Costasiella kuroshimae are shell-less marine opisthobranch gastropod mollusks in the family Costasiellidae.

[4] They have two dark eyes and two rhinophores that emerge from the tops of their heads that look similar to sheep's ears or insect antennae.

Costasiella kuroshimae are capable of a physiological process called kleptoplasty, in which they retain the chloroplasts from the algae they feed on.

[5] Costasiella kuroshimae is a selective feeder of algae from the genus Avrainvillea, from which it sequesters chloroplasts into its own cells, retaining them for short-term photosynthesis.

Even in the absence of active photosynthesis, chloroplasts provide a nutrient storage or "larder" that facilitates the survival of the slug without food for an extended period of time.