On the growth of bivalve gills initiated from a lobule-producing budding zone

The growth of bivalve gills proceeds at the posterior end of the gill from a meristem-like budding zone, that is, an undifferentiated terminal organ, which continuously proliferates new gill elements in growing bivalves. In representatives of protobranch, filibranch, and eulamellibranch gills (13 species from Protobranchia, Pteriomorphia, Palaeoheterodonta, and Heterodonta), the first growth steps demonstrate a uniform basic pattern. The budding zone produces either transverse folds that split after a transition zone into parallel pairs of lobules (which themselves later differentiate into the inner and outer demibranchs), or it produces the lobules directly, without first forming a transition zone. The lobules elongate, differentiate into lobes, and transform into leaflet-like structures (protobranchs) or into filaments (filibranchs and eulamellibranchs). The filaments represent the differentiated outer margins of each lobe, of which the central tissue (interlamellar septum) becomes incised or fenestrated, or transformed by tissue junctions. A distally located main growth zone for each lobe is suggested. With regard to the delayed onset of the differentiation of the outer demibranch in juvenile unionids, an additional temporary growth zone for filaments is suggested to exist at the anterior end of the outer demibranch.