S3-1.3 Wednesday, Jan. 4 Origins of poecilogony and shifts in larval type in photosynthetic sea slugs: a phylogenetic perspective VENDETTI, J. E. *; KRUG, P. J. ; California State Univ., Los Angeles firstname.lastname@example.org
The Sacoglossa are a clade of specialized algae-eating sea slugs, some of which retain functional chloroplasts in their own tissues for weeks to months. As larvae, sacoglossans vary in developmental mode with many producing lecithotrophic larvae with a brief planktonic period or lecithotrophs that metamorphose before hatching. Larvae can also be planktotrophic and several species are poecilogonous. Interestingly, some taxa make additional maternal investments of nutrient-rich extra-capsular yolk (ECY) to their egg capsules, which influences larval size, independently of egg size. Because of these life-history characters, we have developed the sacoglossa as a model system for studying developmental transitions in molluscs, and specifically to identify factors that favor the evolution of lecithotrophy within or among species. To resolve whether lecithotrophy increases rates of cladogenesis or, alternatively, evolves often and independently, we present a molecular phylogeny of 155 sacoglossan species (of 400 known) based on four genes. Bayesian ancestral character state reconstructions reveal five independent origins of poecilogony and at least 24 origins of lecithotrophy. Bayesian support for correlated models of trait evolution indicates that increased maternal investment in ECY biases a lineage towards evolving lecithotrophy at higher rates. Notably, ECY is the first trait associated with developmental transitions in any invertebrate, which suggests that high levels of per-offspring investment may favor evolutionary shifts in larval type. However, only two of five poecilogonous species have ECY, signifying the likely role of ecological factors in the rapid evolution of lecithotrophy in this clade.