Flustrellidra hispida (Fabricius, 1780) broods lecitotrophic shelled larvae called “pseudocyphonautes”. Such larva is present only in a few species of gymnolaemate bryozoans. Here we provide the new data on its histology and ultrastructure.
The pseudocyphonautes larvae of Flustrellidra hispida possess an elongated transparent bivalve shell 198-350x500-620 μm in size. Unlike the cyphonautes, shell valves are lined by a simple squamous nonciliated epithelium. Larval epithelial cells have different ultrastructure, some of them resemble blastemal cells and may correspond to the polypide or cystide rudiment of the ancestrula as it is in unshelled pseudocyphonautes larva of Alcyonidium polyoum. The apical organ comprises of nervous, ciliated and undifferentiated cells, but no blastemas were found. Corona consists of at least two cell types: locomotory multiciliate cells and primary sensory cells. The rudimentary gut consists of blind-closed oesophagus. Revealed anterior and posterior chambers correspond to inchalant and exchalant chambers of the filter-feeding system of the cyphonautes. Anterior chamber epithelium has no cilia, posterior chamber is lined by ciliated epithelium. At the border with coronal cells epithelium forms papillas with ciliary tufts, especially at the region of the ciliated cleft of pyriform organ. All epithelial cells comprising the corona and the outer larval surface between the valves and the oral pole have apical surfaces with long branched microvilli.
According of this investigation the pseudocyphonautes larvae of Flustrellidra hispida is morphologically similar to unshelled pseudocyphonautes larvae of some ctenostomate bryozoans, but differ from them by an arrangement of blastemal cells.