The sea molly is not a fish, but a type of shell-less marine snail called a nudibranch. More specifically, it belongs to a group known as sacoglossan sea slugs, often within the genus Ercolania. These tiny creatures are popular in the saltwater aquarium hobby, which has contributed to their wider recognition.
Physical Appearance and Habitat
The sea molly has a small, slug-like body that measures less than half an inch in length. Its most noticeable features are the leaf-like appendages, called cerata, that cover its back and contain branches of the animal’s digestive gland. The common green coloration of the sea molly is not produced by the slug itself but is derived directly from the algae it consumes. The body is often covered in numerous brownish-red and white spots.
These slugs inhabit shallow, calm tropical marine waters, such as intertidal pools and coastal lagoons. They are most often found living directly on the specific types of macroalgae that make up their diet. This close association with their food source provides both sustenance and camouflage, helping them blend in with their surroundings.
Diet and Behavior
The sea molly is a specialist herbivore. It feeds primarily on certain kinds of filamentous algae, with a notable preference for species like Bryopsis. This sea slug is a slow-moving grazer, deliberately moving along algal filaments to feed. Its feeding mechanism is highly specialized, allowing it to pierce algal cells and suck out the contents.
The sea molly performs a process called kleptoplasty. Through this mechanism, the sea molly incorporates the chloroplasts from the algae it eats into the cells of its own digestive system. These “stolen” chloroplasts, or kleptoplasts, remain functional and continue to photosynthesize, providing the sea slug with an additional source of energy. This relationship allows the slug to survive for periods without actively feeding.
Aquarium Care Considerations
In a home aquarium, the sea molly is primarily used as a member of the “clean-up crew” to manage outbreaks of nuisance algae, especially Bryopsis. However, their highly specialized diet is also the biggest challenge in their care. Once their specific food source is gone, they will starve.
These animals are not a long-term, sustainable solution for algae control but rather a temporary specialist. They are also sensitive to changes in water quality and can be harmed by pump intakes and drains in an aquarium system. Providing a continuous supply of their required algae is necessary for their survival, making them suitable only for experienced aquarists who can meet these specific needs.