Persistent ancestral feeding structures in nonfeeding annelid larvae

Evolutionary loss of the requirement for feeding in larvae of marine invertebrates is often followed by loss of structures involved in capturing and digesting food. Studies of echinoderms suggest that larval form evolves rapidly in response to loss of the requirement for feeding, but a lack of data from other taxa makes it difficult to assess the generality of this result. I show that many members of a large clade of annelids, the Sabellidae, retain ancestral systems for particle capture despite loss of the need and ability to feed. In at least one species, Schizobranchia insignis, an opposed-band system of prototrochal, food-groove, and metatrochal ciliary bands can concentrate suspended particles and transport them to the mouth, but captured particles are invariably rejected because larvae lack a functional gut. The persistence of particle capture systems in larvae of sabellids suggests that they have lost larval feeding very recently, that opposed bands are inexpensive to construct and operate, or that opposed bands have some alternative function. These observations also suggest a hypothesis on how the ability to feed is lost in larvae of annelids and other spiralians following increases in egg size.