What is a cartilaginous fish?
Cartilaginous fish are a group consisting of sharks, rays and something called ghost sharks. In total, there are about 800 species living on Earth today. Ordinary fish have a skeleton of bone, but the cartilaginous fish skeleton is made of a softer material called cartilage. The cartilage may be reinforced with calcium.
Cartilage is also found in the human body, for example in the ear, and in the wall between the nostrils. Cartilage is more bendable and mobile than bone, making cartilaginous fish more flexible. In addition, cartilage is very good at regenerating. This means that lost tissues and certain body parts grow back.
There are now over 500 living species of sharks. This makes sharks the largest part of the cartilaginous fish group. This photo shows a blacktip reef shark.
Photo: Kakidai-CC-BY-SA
Rays, like this stingray, are also cartilaginous fish.
Photo: Chris-Taklis-CC-BY-SA
The strange-looking ghost sharks consist of 47 now known species. They are part of the cartilaginous fish group, as are their more famous relatives, sharks and rays.
Cartilage is much softer than bone, which makes it very unusual to find whole shark skeletons - because they break down quickly when the fish have died. The photo shows a preserved skeleton from a kitefin shark.
Photo: Ryan-Somma-CC-BY-SA
This is how the skeleton of a common eagle ray looks!
Photo: David-Stang-CC-BY-SA
Skin teeth and extra teeth
Most cartilaginous fish have a typical head shape with a protruding snout, and the mouth on the underside of the head. Instead of scales, cartilaginous fish have small, sharp, backward-pointing dermal denticles (literally “skin teeth”). Just like normal human teeth, these dermal denticles are made up of enamel, dentine and pulp. The denticles allow the cartilaginous fish to glide silently through the water, surprising their prey.
The teeth in the mouth of cartilaginous fish are gradually replaced. A shark may have 10 rows of teeth lined up in the back of its mouth, and when teeth are lost or break off, new ones take their place. In a single year, a shark can lose as many as 6,000 teeth! That is why shark teeth are a very common find when looking for fossils from ancient seabeds. Many species have triangular teeth, with saw-toothed edges.
Most cartilaginous fish have a protruding nose, and the mouth is located on the underside, just like this thornback ray.
Photo: Johanna-Rylander-Malmö-Museer
Close up of the skin teeth of a small-spotted catshark. The teeth consist of the same material, and are replaced in the same way, as the teeth in its mouth.
Photo: Isurus-CC-BY-SA
A traditional Japanese grater made of shark skin, for preparing wasabi.
Photo: Haragayato-CC-BY-SA
The rows of teeth of a nurse shark, with many extra rows to replace lost teeth with.
Photo: Luca-Oddone-CC-BY-SA
A shark can lose as much as 6,000 teeth in a single year! Therefore, fossils of shark teeth are a common find.
Photo: Luca-Oddone-CC-BY-SA
Jaws and teeth from a stingray. The teeth are adapted to crush the food, as in a grinder.
Photo: Autopilot-CC-BY-SA
Oil-filled liver and electrical organ
Regular fish have a swim bladder, an organ filled with gas. By changing the amount of gas in the swim bladder, the fish can adjust the level at which they float in the water. But cartilaginous fish do not have a swim bladder. Instead, they have a large, oil-filled liver to help with buoyancy.
All cartilaginous fish have a super-sensitive sense of smell, and can detect the scent of prey from very far away. They also have an electrical sense organ, which is the most sensitive of any animal.
The pelvic fins of male cartilaginous fish are partially transformed into reproductive organs, which are located just below the cloaca of this zebra shark.
Photo: Colourbox
Fertilised egg of small-spotted catshark, which is twisted into a tuft of seaweed with the help from its curly threads.
Photo: Ecomare-CC-BY-SA
A newly hatched baby thornback ray, and eggs in the background.
Photo: Ellika-Nordström-Malmö-Museer
The basking shark, the world's second largest fish, actually visits Swedish waters from time to time. Despite its size, it is completely harmless to humans, as it only eats plankton.
Photo: Greg-Skomal-Public-Domain
Prehistoric animals and the world’s second largest fish
Cartilaginous fish have internal fertilisation, where the male mates with the female. The male’s pelvic fins are partially transformed into mating organs, which he inserts into the female’s rear cavity, called the cloaca. Some cartilaginous fish lay eggs, while others keep the eggs in their body until the young hatch, and then give birth to live young.
Many of today’s species of cartilaginous fish evolved during the Devonian period, 400 million years ago, and have not changed significantly since then. The cartilaginous fish were the first of the still living jawed vertebrates to evolve.
The most common cartilaginous fish that live in the waters around Sweden are the spiny dogfish, the small-spotted catshark, the thorny skate and the thornback ray. But the world’s second largest fish – the basking shark – also lives in Swedish waters. It is a giant shark that is completely harmless to humans. It lives its life at great depths, feeding on plankton. There is no risk of meeting a live basking shark while swimming!