The unique gravireceptor organs of Pleurodasys helgolandicus (Gastrotricha: Macrodasyida)

Abstract

The drumstick-like organs of the marine interstitial gastrotrich Pleurodasys helgolandicus Remane, 1927 were studied using optical and electron microscopy, in order to widen our knowledge of the sensory organs among gastrotrichs. The drumstick-like sensory organs are paired, symmetrical structures located on the dorsal side of the post-cephalic region of the body. Each sensory organ is formed by an outgrowth of the lamellar exocuticle covering the body surface, and it consists of an elongated, cylindrical hollow stalk surmounted by a hollow bulbous tip, housing in its cavity a dense spherical body. Compared with the three main sensory structures recognized among Gastrotricha, i.e., mechanoreceptors, photoreceptors, and chemoreceptors, the organization of the drumstick-like sensory organ matches that of mechanoreceptors. In particular, we suggest that the drumstick-like sensory organs of P. helgolandicus are gravireceptor organs, analogous to the orthopteran "plumb bob-like" receptors. This is the Wrst description of a putative gravireceptor organ among gastrotrichs.

1. Gastrotricha

“…Each organ consists of a single ciliated receptor cell with a similar cytomorphology as the common mechanoreceptive cells of Gastrotricha, i.e., they possess a long cilium that is basally surrounded by a collar of 10 short microvilli (see above). Because the drumstick-shaped external part of the organ has a jointed connection to the body cuticle and is freely movable and because the sphere is reminiscent to the otoliths of statocysts in other taxa, Marotta et al (2008) conclude that the organs in P. helgolandicus must display gravireceptors. The peculiarity of the gravireceptor organs is the presence of a drumstick-shaped, external protuberance of the epicuticle into which the cilium of the receptor cell projects.…”

“…Species of the derived planktonic taxon Neogossea exhibit a pair of pigmented, kidneyshaped structures on the dorsal side of the anterior portion of the pharynx (e.g., Kieneke & Riemann 2007). However, it is possible that these highly specialized animals benefit from Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 6/16/15 1:05 PM reconstructions based on TEM investigations have been carried out for the cephalic tentacles and regular sensory processes of Chordodasiopsis antennatus (Rieger et al 1974), the anterior and posterior head sensory organs of Turbanella cornuta (Teuchert 1976a) and Dactylopodola baltica (Hochberg & Litvaitis 2003b, Liesenjohann et al 2006, diverse types of sensory cilia ("tactile hairs") of T. cornuta (Teuchert 1976a), the sensory palps of Tetranchyroderma papii (Gagné 1980b), and the unique gravireceptor organs of Pleurodasys helgolandicus (Marotta et al 2008). However, it is possible that these highly specialized animals benefit from Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 6/16/15 1:05 PM reconstructions based on TEM investigations have been carried out for the cephalic tentacles and regular sensory processes of Chordodasiopsis antennatus (Rieger et al 1974), the anterior and posterior head sensory organs of Turbanella cornuta (Teuchert 1976a) and Dactylopodola baltica (Hochberg & Litvaitis 2003b, Liesenjohann et al 2006, diverse types of sensory cilia ("tactile hairs") of T. cornuta (Teuchert 1976a), the sensory palps of Tetranchyroderma papii (Gagné 1980b), and the unique gravireceptor organs of Pleurodasys helgolandicus (Marotta et al 2008).…”

“…Additional ultrastructural data related with certain sensory organs exist for Cephalodasys maximus (Wiedermann 1995), Cephalodasys sp., Crasiella cf. Predominantly based on morphological (ultrastructural) evidence, three functional types of receptor organs have been identified in Gastrotricha: mechanoreceptors (including the gravireceptors of P. helgolandicus), photoreceptors, and chemoreceptors (Teuchert 1976a, Ruppert 1991, Marotta et al 2008). Predominantly based on morphological (ultrastructural) evidence, three functional types of receptor organs have been identified in Gastrotricha: mechanoreceptors (including the gravireceptors of P. helgolandicus), photoreceptors, and chemoreceptors (Teuchert 1976a, Ruppert 1991, Marotta et al 2008).…”

“…Basally, mechanoreceptive sensory cells form an axon like all sensory cell types in Gastrotricha ( Fig. However, it is so far not understood why P. helgolandicus is the only known gastrotrich species with such sensory devices (Marotta et al 2008). In T. cornuta, direct connections of these axons to the neuropil of the brain or with the ventrolateral longitudinal neurite bundles are reported (Teuchert 1976a).…”

“…A quite unusual and among Gastrotricha unique sensory organ is the pair of supposed gravireceptor organs of Pleurodasys helgolandicus that are positioned on the dorsal surface in the anterior part of the animal (Marotta et al 2008). Each organ consists of a single ciliated receptor cell with a similar cytomorphology as the common mechanoreceptive cells of Gastrotricha, i.e., they possess a long cilium that is basally surrounded by a collar of 10 short microvilli (see above).…”