Abstract

Chaetonotidae, the most diverse and widely distributed family of the order Chaetonotida (Gastrotricha) includes both marine and freshwater species. Although the family is regarded as a sister taxon to exclusively marine Xenotrichulidae, the type of environment, marine or freshwater, where Chaetonotidae originated is still not known. Here we infered molecular phylogeny of chaetonotids using multiple molecular markers together with morphological and ecological character tracing. Our results revealed that the freshwater genus Bifidochaetus is the earliest branching lineage in the paraphyletic Chaetonotidae (encompassing Dasydytidae and Neogosseidae). Moreover, we reconstructed the clade grouping Lepidochaetus and Cephalionotus genera in sister relation to the remaing chaetonotids which supports Kisielwski's morphological based hypothesis concerning undifferentiated type of body scales as a most primary character in Chaetonotidae. We also recovered that the reversals to marine habitats occurred independently in different Chaetonotidae lineages, thus marine species in the genera Heterolepidoderma, Halichaetonotus, Aspidiophorus and subgenera Chaetonotus (Schizochaetonotus) or Chaetonotus (Marinochaetus) should be assumed as advanced, derived taxa. Tracing of the eco-morphological characters on the reconstructed tree suggests that ventroterminal mouth ring, separate cephalic plates, separate cephalic sensory cilia arrangement in paired tufts, hypostomium, reduced testis and reduced, unmovable spermatozoa together with parthenogenesis as a main type of reproduction are the synapomorphies defining the whole Chaetonotidae.