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


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 first description of a putative gravireceptor organ among gastrotrichs.

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We are indebted to M. Curini-Galletti, F. Leasi and P. Tongiorgi for helping us during the collection of the Gastrotricha. M Curini-Galletti and E. Ferrero provided part of the literature on flat-worm statocysts. We are grateful to R.M. Kristensen for reading an early version of the manuscript. The Eclipse 90i Nikon microscope was funded by “Fondazione Cassa di Risparmio di Modena.” The research benefited also from a MIUR grant to M.A.T. (PRIN-2004—Diversità e zoogeografia dei Gastrotrichi del Mediterraneo). We are also grateful to Laura Valenti for accurate ink drawings, and to both anonymous reviewers for their constructive feedbacks and suggestions.

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Correspondence to Roberto Marotta.

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Marotta, R., Todaro, M.A. & Ferraguti, M. The unique gravireceptor organs of Pleurodasys helgolandicus (Gastrotricha: Macrodasyida). Zoomorphology 127, 111–119 (2008).

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  • Sensory organ
  • Gastrotricha
  • Microscopy
  • Gravireceptors