KINGDOM: Animalia

PHYLUM: Chordata

CLASS: Chondrichthyes

• Cartilaginous skeleton (derived characteristic)
• Jaws
• Respiration through gills
• Internal fertilization
• May lay eggs or give birth
• Acute senses, include lateral lines

Sharks have a unique feeding adaptation, which involves replacing their teeth periodically. This is possible because their teeth are not attached to any skeletal element of the jaw. They replace their teeth by having them ride up on the inner side of the jaw, where they rotate into position as they reach its crest. The exact mechanism by which the teeth move is not fully understood. Shark teeth are so distinct that the form an important taxonomic characteristic.

Larger sharks and rays are suspension feeders that feed on mostly plankton and small fish. The rest are carnivores.

Sharks have sharp vision, but they cannot distinguish between colors. They have such a keen sense of smell that their noses are often termed "living noses". Sharks also have batteries of receptors, which allow them to respond to mechanical forms of energy such as touch, vibration, water currents, sound, and hydrostatic pressure. Sharks also have a sensitive electrosensory system. This system allows them to detect electromagnetic fields. It is believed that this sense aids the animal in their final stages of prey capture.

Sharks have a streamlined body that reduces turbulence. Sharks have special paired pectoral and pelvic fins that provide them with lift in the water. The caudal fins propel them forward. The dorsal fin acts as a stabilizer. They do not have swim bladder, therefore if they were to stop moving they would most likely sink to the bottom of the ocean.

Chondrichthyes had an origin about 400 million years ago and have a common ancestor to the bony fishes. The earliest sharks had terminal mouths and long upper jaws. During the Carboniferous, about 310 million years ago, a Chondrichthyes developed. In the late Paleozoic and during the Mesozoic, many sharks achieved new levels of organization that included:

• Pectoral fin skeleton
• Development of anal fin
• New ventral element of the vertebral column

Then in the lower Jurassic of the Mesozoic, about 170 million years ago, they formed modern hyostylic jaw suspensions, a shortened upper jaw, and well-developed vertebrae.

Compared to other fish, sharks have a unique form of reproduction that is almost mammalian. They have internal fertilization. The male transfers the sperm into the reproductive tract of the female using its claspers on its pelvic fins. Depending on the species of shark, it can either be oviparous (the eggs hatch outside the mother’s body), ovoviviparous (the fertilized egg is retained in the oviduct), or viviparous (the young develops in the uterus and nourished by the mother prior to birth).

Reference:

Campbell, Niel A. Biology 5^th ed. Addison-Wesley Longman, Inc.: Menlo Park, California 1987 pg636-638.

Moss, Sanford A. Sharks: An Introduction for the Amateur Naturalist. Prentice – Hall, Inc., Englewood Cliffs, New

Jersey (1984).