Here are some of the terms commonly associated with Elysia. I have provided links to Wikipedia or the Sea Slug Forum for those wishing to know more.
Chloroplast: Components (organelles) of plants cells that confer the ability to produce energy from sunlight (photosynthesis). Elysia species are able to maintain the chloroplasts from their food plants within their digestive systems, and thereby perform photosynthesis themselves.
Clade: An evolutionary group, consisting of an ancestor and all its descendants.
Diverticulum: A branching of a tube within the body. In Elysia, kleptoplasts are distributed throughout the body via diverticula from the digestive system.
Endosymbiosis: Literally “living together inside.” In Elysia, it refers to the maintenance of chloroplasts inside cells of the digestive diverticula.
Horizontal Transfer: The movement of genetic material between genomes, often of distantly related organisms. This stands in contrast to vertical transfer of genetic material between generations of the same species. Horizontal transfer is well-documented in bacteria (being observed in such phenomena as acquisition of antibiotic resistance), but has been controversial in multicellular oragnisms. Evidence has been produced for horizontal transfer of algal genes for maintenance of kleptoplasts in Elysia (e.g., Pierce et al., 2012), but the proposal remains controversial.
Kleptoplasty: Theft of chloroplasts. The major claim to fame of Elysia species is that they take the chloroplasts from their food plants and sequester them inside their own cells. Experiments clearly demonstrate that kleptoplasts continue to perform photosynthesis, but it remains controversial as to how much this contributes to the slugs’ overall metabolism.
Macroalgae: Macroalgae comprise the marine plants most people refer to as “seaweed.” In contrast to microalgae, which are single celled and microscopic, macroalgae are usually multicellular, highly visible members of their ecological communities. Macroalgae are divided into rhodophytes (red algae), chlorophytes (green algae), and Phaeophyceae (brown algae). With the exception of a Japanese species that feeds on red algae (Trowbridge et al., 2010) and unconfirmed reports of Elysia diomedea feeding on brown algae (Hernández-Almaraz, et al., 2014), most Elysia species feed on green algae in the following genera:
Nudibranch: Sea slugs in the clade Nudibranchia. Literally “naked gill,” referring to the exposed gills on the posterior dorsal surface. Elysia is often referred to as a nudibranch, but is only distantly related. Unlike Sacoglossans like Elysia, which eat plants, nudibranchs feed on animals such as sponges and bryozoans, and are often specialized for a single prey species.
Opisthobranchia: The order within which Elysia (and the family Plakobranchidae) used to reside. The validity of this order is now in question.
Parapodia: The “flaps” on either side of the slug. In Elysia, parapodia are essentially continuous with the main body, giving the slug the shape of a pancake, or perhaps more biologically appropriate, a leaf.
Phagocytosis: Cellular engulfment. In the context of Elysia, the uptake of chloroplasts by cells of the digestive glands.
Photosynthesis: Production of complex organic molecules, i.e., sugars, from sunlight.
Rhinophores: Sensory appendages on the anterior end of a sea slug, often giving the appearance of bunny ears. They are predominantly chemosensory organs, mediating smell and taste.
Sacoglossa: The clade within which Elysia and its relatives have been placed. Sacoglossan slugs are herbivores that suck the sap from their food plants.