Abstract
Understanding biological diversity is crucial for ecological and evolutionary studies. Even though a great part of animal diversity has already been documented, both morphological surveys and metabarcoding analyses have previously shown that some animal groups, such as Platyhelminthes, may harbour hidden diversity. To better understand the molecular diversity of Platyhelminthes, one of the most diverse and biomedically important animal phyla, we here combined data from six marine and two freshwater metabarcoding expeditions that cover a broad variety of aquatic habitats and analysed the data by phylogenetic placement. Our results show that a great part of the hidden diversity is located in early-branching clades such as Catenulida and Macrostomorpha, as well as in late-diverging clades such as Proseriata and Rhabdocoela. We also report the first freshwater record of Gnosonesimida, a group previously thought to be exclusively marine. Finally, we identified two putative novel freshwater Platyhelminthes clades that branch between well-defined orders of the phylum. Thus, our analyses of several environmental datasets confirm that a large part of the diversity of Platyhelminthes remains undiscovered, point to groups with more potential novel species and identify freshwater environments as potential reservoirs for novel species of flatworms.
Highlights
To understand past and present biological processes and to make meaningful decisions for the future, it is of pivotal importance to decipher extant biodiversity [1]
We collected a total of 1380 representative sequences of clustered operational taxonomic units (OTUs) identified as Platyhelminthes in six marine and two freshwater environmental surveys
We constructed a reference tree of complete 18S rDNA sequences retrieved from the National Center for Biotechnology Information (NCBI) nucleotide database to use as a backbone for phylogenetic placement
Summary
To understand past and present biological processes and to make meaningful decisions for the future, it is of pivotal importance to decipher extant biodiversity [1]. Flatworms are biomedically relevant, being 75% of the described species of obligate parasites of vertebrates [15,16] They are key meiofaunal taxa in both marine and freshwater aquatic environments [17,18]. Flatworms have rarely been taken into consideration in traditional biodiversity studies, given that their morphological identification is tedious, requiring fixation and histological processing [19] or live examination when fixation can destroy their taxonomically informative internal reproductive anatomy [18]. To our knowledge, a thorough analysis of metabarcoding data on Platyhelminthes has never been done To fill this gap, we here analysed different environmental datasets from six marine and two freshwater habitats to both explore the diversity of Platyhelminthes at the level of orders and detect potential novel, undescribed molecular diversity
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