Costasiella ocellifera is a small (5–13 mm) species of sea slug, a shell-less marine gastropod mollusk in the family Costasiellidae.

[2][9][10] Costasiella ocellifera inhabits shallow coastal waters and is almost always found on the plant species Avrainvillea nigricans.

[15] In order to protect themselves from these predators, C. ocellifera was able to develop a chemical defense system by using secondary metabolites that come from the algae they feed on, Avrainvillea.

This chemical defense is so effective that in a laboratory test, several slugs were placed into a tank filled with predatory fish (bluehead wrasses), and it was observed that whenever a sea slug was eaten by a fish it was almost immediately spit back out, unharmed.

[7] Costasiella ocellifera is a strictly herbivorous sea slug that feeds almost exclusively on the algae species, Avrainvillea.

[11] By taking part in this process, these animals are able to establish a unique symbiosis with the algal plastids, known as photosymbiosis.

Some sacoglossan species have non-retention (NR), meaning they are unable to incorporate the plastids and photosynthesize.

[7] Lastly there are the sea slugs, such as C. ocellifera, who have long-term retention (LtR), which means they can stay alive from one to several months solely on photosynthesis.

It has been found that C. ocellifera can survive for up to 65 days on photosynthesis alone, but, it is unclear why there are these differences in the retention periods.

Newly hatched sea slugs must feed on algae for a certain amount of time before they are able to photosynthesize.

They reproduce through internal fertilization, and then spawn their eggs in a unique spiral pattern onto the algae that they eat, Avrainvillea nigricans.