Ultrastructural features associated with ambush predation in the sessile rotifer Collotheca monastica Franch, 2025

Abstract We evaluate the hypothesis that specialization for macrophagous ambush feeding in sessile rotifers is reflected not only in external morphology but also in ultrastructural traits apparently absent from currently described filter-feeding taxa. Using transmission electron microscopy, we examined the tube, epidermis, and digestive epithelia of Collotheca monastica . Immotile cilia on the infundibular rim and buccal tube correspond to the absence of the outer dynein arm and apical mitochondria. The infundibular lining is a syncytium with a thin dense intracellular layer, fibrous–granular glycocalyx, and conical pores with a bulb suggestive of secretion. The buccal cleft is formed by ciliated cells with immotile cilia arranged in a ring, joined by junctional complexes to the infundibular lining above and to flattened buccal epithelial cells below. Beneath them, strongly folded flattened cells likely produce a detachable wall, potentially homologous to the buccal velum; both the cells and the wall continue into the proventricular epithelium. The stomach shows active intracellular digestion, and the intestine contains partially digested material embedded in a fibrous matrix. The ultrastructure of the excretophores is described for first time. These integrated ultrastructural features support the view that macrophagy in Collothecaceae has shaped distinctive tissue organizations, not reported in filter-feeding rotifers.