Development and larval feeding in the capitellid annelid Notomastus cf. tenuis

Making inferences about the evolution of larval nutritional mode and feeding mechanisms in annelids requires data on the form and function of the larvae, but such data are lacking for many taxa. Though some capitellid annelids are known or suspected to have planktotrophic larvae, these larvae have not previously been described in sufficient detail to understand how they feed. Here we describe embryos and larvae of the capitellid Notomastus cf. tenuis from San Juan Island, Washington State. Fertilized oocytes average about 58 μm in equivalent spherical diameter. Early embryos undergo spiral cleavage and develop into larvae that feed for about 5 weeks before metamorphosis. Larvae of N. cf. tenuis capture food particles between prototrochal and metatrochal ciliary bands and transport them to the mouth in an intermediate food groove; this arrangement is typical of "opposed band" larval feeding systems. Surprisingly, however, larvae of N. cf. tenuis appeared to have only simple cilia in the prototrochal ciliary band; among planktotrophic larvae of annelids, simple cilia in the prototroch were previously known only from members of Oweniidae. The anteriormost tier of prototrochal cilia in N. cf. tenuis appears to be non-motile; its role in swimming or particle capture is unclear. Like some planktotrophic larvae in the closely related Echiuridae and Opheliidae, larvae of N. cf. tenuis can capture relatively large particles (up to at least 45 μm in diameter), suggesting that they may use an alternative particle capture mechanism in addition to opposed bands of cilia.