Ultrastructure of the "statocysts" in Protodrilus species (Polychaeta): Reconstruction of the cellular organization with morphometric data from receptor cells

Abstract

The "statocysts" in Protodrilus ciliatus, P. oculifer, P. haurakiensis and P. helgolandicus are situated in the prostomium anterior to the palps and have been investigated by electron microscopy. The sensory organs were reconstructed from serial sections, volumes were calculated from areas of consecutive section profiles, and additional data on surface area of distal receptor elements have been determined. In spite of variations in size (diameter 8-20 μm) their structure is nearly identical. The organs consist of one cup-shaped supportive cell, one large bi- or multiciliated sensory cell and two small uni- or biciliated sensory cells forming an extracellular cavity. This cavity is completely filled with microvillus-like or paracrystalline structures and there are no signs of statoliths composed of extracellular material. The most striking feature is the occurrence of paracrystals made up of undulating ciliary membranes extending from the large sensory cell and occupying 75-90% of the cavity in P. ciliatus, P. oculifer and P. haurakiensis. The remaining space is filled with microvilli or dendritic processes of the sensory cells. In P. helgolandicus the ciliary paracrystals are almost completely replaced by microvillus-like branches of cilia of the corresponding sensory cell. Paracrystals fill less than 10% of the cavity and are formed of flattened membranes. These sensory organs enclose large surface areas of membranes (15,000-38,000 μm2). The surface areas of the paracrystals composed of undulating membranes is almost identical to that of densely arranged arrays of microvilli (about 25 μm2 per μm3). These sensory organs are so different from all known statocysts that it is likely that they have another function. Their greatest structural correspondence is to light-receptive organs, especially in the structure and arrangement of microvilli. The role the paracrystals play is discussed: they might bear photopigments or simply represent a lens - a transparent, refractile and crystalline structure. These sensory organs are completely different from pigmented ocelli and phaosomes occurring in some protodrilids and represent a type of sensory organ thus far undescribed in polychaetes. © 1990 Springer-Verlag.