Three More Poecilogonous Sacoglossans: Why Is Egg‐Size Dimorphism Generally Rare Except in One Lineage of Sea Slugs?

Theory and meta‐analyses suggest changes from feeding to non‐feeding larvae in marine animals may be an adaptation to conditions such as ocean productivity or circulation, but testing such predictions is difficult because intraspecific dimorphism in larval type (poecilogony) is rare. In gastropods, known examples are three caenogastropods and six heterobranch sea slugs, including five of ∼400 species in clade Sacoglossa. Here, we describe poecilogony in three more photosynthetic sea slugs from the Western Atlantic: Elysia velutinus , Elysia subornata , and Elysia papillosa . Data on egg and larval shell sizes were collected for both developmental morphs, and molecular analyses confirmed individuals differing in larval type were conspecific. Planktotrophic larvae of E. velutinus were cultured to metamorphosis and compared with lecithotrophic larvae in settlement assays. Poecilogonous taxa expressed planktotrophy in areas of enhanced productivity, but were lecithotrophic in enclosed lagoons or strong, asymmetric current regimes where circulation could limit successful dispersal. A meta‐analysis of egg size and development mode data for 437 heterobranch and 57 calyptraeid species revealed that sacoglossans have unusually small lecithotrophic eggs. We hypothesize that the compressed range of egg sizes explains why poecilogony is 200 times more frequent in Sacoglossa than other gastropod clades. Traits such as suctorial feeding, kleptoplasty, and maternal investment in yolk reserves may offset fitness costs of smaller eggs in Sacoglossa. Our findings highlight new systems for studying transitions in life‐history evolution, including intermediate larval forms and links between the environment and expression of development mode.