Intercoronal cell complex of larvae of the bryozon Watersipora arcuata (cheilostomata: Ascophora)

The coronate larva of the ascophoran bryozoan Watersipora arcuata has a ring of 32 large, multiciliated coronal cells that are used for swimming. Fourteen pairs of small cells are intercalated between the lateral margins of adjacent coronal cells. These intercoronal cells are arranged in a precise pattern and are polymorphic: seven pairs have multiple cilia and seven pairs are mono- or oligociliated. Three pairs of multiciliated intercoronal cells have their cilia arranged as a whorl that is recessed in a pocket formed between the adjacent coronal cells, and they are thought to be photoreceptors that sense general light intensity. Two other pairs of multiciliated cells with cohesive tufts of cilia may be chemo- or mechanoreceptors. Roles of the other intercoronal cells in this species are not evident, but it is proposed that the majority, if not all, of them are sensory. The close proximity of all the intercoronal cells to the equatorial nerve ring is compatible with this interpretation. Analyses of the literature on cleavage patterns, pigment cup ocelli, and flagellar tufts that serve as balancers in coronate larvae lead us to propose that (1) an intercoronal cell is the sensory element of most, if not all, pigment cup ocelli of bryozoan larvae; and (2) intercoronal cells are not modified coronal cells but probably are specialized supra- and/or infracoronal ones that have migrated to an intercoronal position.