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Chondrichthyes – Sharks and Rays

Chondrichthyes - Shark

By on November 11, 2017

Chondrichthyes

The Chondrichthyes are a class of fish that have a skeleton made up of cartilage rather than bones. It includes sharks, rays and Chimaeras.  Chimaeras (in the sub-group Holocephali), are predominantly a lesser known group associated with deepwater. The more common Sharks and Rays, are collectively called Elasmobranchs.

Chondrithyes have been around on Earth for approximately 435 million years and have remained relatively unchanged since then. To put that into perspective, the Atlantic Ocean has only been around for 135-195 million years! Sharks have been around 3 times as long as The Atlantic Ocean! With that being said, the ancestral Chondrichthyes still had bones, however these were lost over time to reduce weight and improve buoyancy control, which is the main way in which they differ from the bony fish.

Most species do have calcified areas around the vertebrae and skull, to protect vital organs. Depending on lifestyles, some may also have strengthened fin girdles. The fins are lined with stiffened radial struts at the base and a network of rods, called Ceratotrichia, allow greater flexibility along the edges. The Ceratotrichia are made from a protein called Elastin.1

Sharks and rays also lack a swim bladder and so instead, use oils within their liver.  These oils make them slightly buoyant and so the upper lobe of their caudal (tail) fin is generally longer than the lower lobe. This aids in buoyancy control by providing downward momentum.  Another key difference, is that instead of having scales like fish, dermal denticles cover the bodies of Chondrichthyes.  This is similar to having tiny teeth embedded into their skin. The skin is so tough, that in ancient times, it was used as sandpaper in Greece and Persia. Their teeth are buried in their gums rather than being attached to the jaws and their mouth and nostrils are on the underside of their head (instead of the front).1

Chondricthyes

Sharks

Despite the majority of sharks being rather small and harmless, they are often feared by humans, due to popular misconceptions presented in media. On average, there are approximately 10 deaths a year that are attributed to shark attacks, worldwide. More people die from coconuts falling on their heads, ant bites, even horses kill more people, annually.

Sharks are an apex predator, vital to the health of our oceans, yet we are still killing over 100 million sharks each year, predominantly in the Shark Finning Industry.2,3 In fact, one study by Myers et al. (2007), showed that in the South Eastern coast of USA, there has been a significant increase in numbers of Cownose Ray (Rhinoptera bonasus). This has been in response to a decline in populations of large sharks, which normally prey on these rays. As a result, these rays have decimated seagrass beds and contributed to the collapse of the Scallops, Argopecten irradians.1

Sharks inhabit every ocean worldwide, with some species also being present in rivers and estuaries. They are generally considered to be coastal predators, or lurking off the continental slopes, with very few reports of them being found in the abyssal depths. Sharks are stream-lined for hydrodynamic efficiency and are considered to be one of the most successful predators The Earth has seen. They are perfectly adapted to their relative habitats, whether they be benthic bottom dwellers, or oceanic wanderers.1

Most sharks are active hunters, or scavengers, however some are planktonic feeders and even ecto-parasitic, biting off chunks of flesh from larger fish or mammals.

Learn more about the decline in shark populations, worldwide and the shark finning industry, by watching the Sharkwater Movies, by Rob Stewart.

https://www.sharkwater.com

Rays

Whilst sharks have conquered the open ocean, Rays have mastered a bottom-dwelling lifestyle. There are always exceptions to the rule however, with some sharks also adapted to benthic life, there are similarly some rays, like the Manta, which have adapted perfectly to the open ocean. Also like sharks, rays are present worldwide, in all oceans and some rivers and even lakes. There are generally coastal feeders, or found on the continental slopes, with very few records from the abyssal depths.1

Rays have a flattened, disc-shaped body, created by large pectoral fins and the tail is often long and whip-like. Since their mouths and gills are located on the ventral side, large spiracle holes are located just behind either eye to draw in water and prevent the gills from getting clogged with sediment.1

Whilst mainly sifting through the sediment in search of food, rays are capable of taking a fish mid-water. Others are sit-and-wait ambush predators and then there are the pelagic filter-feeders. They are often seen buried in the sand, with just their eyes exposed. This may be a great camouflage, but not good enough to help them hide from predators, such as Hammerhead Sharks.1

Order – Fancy science word used to group organisms with similar characteristics