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

Horneridae (Cyclostomatida: Cancellata) is a family of marine bryozoans that forms tree-like colonies bearing functionally unilaminate branches. Colony development in this clade is not well understood. We used micro-computed tomography and scanning electron microscopy 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 frontal budding centered 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. Patterns of exomural budding vary among hornerid taxa, and future studies of Cancellata taxonomy and phylogeny may benefit from morphological concepts presented here.