Animals

Cartilaginous Fish – Sharks and Rays

Cartilaginous fish (Chondrichthyes) are jawed fish that have skeletons that are made out of cartilage, a type of connective tissue that is less rigid than bone.

Within Chondrichthyes, there are two living clades - Elasmobranchii (sharks, rays and sawfish) and Holocephali (chimeras).

A clade is a group of organisms with a common ancestor.

Like all jawed fish, cartilaginous fish are jawed vertebrates (Gnathostomes).

Blue sharkThe first cartilaginous fish that we know of appeared on Earth nearly 450 million years ago.

Physical Characteristics

Although cartilaginous fish do not have any true bones, some parts of their skeletons, particularly their vertebrae, are calcified - hardened by a buildup of calcium salts.

Cartilaginous fish have calcified materials in their teeth, and Elasmobranchii have calcified material in their scales.

Because cartilaginous fish do not have bones, they do not have bone marrow. Their red blood cells are produced in their spleens and in tissues around their gonads (sex organs).

In animals with bony skeletons, red blood cells are produced in bone marrow.

Cartilaginous fish have no ribs. A large cartilaginous fish would not be able to support its own weight out of water.

A cartilaginous fish has a caudal fin (tail fin), one or two dorsal fins (back fins), and an anal fin, which lies on its underside, between its anus and its tail.

Spotted eagle rayIt also has two sets of paired fins - a pair of pectoral fins and a pair of pelvic fins.

The pectoral fins lie on each side of the fish's body. The pelvic fins, which are also known as ventral fins, lie below the pectoral fins, closer to the fish's abdomen.

Unlike many bony fish, cartilaginous fish do not have swim bladders.

A swim bladder is filled with gas. If allows a bony fish to stay afloat while remaining in the same place.

Cartilaginous fish must swim to remain afloat. They use their pectoral fins to provide lift in the water in the same way that birds use their wings to provide lift in the air.

Every cartilaginous fish has a pair of nostrils and a two-chambered heart.

Scales

Sharks, rays and sawfish have scales that are made from the same materials as the teeth of jawed vertebrates  and have the same structure as the teeth of jawed vertebrates.

These scales are known as placoid scales, or dermal denticles.

The word "dermal" refers to skin, and the word "denticle" means "small tooth".

A placoid scale, like the tooth of a jawed vertebrate, consists of a pulp cavity with nerves and blood vessels that is surrounded by dentin - a type of calcified tissue. The outermost layer of the scale (or tooth) consists of enamel.

Chimeras have no scales.

Respiration

Cartilaginous fish breathe through gills.

Each fish has between four and seven pairs of gills.

Some species, particularly those that spend time resting on the bottom of the ocean, are able to pump water in and out of their gills using spiracles, holes on each side of the head behind the eye.

Other species must constantly swim in order to keep water moving through their gills.

Cartilaginous fish do not have operculums.

An operculum is a bony flap that covers the gills of bony fish.

Some bony fish move their operculums in order to change the amount of pressure in their mouths. This helps to force water through the gills, and allows the bony fish to breathe more efficiently.

Reproduction

Cartilaginous fish practice internal fertilization.

Male cartilaginous fish have organs called claspers at the ends of their pelvic fins.

A clasper has a function that is similar to that of a penis.

During mating, the male uses one of his claspers to transfer semen into the cloaca of the female.

A cloaca is an opening that is used for both excretion and reproduction. Both male and female cartilaginous fish have cloacas.

Some species of cartilaginous fish are oviparous - eggs are laid outside the mother's body, while some are viviparous -the young develop inside the body of the mother.

Many species are ovoviviparous. This means that the eggs remain in the mother's body, and the young growing within the eggs are nourished by a yolk.

In some viviparous species of shark, the young growing inside the body are nourished by means of a placenta.

Gases, nutrients and waste pass from the mother's blood to the blood of the young and back via the placenta.