Elysia diomedea is a species of sea slug found throughout the Pacific coast of Central America.

[1][2] This sea slug resembles a nudibranch but is not closely related to that order of gastropods, instead belonging to a closely related clade, Sacoglossa, the "sap-sucking" sea slugs.

They are not joined at the front and when they are spread apart, they reveal blue spots underneath.

Parts of the gut ramify into the foot, causing green patches which are interspersed with translucent white streaks.

The eggs are very tiny, which suggests that they pass through a veliger larval stage as part of the zooplankton before settling on the seabed, undergoing metamorphosis and becoming juvenile crawling slugs.