A Heterolepidoderma and Halichaetoderma gen. nov. (Gastrotricha: Chaetonotidae) riddle: integrative taxonomy and phylogeny of six new freshwater species from Central Europe

Abstract

Heterolepidoderma is a widely distributed gastrotrich genus encompassing 22 freshwater and 15 marine species. In the present study, two new Heterolepidoderma and four new Heterolepidoderma-like species were discovered in the inland waters of Central Europe. Although Heterolepidoderma-like species exhibited all the morphological features typical of Heterolepidoderma, both nuclear and mitochondrial genes robustly showed their sister-group relationship to the marine genus Halichaetonotus. Most Heterolepidoderma species, including the new ones, were grouped with some Chaetonotus and Ichthydium species in a strongly supported cluster (so-called Heterolepidoderma s.l. clade). Since statistical tree topology tests robustly refuted the close affinity of Heterolepidoderma-like species to the Heterolepidoderma s.l. clade, a new genus, Halichaetoderma gen. nov., was proposed for them. Stochastic mapping suggested that the last common ancestor (LCA) of the Chaetonotidae‒Neogosseidae‒Dasydytidae clade had scales carrying keels with spines. Scales became oblong in the LCA of the Halichaetoderma + Halichaetonotus clade and three times independently within the Heterolepidoderma s.l. clade. Spines were lost in the LCAs of the Halichaetoderma + Halichaetonotus and Heterolepidoderma s.l. clades convergently. State-by-state associations of cuticular characters suggested that particular scale morphologies tend to be correlated with particular numbers of keels and the presence/absence of spines during the chaetonotid phylogeny.