Though Oyster Leeches are called leeches they are not to be mistaken with common blood-sucking leeches. Most live under rocks and oyster shells. They also live in other invertebrates. They are a parasitic organism. This means they feed off a host until the host dies and then move on.Oyster leeches tend to hide under debris or in shells of oysters and barnacles. After mating, which can last nine hours and involve more than four partners, an individual oyster leech can deposit seven thousand to twenty-one thousand eggs. Egg masses usually are attached to clean oyster shells. The worms cover their eggs to protect them. Larvae with six eyes hatch from the eggs. The larvae swim up and away from the bottom.