Abstract
BackgroundGastrotricha systematics is in a state of flux mainly due to the conflicts between cladistic studies base on molecular markers and the classical systematisation based on morphological traits. In sandy samples from Thailand, we found numerous macrodasyidan gastrotrichs belonging to an undescribed species of difficult taxonomic affiliation. The abundance and original nature of the specimens prompted us to undertake a deep survey of both morphological and molecular traits aiming at a reliable systematisation of the new taxon.Methodology/Principal FindingsUsing several microscopical techniques we investigated the external and internal anatomy, including the muscular and nervous systems of the new species. Additional specimens were used to obtain the 18S rRNA gene sequence; molecular data was analysed cladistically in conjunction with data from additional species belonging to the near complete Macrodasyida taxonomic spectrum. Specimens are vermiform, up to 806 μm in total length, and show a well-defined head equipped with peculiar leaf-like sensorial organs and a single-lobed posterior end. The adhesive apparatus includes anterior, ventrolateral, dorsal and posterior tubes. Pharynx is about 1/4 of the total length and shows pores at its posterior 3/4. Adult specimens exhibit maturing eggs and a bulky, muscular caudal organ, but do not show sperm nor the frontal organ. Musculature and nervous system organisation resemble the usual macrodasyidan plan; however, the somatic circular muscles of the intestinal region surround all other muscular components and a third FMRFamide-IR commissure ventral to the pharyngo-intestinal junction appear to be an autoapomorphic traits of the new species.Conclusions/SignificanceWhile the anatomical characteristics of the Asian specimens appear so unique to grant the establishment of a new taxon, for which the name Thaidasys tongiorgii gen. et sp. nov. is proposed, the result of phylogenetic analyses based on the 18S rRNA gene unites the new genus with the family Macrodasyidae.
Highlights
Gastrotricha is a phylum of aquatic microinvertebrates, which contains about 820 accepted species divided into two orders: Chaetonotida that includes tenpin-shaped, hermaphroditic and/or parthenogenetic species found in marine, brackish or freshwater habitats, and Macrodasyida, a group of vermiform, hermaphroditic species that live interstitially, mostly in marine sand (e.g., [1, 2])
Gastrotricha systematics is in a state of flux mainly due to the conflicts between cladistic studies base on molecular markers and the classical systematisation based on morphological traits
Allowing for the high number of individuals examined, of different sizes/ages, the absence of sperm appears to be the normal condition of T. tongiorgii gen. et sp. nov
Summary
Gastrotricha is a phylum of aquatic microinvertebrates, which contains about 820 accepted species (as of April 2015) divided into two orders: Chaetonotida that includes tenpin-shaped, hermaphroditic and/or parthenogenetic species found in marine, brackish or freshwater habitats, and Macrodasyida, a group of vermiform, hermaphroditic species that live interstitially, mostly in marine sand (e.g., [1, 2]). Knowledge on the alpha biodiversity of the entire phylum is growing at a fast pace due to the continual description of new species (e.g., freshwater: [3,4,5,6,7]; marine: [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]), whereas recent cladistics studies challenging the phylogenetic congruence of the classical systematisation have notably increased the number of recognised genera and families [21, 23,24,25,26]. Since the process of re-systematisation benefits from additional surveys of insufficiently known taxa [33], the discovery of new species with novel characteristics could help to identify plesiomorphy in these morphologically diverse animals, providing a more solid ground for their natural grouping [1, 21]. The abundance and original nature of the specimens prompted us to undertake a deep survey of both morphological and molecular traits aiming at a reliable systematisation of the new taxon
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