Ultrastructure of the epidermis and tube of two sessile species of the genus Limnias (Rotifera: Monogononta: Gnesiotrocha: Flosculariidae)

Abstract At the ultrastructural level, the distinctive feature of the phylum Rotifera is the presence of a syncytial epidermis with a dense intracytoplasmic lamina (ICL). Using transmission electron microscopy (TEM), we studied the ultrastructure of the different regions of the epidermis of the sessile rotifers Limnias ceratophylli Schrank, 1803 and Limnias melicerta Weisse, 1848, as well as the peduncle and tube of L. ceratophylli . The ultrastructure of the epidermis exhibits a consistent cytological pattern across different body regions in the two Limnias species. Regional variations mainly affect glycocalyx morphology, ICL thickness, and surface details. Compared to other sessile species, the alternating cilia and microvilli rows in the buccal cavity, the surface mounds on the trunk and foot, the types of glycocalyx, and the ultrastructure of the layers just below the apical plasma-membrane could be distinctive traits that may have taxonomic significance. The little-known ultrastructure of two unique features of sessile rotifers is described for the first time in Limnias ceratophylli : the peduncle that attaches the animal to the substrate, and the tube. The peduncle at least partially maintains the microtubules of the caudal cilia of the free-swimming larva. The tube is made up of an inner hyaline layer and an outer layer with exogenous particles, although in the basal region there is a fibrous layer between them. Such findings enhance understanding of the adaptive significance of these organs. In summary, the ultrastructure of the epidermis of the different body regions is consistent in the two species of Limnias , reflecting the different biological roles of various body parts. On the other hand, there are ultrastructural differences with other genera of sessile rotifers that may have taxonomic importance.