Kauf, L. | Search bioRxiv

bioRxiv2026

Cave-dwelling Planariidae in Croatia exhibit differing levels of cave trait evolution

The Dinaric karst of Croatia encompasses a network of over 10,000 caves and represents one of the worlds most important subterranean biodiversity hotspots. It is inhabited by remarkably diverse and often endemic species, including planarian flatworms, which are among the rarest macroinvertebrates encountered in cave habitats. Although the presence of cave planarians has long been known, no integrative research on this group has been conducted to date, and the evolutionary relationships between these animals and their surface water counterparts are currently unresolved. To address these gaps, we combined field sampling, phylogenetic analysis based on COI and 18S genes, and phenotypic characterization. Our results show that cave planariids in Croatia belong to at least three genera and are more widespread and diverse across both Croatia, and the broader Dinaric karst, than previously assumed. We increased the number of cave records in the Dinaric karst from 26 to 37 and documented cf. Atrioplanaria and Phagocata in Croatian caves for the first time. Phylogenetic reconstructions suggest numerous independent cave colonization events, including multiple instances within the genera Crenobia and cf. Atrioplanaria. Variation in pigmentation and eye reduction, both within and between populations, further reveal heterogeneous evolutionary trajectories of cave-associated phenotypes. The biogeographical patterns and high genetic diversity we report here point to a complex evolutionary history of planariids in the Dinarides. Our newly generated molecular phylogenies and systematic documentation of trait variability establish Planariidae as a valuable model for studying mechanisms underlying convergent evolution of pigment loss and eye reduction in cave environments.

zoology↗

bioRxiv2025

An integrative taxonomic approach reveals unexplored diversity in Croatian planarians

BackgroundFreshwater ecosystems are among the most endangered habitats on Earth, with approximately one-fourth of aquatic species at risk of extinction. Effective conservation efforts require comprehensive monitoring and accurate species identification, including often overlooked groups. DNA barcoding promises rapid and accessible species identification but requires the availability of "universal" primer pairs and robust, taxonomically curated reference libraries. Planarian flatworms are one such group for which these resources are currently lacking, even though they are common constituents of freshwater ecosystems worldwide. As a result, the true extent of planarian diversity remains underappreciated in many areas. ResultsMotivated by the highly skewed representation of planarian diversity in current GenBank records of the barcoding gene mitochondrial cytochrome c oxidase subunit I (COI), we optimised existing barcoding primers and tested them on planarians collected in a species-rich but, so far, poorly surveyed region of Croatia. Using an integrative approach combining the new primers, traditional taxonomy and karyology, we generated new taxonomically curated COI barcode sequences for European dugesiids, dendrocoelids and planariids, which, in the case of the latter two groups, represent a substantial increase of the number of entries for those families. Our efforts resulted in several significant findings, including the description of a new pigmented Dendrocoelum species, Dendrocoelum pigmentatum sp. nov, the discovery of two highly differentiated haplotypic clades in Schmidtea lugubris, and the rediscovery of Polycladodes alba in Croatia after a century. ConclusionsOverall, our work extends the utility of DNA barcoding for species-level identification of planarians to previously inaccessible groups. Additionally, it integrates Croatia as an underexplored but species-rich region into the endeavour to systematically describe the planarian fauna of Europe. The expansion of a known number of Croatian planarian species from eight to sixteen and the discovery of a new, large planarian species in continental Europe, Dendrocoelum pigmentatum, demonstrate the effectiveness of our integrative approach. Overall, our work highlights the underappreciated diversity of planarians, even in continental Europe and supports practical conservation efforts to preserve aquatic biodiversity.

zoology↗