Meeting Abstract
The nemertean pilidium larva, produced by members of the clade Pilidiophora, is generally known to be planktotrophic and comes in many distinct forms. The typical pilidium is characterized by a large inflated episphere, a tucked-under hyposphere and lobes and lappets along the margin. It resembles a deer-stalker cap and feeds on unicellular algae using a ciliated band spanning its lobes and lappets. Pilidial development epitomizes maximally-indirect development: the juvenile arises from several independent rudiments within the larval body, and emerges during catastrophic metamorphosis, devouring the larval body. Other nemerteans (including the Hoplonemertea and the basal Palaeonemertea) develop more or less directly, producing uniformly ciliated superficially planula-like larvae, which may be non-feeding or macrophagous, i.e. feeding on macroscopic prey, such as other invertebrate larvae. Intriguingly, some pilidiophorans have superficially planula-like or trochophore-like larvae that are lecithotrophic. Recent studies by us and others suggest that at least 17 pilidiophoran species exhibit or are expected to have lecithotrophic development. Despite superficial resemblance to planuliform larvae of other nemerteans, or trochophores of other spiralians, these still develop via imaginal discs and undergo dramatic metamorphosis, thus retaining important features of pilidial development. Here we present our data on the larval forms of numerous pilidiophoran species, identified through DNA-barcoding, or embryologically. By placing this information into a phylogenetic context we show that lecithotrophy evolved many times independently within the Pilidiophora. Convergent evolution of non-feeding planuliform and trochophore-like larvae in nemerteans underscores the success of this body plan and life history.
