Ciliary receptors associated with the mouth and pharynx of Acoela (Acoelomorpha): a comparative ultrastructural study

Abstract

Abstract Todt, C. and Tyler, S. 2007. Ciliary receptors associated with the mouth and pharynx of Acoela (Acoelomorpha): a comparative ultrastructural study . -Acta Zoologica (Stockholm) 88 : 41-58The ultrastructure and distribution of receptor cells near the mouth and (where present) the pharynx of Hofstenia miamia , Proporus bermudensis , Conaperta thela , and Convoluta convoluta (Acoela) were investigated by transmission electron microscopy and confocal laser scanning microscopy of specimens stained with a fluorescence marker for actin. Five types of monociliary receptors were identified: (1) non-collared receptors with a single long and narrow ciliary rootlet; (2) non-collared receptors with a wide main ciliary rootlet and a smaller posterior rootlet; (3) non-collared receptors with a single wide and hollow ciliary rootlet with a granulated core; (4) Collar (?) receptors with obliquely radial filament bundles in the cell apex and with a single hollow ciliary rootlet composed of numerous strand-like elements; and (5) Collar receptors lacking a striated rootlet but with a granular body (swallow's nest rootlet). While H. miamia bears the first two receptor types, P. bermudensis has receptors of type 1, 3 and 5, and Cona. thela and Conv. convoluta have receptors of type 3, 4 and 5. The density of receptors is generally highest at the anterior body tip, regardless of where the mouth is located. Most receptor types occur scattered over the whole body but type 2 receptors of H. miamia are restricted to the pharynx and mouth region. The lack of a common receptor type specific for the mouth and pharynx of the investigated species points to an independent origin of the pharynges in Hofsteniidae and in Proporidae and of the mouth tube in Convolutidae. Moreover, the homology of the so-called collar receptors in Acoela with typical collar receptors in other invertebrates is questioned.

Systematic revision of acoels with 9+0 sperm ultrastructure (Convolutida) and the influence of sexual conflict on morphology

“…Sensory organs in the Convolutida include sensory cilia (Todt and Tyler 2007) and eyes. The latter comprise pigment cells that contain vacuoles with refractive concrements and are innervated by a few nerve cells (Popova and Mamkaev 1985; Yamasu 1991).…”

Structure and evolution of the pharynx simplex in acoel flatworms (Acoela)

“…On the basis of the ultrastructure of receptor cells associated with the mouth and pharynx, Todt and Tyler (2007) proposed a close relationship of Hofsteniidae to Solenofilomorphidae. Recently Hooge et al (2007) presented an overall wellresolved tree based on 18S and 28S rDNA sequences, which includes several of the pharynx-bearing taxa.…”

Ultrastructure of epidermal sensillae in three species of Acoela

“…E.g., Raikova (1989) described four types of ciliary receptors in four species of Acoela: monociliary sensillae with a thin rootlet, monociliary sensillae with a thick rootlet, monociliary collared sensillae and multiciliary sensillae. The receptors of another four species of Acoela, studied by Todt & Tyler (2007) were found to bear the single cilium. Among them there are sensillae with a thin rootlet, sensillae with two rootlets, sensillae with a thick rootlet (all of them are non-collared), collared sensillae with a rootlet composed of the strand-like elements and collared sensillae without a rootlet.…”

“…The primitive features of nervous system of Acoela are combined with the high diversity of epidermal ciliary receptors, most likely responsible for tactile and chemosensory functions. As a rule, they are scattered over the whole body, forming concentrations on the anterior end around the glandular frontal organ (Smith, Tyler, 1985, 1986) and on the ventral side around the mouth opening and pharynx (Todt, Tyler, 2006, 2007.…”

“…The most relevant and detailed reviews on ciliary receptors of Acoela were provided by Todt & Tyler (2007) and Haszprunar (2016). E.g., Raikova (1989) described four types of ciliary receptors in four species of Acoela: monociliary sensillae with a thin rootlet, monociliary sensillae with a thick rootlet, monociliary collared sensillae and multiciliary sensillae.…”