A large percentage of red algae inhabit marine environments, and only about 10% occur in a limited number of freshwater habitats, due to their stenovalence (Kumano 2002; Crowell et al. 2025). In particular, they are mainly found in rivers with running waters, where they colonise rocks exposed to turbulent flow conditions (Sheath and Hymes 1980). A consequence of this ecological preference is that some of these algae appear to be rather rare and restricted to specific habitats and are unable to survive in anthropised areas (Eloranta and Kwandrans 2002; Simić et al. 2010).

A significant percentage of freshwater red algae (about 70%) belongs to the order Batrachospermales, one of the four orders composed exclusively of freshwater taxa that primarily inhabit flowing waters (Kumano 2002; Vis and Necchi 2021). The members of this order generally have macroscopic uniaxial gametophytes with whorled branches, giving a bead-like appearance or forming a pseudoparenchymatous thallus with mucilaginous or cartilaginous consistency. Most members have a heteromorphic triphasic life history consisting of the sporophyte (Chantransia stage), the gametophyte (monoecious and/or dioecious), and the carposporophyte phases (Sheath 1984; Sheath et al. 1996; Necchi and Carmona 2002; Vis and Necchi 2021).

According to a molecular phylogenetic analysis based on two molecular markers, the order Batrachospermales currently comprises a single family, the Batrachospermaceae (Entwisle et al. 2009). This family comprises 21 genera (Vis and Necchi 2021), including Lemanea Bory and Paralemanea (P.C. Silva) M.L. Vis & Sheath. These entities have likewise been subject to an intricate taxonomic history culminating in their separation and present status as distinct genera, as described by Xie et al. (2004). Mitochondrial genome investigations further corroborate the modern separation of these two genera. Paralemanea species possess the largest genomes and most extensive intergenic regions sequenced to date within the Batrachospermales. Notably, they are the only taxa within the order to contain introns, which are absent in all other members, including the sister genus Lemanea (Lemes Da Silva et al. 2025).

Morphologically, the two genera Lemanea and Paralemanea can be distinguished mainly by the following characters: Lemanea has spermatangia arranged in circular papillae at the nodes and internal organisation with T- or L- shaped ray cells abutting the cortex and a main axis without cortication; Paralemanea has spermatangia arranged in rings around nodes and internal organisation with simple ray cells and cortication around the main axis. In particular, axial filaments are surrounded by cortical filaments, and the simple ray cells consist of two layers; the proximal one does not touch the outer cortex, and the distal one is generally “Y” branched and connected to the cortex. Furthermore, the genus Paralemanea includes species that lack stalks (Vis and Sheath 1992; Sheath et al. 1996; Vis and Necchi 2021).

In Italy, reports of species belonging to these genera are scarce and scattered (Barone et al. 1995; Mannino 2001; Mannino et al. 2003; Ceschin et al. 2012, 2013; Saber et al. 2016; Campisi et al. 2020). Indeed, the taxonomy, distribution, and ecological role of these red algae have been poorly investigated, especially in Sicily. Only three records of Lemanea are available for this region, all referring to Lemanea fluviatilis (Linnaeus) C. Agardh (Barone et al. 1995; Mannino 2001; Campisi et al. 2020). As far as we know, there are currently no published reports of Paralemanea species in Sicily, creating a research gap in this field. Moreover, species of Batrachospermaceae have never been reported from the Alcantara River, one of the most important freshwater bodies in Sicily. Although this river has been the object of several studies, particularly regarding its ecological status (Gugliandolo et al. 2009; Guglielmo et al. 2013), no detailed investigations have yet been carried out on its algal flora.

During field surveys along two sites of the Alcantara River, numerous thalli of freshwater red algae referable to the genera Lemanea and Paralemanea were found. Considering the scarcity of data on these genera in Sicily, we deemed it worthwhile to investigate these specimens in greater detail in order to fill the existing knowledge gaps and to provide new information on the ecology and habitat of these species.

Based on the diacritical characters summarised in Table 1, two species of Batrachospermaceae, Lemanea fucina Bory and Paralemanea catenata (Kützing) Vis & Sheath, were identified.

This study reports the first record of Lemanea fucina for Italy and Paralemanea catenata for Sicily. Specimens of L. fucina showed the same branching patterns reported by Vis and Necchi (2021) and John et al. (2002), as well as the “false” branches, as reported by Mitrović et al. (2021). Similarly, the P. catenata specimens showed the diacritic characters reported in all previous studies (Mannino et al. 2003; Xie et al. 2004; Carmona et al. 2011; Simić and Đorđević 2017; Vis and Necchi 2021). Our samples of both species, contrary to what has been reported in the literature, showed smaller carposporangia (Vis and Necchi 2021). These findings contribute to improving knowledge of the distribution of these species in Europe and Italy, indicating that both extend their range further south than previously reported.

The taxonomy of the Batrachospermaceae is complex, since diagnostic characters are often ambiguous or insufficiently defined. Kučera and Marvan (2004) noted that features considered diagnostic may vary depending on the age and maturity of the thallus, which can lead to misidentification. The identification of species within the Batrachospermaceae is further complicated by the numerous taxonomic revisions over time (Xie et al. 2004). In fact, this taxonomic confusion has often led to the same species being described under different synonyms, as exemplified by the cases of L. fucina and L. mamillosa (Kumano 2002; Eloranta et al. 2011).

Beyond a reliable taxonomic identification, it is essential to understand ecological preferences and resilience to anthropogenic pressures. Indeed, species of the genus Lemanea are known to be adapted to specific environmental conditions, making them vulnerable to human-induced disturbances (Mitrović et al. 2021). In our study, L. fucina was restricted to Monte San Nicola, a natural site with low levels of anthropisation. In contrast, P. catenata was observed at both Monte San Nicola and Passo Mojo, a site more affected by human activities. The limited abundance of L. fucina compared to P. catenata, suggests that the latter species may have a broader ecological tolerance, being more resilient to environmental impacts. Furthermore, the presence of carposporangia in both species may indicate the healthy state of populations inhabiting non-anthropised areas along the Alcantara River.

Monitoring sensitive species allows for the early detection of environmental changes, enabling the identification of their causes and timely intervention (Gutowski et al. 2004). In this context, river dynamics strongly influence community composition. Indeed, summer variations in the river flow dynamics caused both the desiccation of L. fucina and P. catenata thalli and the subsequent dominant growth of Cladophora sp. and Vaucheria sp. These algae also colonise the less turbulent portions of the river. They are likely to be competitively advantaged when the limited flow of the river does not allow the establishment of Lemanea sp. and Paralemanea sp, which require turbulent water. Nostoc sp., on the other hand, mainly grows in isolated colonies on riverbanks or in areas where the water flow is less turbulent. Under climate change scenarios predicting reduced precipitation and river flow (Billi and Fazzini 2017), these dynamics could be further impacted.

In conclusion, additional investigations are needed to gain better knowledge about the distribution and environmental sensitivity of freshwater algae within river systems. Such research will be crucial for developing effective conservation and monitoring measures aimed at protecting freshwater habitats, which are becoming increasingly susceptible to anthropogenic pressures and climate change.