Elysia pusilla feeds on the calcified green alga Halimeda and incorporates functioning chloroplasts into its body, thus it is known as a solar-powered sea slug.

Its colour and shape both match that of the alga on which it is feeding; on older foliage it is flattened and a mottled pale green, on fresh new growth it is a brighter shade of green and on the cylindrical shoots that sometimes develop, it adopts a more circular cross section.

Elysia pusilla are found exclusively on calcareous green algae, called Halimeda taenicola.

Many of the Elysia species have parapodia, wing-like flaps, that are spread out and flattened to increase surface area and cephalic tentacles.

[7] Located on top of its head are white, club-shaped sensory organs called rhinophores or cephalic tentacles.

As part of the mollusca phylum and under the class Gastropod, Elysia pusilla has a soft body, ventralized foot for moving, radula and long guts used for feeding, and  a mantle used for protection.

In another nudibranch species, such as the aeolids, the cerata contain sacs called cnidosacs that store nematocysts, stinging cells from cnidarians which have been acquired from feeding.

[6] They are found between the depths of 0.5 and 0.9 m.[9] As a member of the class gastropod, Elysia pusilla uses a radula, which is a ribbon of chitinous teeth to feed.

Sea slugs extract the green algae by piercing the cell wall and storing the plastids in their digestive glands.

Elysia pusilla and other sea slugs in this superorder are able to take the plastids from the algae they eat and incorporate it into their living tissue in a process called kleptoplasty.

The chloroplast lines the digestive tract, which allows the organisms to survive for several months solely relying on kleptoplasty.

[12] Specifically, the algae is digested, but the plastids are still intact, allowing it to be incorporated into the host organism.

The proteins synthesized by the sea slugs are moved into the chloroplast, allowing it to survive for long periods in the host cells.

[20] The smaller size of the lecithotrophic larvae in sacoglossans in comparison to other heterobranchs may allow for developmental plasticity in response to the environment.

[21] During mating, both individuals are fertilized and involve the transfer of love darts which includes stabbing and hormonal manipulation.