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

The drumstick-like organs of the marine interstitialgastrotrich Pleurodasys helgolandicus Remane, 1927were studied using optical and electron microscopy, inorder to widen our knowledge of the sensory organs amonggastrotrichs. The drumstick-like sensory organs are paired,symmetrical structures located on the dorsal side of thepost-cephalic region of the body. Each sensory organ isformed by an outgrowth of the lamellar exocuticle coveringthe body surface, and it consists of an elongated, cylindricalhollow stalk surmounted by a hollow bulbous tip, housingin its cavity a dense spherical body. Compared with thethree main sensory structures recognized among Gastrotricha,i.e., mechanoreceptors, photoreceptors, and chemoreceptors,the organization of the drumstick-like sensoryorgan matches that of mechanoreceptors. In particular, wesuggest that the drumstick-like sensory organs of P. helgolandicusare gravireceptor organs, analogous to the orthopteran“plumb bob-like” receptors. This is the firstdescription of a putative gravireceptor organ among gastrotrichs.