An integrate approach to the description and systematization of a new genus and species of marine Gastrotricha

In sandy samples from South-West Thailand we found numerous macrodasyidan gastrotrichs belonging to an undescribed species. The abundance and originality of the specimens prompted us to undertake a deep survey of both morphological and molecular traits aiming at the unbiased systematization of the new taxon. Using several microscopical techniques (DIC, SEM and CLSM) we investigated the external and internal anatomy plus the muscular and nervous systems. The Thai gastrotrichs are vermiform, up to 800 μm in total length; the head is well defined and, provided at its posterior edges with a pair of leaf-like sensorial organs; the posterior half of the body appears slightly wider than the anterior region and terminates in a single lobe. The adhesive apparatus includes tubes of the anterior, ventrolateral, posterior and dorsal series. Pharynx is about ¼ of the total body length and shows pores at its posterior 3/4. Adults exhibit maturing eggs and a bulky, muscular caudal organ but, surprisingly, not the frontal organ nor the (usual) spermatozoa. Falloidin reaction indicated that the muscular system organization follows the usual macrodasyidan plan. The nervous system, revealed with antibodies against RF-amide and serotonin (5HT), is present in the central, peripheral and stomatogastrich compartments. The brain consists of paired RF-positive perikarya connected by three dorsal- and a single ventral commissures, plus paired 5HT-positive cells joined by a dorsal commissure. Paired RF- and 5HT-positive neurites run ventrally along all the body length and coalesced at the posterior end. While the anatomical details of the new species appear unique among known genera, ongoing molecular analysis will help (us) clarify its phylogenetic position within the order.