Fine details of the choanocyte filter apparatus in asconoid calcareous sponges (Porifera: Calcarea) revealed by ruthenium red fixation.

Sponges (phylum Porifera) are highly specialized filter-feeding metazoans, pumping and filtering water with a network of canals and chambers, the aquiferous system. Most sponges have a leuconoid aquiferous system, characterized by choanocytes organized in small spherical chambers connected with ambient water by a complex net of canals. Such organization requires substantial pressure difference to drive water through an elaborate system of canals, so the choanocytes in leuconoid sponges have several structural features to generate pressure difference. In contrast, it is generally accepted that asconoid and syconoid sponges with long choanocyte tubes or large choanocyte chambers have no similar structures in their choanocytes. The present study is devoted to the detailed ultrastructural analysis of the choanocytes and their filter apparatus in the asconoid calcareous sponge Leucosolenia variabilis. The general structure of L. variabilis choanocytes is similar to that described for other sponge species. However, the fixation with 0.1% ruthenium red allowed us to reveal for the first time a complex of glycocalyx structures (vanes on the flagella, a fine glycocalyx sealing microvilli in the collar, and a glycocalyx strainer, embedding the apical parts of neighboring collars) in the choanocytes of L. variabilis, which are traditionally associated with the pumping and filtration process in leuconoid demosponges. All revealed glycocalyx structures have dimensions and locations similar to those found in the choanocyte chambers of some demosponges. The data suggest that L. variabilis utilizes the principles of water pumping and filtration similar to those in demosponges and revealed glycocalyx structures are potentially crucial for these processes. It seems that sponges from distant phylogenetic lineages and with different body plans rely on common principles of choanoderm organization for effective pumping and filtration of water. However, while some adaptation for effective pumping and filtration of water have possibly arisen before the diversification of Porifera, others have appeared independently in different lineages.