[1] This species is most commonly found in salt marshes, tidal marshes, pools, and shallow creeks, at depths of 0 m to 0.5 m.[1] Adult Elysia chlorotica are usually bright green in color owing to the presence of Vaucheria litorea chloroplasts in the cells of the slug's digestive diverticula.

[2] By taking on the green color from the chloroplasts of the algal cells, the slugs are able to blend in with the sea bed, helping them improve their chances of survival and fitness.

However, they can occasionally appear reddish or greyish in colour, which is thought to depend on the amount of chlorophyll in the branches of the digestive gland throughout the body.

[3] A juvenile, prior to feeding on algae, is brown with red pigment spots due to the absence of chloroplasts.

[4] Elysia chlorotica have a typical elysiid shape with large lateral parapodia which can fold over to enclose the body.

[5] Juvenile slugs are brown with red pigment spots until they feed upon the algae, at which point they become green.

The incorporation of chloroplasts within the cells of Elysia chlorotica allows the slug to capture energy directly from light, as most plants do, through the process of photosynthesis.

They starved six specimens of P. ocellatus for 55 days, keeping two in the dark, treating two with chemicals that inhibited photosynthesis, and providing two with appropriate light.

After the eight-month period, despite the fact that the Elysia chlorotica were less green and more yellowish in colour, the majority of the chloroplasts within the slugs appeared to have remained intact while maintaining their fine structure.

This once again leads to a conservation of energy, as stated earlier, allowing the slugs to focus on more important activities such as mating and avoiding predation.

[13] However, these results have since been called into question, as FISH analysis can be deceptive and cannot prove horizontal gene transfer without comparison to the Elysia cholorotica genome, which the researchers failed to do.

[14] The exact mechanism allowing for the longevity of chloroplasts once captured by Elysia cholorotica despite its lack of active algal nuclear genes remains unknown.

[15] Genomic analysis of Acetabularia acetabulum and Vaucheria litorea, the primary food sources of Elysia timida, has revealed that their chloroplasts produce ftsH, another protein essential for photosystem II repair.

At the end of cleavage, the embryo forms a stereoblastula, meaning a blastula without a clear central cavity.