# Romanian Dendrocoelidae Hallez, 1892 (Platyhelminthes, Tricladida, Dendrocoelidae) Revisited: A Tribute to Radu Codreanu and Doina Balcesco

## TL;DR

This paper revisits the taxonomy and history of Romanian freshwater flatworms, highlighting the need for modern research to clarify their classification and evolutionary relationships.

## Contribution

The paper emphasizes the need for integrative research combining morphology and genetics to resolve the taxonomy of Romanian Dendrocoelidae.

## Key findings

- The Romanian Dendrocoelidae remain a poorly understood group due to lack of type specimens and morphological overlap.
- Integrative approaches are necessary to clarify the phylogeny and evolutionary history of the group.
- The study highlights the historical contributions of Radu Codreanu and Doina Balcesco to the understanding of the group.

## Abstract

The European Dendrocoelidae are an important group of freshwater flatworms. From the first species described by Müller in 1774, Dendrocoelum lacteum, the current account comprises nearly 100 species; many of them are endemic, with a geographical range restricted to a small area, and blind, inhabiting dark environments. The species were historically included in a system of nine units (taxa) that were difficult to distinguish clearly based on their morphology, as one or more character were often shared among units. The classical literature operates with the taxa Polycladodes, Dendrocoelum, Dendrocoelides, Paradendrocoelum, Palaeodendrocoelum, Eudendrocoelum, Neodendrocoelum, Bolbodendrocoelum, and Apodendrocoelum. The endemicity and the diversity of the morphological characters make it difficult to achieve a clear understanding of the taxonomic diversity and natural history of the group. How many species are there? What is their history? When did they evolve? To what extent are they related? The Dendrocoelidae of Romania are part of Europe’s freshwater diversity. A major knowledge gap is the lack of type specimens, the specimens upon which the species description was made. The Romanian Dendrocoelidae are a missing piece of a blurred, unresolved puzzle. These questions can be addressed by combining classical morphology with modern genetic tools.

This paper presents the current state of knowledge on the Romanian Dendrocoelidae as part of the European/Palearctic Dendrocoelidae, emphasizing the contributions of the Romanian zoologists Radu Codreanu and Doina Balcesco. The main objective of this work was to identify the knowledge gaps for future alignment with current standards. This article presents the species inventory and a short historical overview of the classical phylogenetic system and discusses some morphological characters used in the systematics of the group. This study also analyzes the arguments (and hypotheses) put forward by Codreanu, Balcesco, and other authors regarding the phylogenetic value of various factors, including (a) the position of the oviducts between the male atrium and the bursal canal (typical for Paradendrocoelum); (b) the eyes and the penial flagellum in relation to the palaeogeographical context governed by the Quaternary Glaciation; and (c) the point of view of Codreanu and Balcesco on the origin and composition of the actual Romanian Dendrocoelidae fauna. The major key finding is that the Dendrocoelidae species in Romania should be reinvestigated in an integrative way, and specific research needs and future directions are suggested.