from Norway to South Africa. They are a benthic species; preferring continental shelves with muddy substrate, and are usually found from 100 - 600 m depths. Very, very little is known about this enigmatic little shark and why it has evolved this shape is still being investigated (Compagno, 1984; Henneman, 2007).
This highlights how little we know about certain species of sharks- there is always more to learn and more to discover!
If you enjoyed learning about these bizarre adaptations, you can discover more about shark evolution at On the Origin of (Shark) Species and The Magnificent 9.
References
Compagno LJV (1984). Sharks of the world. An annotated and illustrated catalogue of shark species known to date. FAO Fisheries Synopsis, 125. Access online.
Hennemann RM (2007). Sharks and Rays: Elasmobranch Guide of the World, 2nd Edition. IKAN Unterwasserarchiv, Germany.
By Sophie A. Maycock for SharkSpeak
this is a great article .. love the hammerheads