Overall, the evolution of light producing cells (photocytes) is believed to have happened twice in sharks through convergence. Evidence suggests that the bioluminescent properties of the shark, Etmopterus spinax, came about as a mechanism of camouflage. It is thought that luminescence has other functions as well due to camouflage not being a logical explanation for the luminescence on the lateral sides of the shark. Bioluminescence is believed to have only evolved in sharks among the cartilaginous fishes. The function of bioluminescence among sharks has not been fully ascertained.