An oral brush of cilia in the feeding larvae of Micronephtys cornuta (Annelida, Nephtyidae)

Larvae of many phyllodocidan annelids are planktotrophic, but the feeding mechanisms of larvae in this diverse clade are poorly known. Many larvae belonging to one large clade of phyllodocidans, Aphroditiformia, bear a bundle of long cilia attached to the left side of the prototroch, the oral brush, which they use in feeding. In 1936, D.P. Wilson observed that trochophore larvae of Nephtys hombergi , a member of the phyllodocidan family Nephtyidae, bore a strikingly similar bundle of long cilia on the left side of the body. Since Wilson's observation, numerous descriptions of nephtyid larvae have been published, but none remark on the presence of an oral brush. Here I show that metatrochophore I and II larvae of Micronephtys cornuta bear an oral brush, but that it is lost in the transition to the nectochaete stage, during which the larval mouth and foregut are also being remodeled to function in benthic feeding by juveniles. That an oral brush is clearly present in at least some larval stages of two genera suggests that oral brushes may be widespread in the feeding larvae of nephtyids, but have simply been overlooked for more than 80 years. Additional work is needed to make inferences on the evolutionary history of the oral brush in phyllodocidan annelids, and to distinguish among several hypotheses on the function of this peculiar group of cilia in larval feeding.