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Elasmobranchs

Sharks and rays are elasmobranchs, and differ from true bony fishes by having a cartilaginous skeleton, only parts of which are calcified (e.g., the jaws of a shark). Gill structure—elasmobranch means "plate-gilled"— and other physical features differ between bony fish and sharks and rays, which are considered a more primitive form.

Sharks. There are many species of sharks in Indonesian waters, but those most commonly seen by divers are the reef white-tip shark (Triaenodon obesus), the gray reef shark (Carcharhinus amblyrhynchos), and the reef black-tip shark (C. melanopterus).

The reef white-tip shark grows to 1.7 meters, and is a thin, gray fish with white tips to its dorsal and tail fins. This is the most commonly seen shark on the Indonesian reefs. This small shark can be often be found hiding under overhangs.

The gray reef shark grows up to 2.3 meters, and has a very dark trailing edge to its tail. Although this animal is known to be aggressive in some areas, it is not considered dangerous in Indonesia.