Larval morphology of the bryozoan Watersipora arcuata (Cheilostomata: Ascophora)

The larva of the ascophoran cheilstome Watersipora arcuata is described on the basis of serial 1-μ sections, light microscopy of whole mounts, and scanning electron microscopy. Using lightly osmicated specimens, it was possible to map almost every cell on the larval surface. Limited observations on hatching and larval behavior are provided in conjunction with the anatomical description. Tissues of the larva are partitioned between those that function exclusively during the larval period and are degraded at metamorphosis as transitory tissues and those that will have postmetamorphic fates in formation of the ancestrula. Significantly, W. arcuata has two possible anlagen for the ancestrular polypide, the infracoronal cells in the oral hemisphere and the epidermal blastemal cells in the aboral hemisphere, rather than only one or the other of these as reported in other species. Also detailed are the supracoronal flange and groove, which are unique to this genus and are involved in the transmission of mycoplasma-like organisms between successive generations of adults; two pairs of complex pigment cup ocelli; multiple intercoronal cells that are presumed to have varied sensory and mechanical functions; and the sensory, adhesive, and locomotory components of the pyriform organ. The larval anatomy of W. arcuata is compared with that of the larvae of the ctenostomes Alcyonidium gelatinosum (coronate), Bowerbankia imbricata (coronate), B. gracilis (coronate), and Flustrellidra hispida (shelled lecithotrophic) and of the cheilostomes Bugula neritina (coronate), Electra pilosa (cyphonautes), and Membranipora membranacea (cyphonautes). This study is the first detailed analysis of the larval structure of any ascophoran bryozoan and provides a necessary platform for subsequent analyses of embryology and metamorphosis.