The family Spondylidae contains 66 species in one genus - Spondylus. They are mostly shallow water inhabitants of tropical and sub-tropical oceans but there are some deep sea species. Unusually amongst the bivalves, the two valves articulate with a ball and socket joint rather than the hinge and ligament arrangement found in the other bivalves. Like the true oysters, the spiny oysters cement themselves to their substrate. Spiny oysters possess many eyes and have an enlarged portion of their "brain" for optical processing. On many occasions I have only noticed a spiny oyster by the movement of the valves as the shell closes. They are very difficult to photograph open because their eyes are so effective. When they do close, it is sometimes possible to feel a jet of water from the animal as the valves shut.
Spiny oysters can produce pearls. Some Mediterranean species are eaten but tropical Pacific species accumulate saxitoxin which not just makes them inedible but potentially lethal. Some species in the genus are referred to as thorny oysters.
Previously, classification to species used to depend on the shells which are highly variable. As a result many species have subsequently been synonymised after other characteristics (including DNA) were used in species' analysis.
Spondylus varians Variable thorny oyster
Spondylus varians Variable thorny oyster mantle detail Raja Ampat West Papua IMG_1475
Spondylus varians Variable thorny oyster mantle detail Raja Ampat West Papua IMG_1934
Spondylus varians Variable thorny oyster, Crackerjack, Great Barrier Reef IMG_8585
Spondylus varius Variable thorny oyster Puerto Galera, Philippines IMG_5438