Cartilaginous fish are fish that have skeletons made of cartilage instead of bone. Cartilage is a tough, flexible material, like what you find in your nose or ears. Examples of cartilaginous fish include sharks, rays, and skates.
These fish usually have other features like tough skin with tiny tooth-like structures, and they often have multiple gill slits on the sides of their heads.
The internal skeleton of cartilaginous fish is made up of cartilage only. It does not contain bones. They have two dorsal fins, a caudal fin, an anal fin, and ventral fins. These fins are supported by girdles. There is no swim gland.
Their skin is very rough because they are covered with denticles. They are rough like sandpaper. Hence their skin was used as sand-paper for many years. They have no bone marrow. So the spleen and the special tissue around the gonads produce red blood cells.
Some of them have an organ called leydis that produces red blood cells. The tiger shark, whale shark, and sting-ray belong to this category. The whale shark is a slow-moving filter-feeding shark. They can live up to seventy years. The sharks have tiny teeth. They take the respiratory water through the mouth, extract the oxygen from the water in the gills and pass it out through the gill slits.
Sharks have five-gill slits one spiracle, dermal teeth on the upper body surface, a toothed jaw and an upper jaw while chimeras have one gill opening on each side, tooth plates and a skull with a firmly attached upper jaw.
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Comparison between Cartilaginous Fish and Bony Fish using External Anatomy
You can tell a bony fish from a cartilaginous fish without looking at its skeleton. The way the fish looks on the outside gives you many clues.
1. Scales: Most fish have scales. In sharks, the scales are called dermal denticles. They are tiny tooth-like structures in the skin. They give the shark’s skin a smooth appearance that feels like sandpaper. Bony fish scales are made of bone and look like shingles on a roof.
2. Fins: Fins move, stabilize and sometimes protect the fish. A fish may have paired fins (pectoral and pelvic fins), and unpaired fins (anal, caudal, and dorsal fins). Some fish do not have all of these fins, and their placement shows great variability.
The very flexible fins of most bony fish have visible supporting rays and spines. The skeletal supports of cartilaginous fish fins are not visible, and these fins are fairly stiff.
3. Gills: Oxygen enters the bloodstream at the gills. The gills are feathery structures found along the sides of the head. The gills of a healthy fish are bright red due to a large amount of blood present.
In bony fish the gills are usually covered by a bony plate called an operculum. In sharks, there are five to seven gill slits on the sides of the head which allow water to pass out of the gill cavity.
In summary, the internal skeleton of this fish is made up of cartilage only. It does not contain bones. They have two dorsal fins, a caudal fin, an anal fin, and ventral fins. These fins are supported by girdles. There is no swim gland.
There are two groups of fish in the class that differs in the composition of their bones. These are the bony fish (fish with well-developed hard bones) like tilapia, and catfish and the cartilaginous (fish with soft flexible fibrous bones called cartilage) fish like the shark. The most obvious external anatomical differences between bony and cartilaginous fish are seen in their gills, scales, and fins.
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