The Sea Bat (1930)

"Sea Bat" is another name for the poisonous sting rays that trouble swimmers in warmer ocean climes. The story is set upon a tropical island and centers on the sister of a reef diver who was attacked under water by another diver and left to be eaten by an enormous sea bat (in reality the rays eat plankton). The distraught young woman, looking for solace, goes to a recently arrived priest, who, unfortunately is an escaped convict from Devil's Island in disguise. He and the girl's attempts to solve the murder are constantly thwarted until the title creature gets involved and sees that deep sea justice is served.