The gastropod Lamellaria mopsicolor utilizes ascidians of the family Didemnidae for food and as an incubatory site for its eggs. Individuals always mimic the colour of their host ascidian. At a study site in south-eastern Brazil, the number of incubatory pouches in each ascidian colony ranged from 3 to 53, and the number of eggs in each pouch ranged from 13 to 164. Larval development inside the colony lasted about six days, and caused the death of the zooids around the incubatory pouch or even a part of the colony. Lamellaria mopsicolor is, therefore, a specialized parasite of ascidians.