Composite branch construction by dual autozooidal budding modes in hornerids (Bryozoa: Cyclostomatida)

Horneridae (Cyclostomata; Cancellata) is a family of marine bryozoans that form tree-like colonies bearing functionally unilaminate branches. Colony development in this clade is not well understood. We used micro-CT and SEM to trace zooidal budding in Hornera from the ancestrula onwards. Results show that hornerid branches are constructed by dual zooidal budding modes occurring synchronously at two separate budding sites at the growing tips. Frontal autozooids bud from a multizooidal budding lamina. Lateral autozooids bud from discrete abfrontal budding loci by exomural budding, a previously undescribed form of budding centred on hypostegal pores in interzooidal grooves on the colonial body wall. These two budding modes are integrated during primary branch morphogenesis, forming composite, developmentally bilaminate, branches. Tracing development back to the founding zooids shows that the ancestrula and adventitious periancestrular autozooids are the first-formed lateral autozooids. They grow as a vertical stem onto which new laterals bud exomurally, intercalating with earlier autozooids to form an expanding funnel. The inner wall of this funnel becomes a ring-shaped budding lamina, from which the first frontal autozooids bud centripetally. When the branch crown splits, the ring lamina is subdivided, each fragment becoming a paramedial budding lamina in its own branch, while new laterals continue to be budded exomurally onto the frontal walls of existing laterals. Co-occurrence of two rare cyclostome frontal budding types--adventitious budding on the ancestrula and exomural budding on the branches--suggests they may be homologous. Patterns of exomural budding vary among hornerids. Narrow-branched taxa possess a single medial line of budding sites on the abfrontal wall; wide-branched species have up to six parallel lines of loci. Exomural budding also occurs sporadically in Calvetia osheai, a radial-branching hornerid. Future studies of cancellate taxonomy and phylogeny may benefit from morphological concepts presented here.