Elasmobranchii is the largest group of cartilaginous fish, containing Sharks (Selachii) and Rays (Batoidea).
Unlike bony fishes, cartilaginous fishes have a skeleton made of cartilage, which is light-weight, flexible and durable, and only about half the normal density of bone.
This lighter-weight skeleton allows cartilaginous fish to swim faster and use less energy than bony fishes.
Sharks and Rays have five to seven pairs of gill clefts which can open independently, rigid dorsal fins and small bony, pointed, tooth-shaped scales on the skin.
This sheath of tiny flat V-shaped scales covering the skin are called denticles. These scales are more like teeth than fish scales, helping sharks and rays to swim quickly through the water and providing protection from parasites.
Elasmobranch teeth are arranged in multiple rows and are replaced continually.
Members of this class don't have swim bladders, but instead their large livers are full of oil to provide buoyancy. The oil-filled liver can be up to 20% of the total animal weight.
All cartilaginous fishes employ internal fertilization and usually lay large, heavy-shelled eggs or give birth to live young.
Requiem Sharks are migratory, live-bearing sharks of warm seas (sometimes of brackish or fresh water) and include the Blacktip Reef Shark which is found around both the Bida Islands and Koh Haa Islands.
The Zebra Shark family contains only one member, the Zebra, or Leopard Shark. Whilst not as common in recent years, there are still a few dive sites where we have a good chance to see these sharks, for example Hin Bida, or Hin Klai. These sharks are often found resting on the sandy bottoms close the the reef edge.
The largest known fish species, the Whale Shark (Rhincodon typus), is a slow-moving, filter-feeding carpet shark. The Whale Shark is the only member of the genus Rhincodon and the only member of the family Rhincodontidae.
The Mobulidae (manta rays and devilfishes) are a small family of rays consisting mostly of large species living in the open ocean rather than on the sea bottom. There are 11 species in this family.