Two freshwater red algae make surprise first appearances in Sicily’s Alcantara River

Freshwater red algae are among the most overlooked organisms in European rivers, and a new study along Sicily’s Alcantara River has just filled two conspicuous gaps on the map. Writing in Italian Botanist, researchers report the first record of the stalked, highly branched red alga Lemanea fucina for Italy as a whole, and the first Sicilian record of its unstalked relative Paralemanea catenata. Both species were found in the Alcantara River, one of the main freshwater bodies of eastern Sicily and the island’s only river park, where no members of the family Batrachospermaceae had ever been documented before.

Red algae are overwhelmingly marine: only about ten percent of the group occurs in freshwater, and those species are stenovalent, meaning they tolerate only a narrow range of environmental conditions. They favour rivers with running water, where they colonise rocks exposed to turbulent flow. This ecological pickiness makes some of them rare, restricted to specific habitats, and unable to survive in anthropised areas. Roughly seventy percent of freshwater red algae belong to the order Batrachospermales, whose members typically form macroscopic uniaxial gametophytes with whorled branches that give a bead-like appearance, or a pseudoparenchymatous thallus with a mucilaginous or cartilaginous consistency. Most have a heteromorphic triphasic life history comprising a sporophyte known as the Chantransia stage, a gametophyte, and a carposporophyte phase.

Molecular phylogenetics based on two markers now places the entire order in a single family, the Batrachospermaceae, with 21 genera including Lemanea and Paralemanea. The two genera have an intricate taxonomic history that culminated in their separation as distinct genera. Mitochondrial genome work has corroborated that split: Paralemanea species possess the largest genomes and most extensive intergenic regions sequenced to date within the Batrachospermales, and they are the only taxa in the order to contain introns, which are absent in all other members, including the sister genus Lemanea.

Morphologically, the genera are distinguished by a handful of diacritical characters. Lemanea carries spermatangia arranged in circular papillae at the nodes, has T- or L-shaped ray cells abutting the cortex, and a main axis without cortication. Paralemanea instead has spermatangia arranged in rings around the nodes, simple ray cells organised in two layers — a proximal layer that does not touch the outer cortex and a distal, generally Y-branched layer connected to the cortex — and cortical filaments surrounding the axial filaments. Some Paralemanea species also lack stalks. In Italy, reports of both genera are scarce and scattered, and in Sicily only three records of Lemanea exist, all referring to Lemanea fluviatilis. No Paralemanea species had ever been published for the island.

During field surveys from May to early July 2022 at two sites along the Alcantara River — Passo Mojo at 547 metres above sea level and Monte San Nicola at 400 metres, both in the territory of Castiglione di Sicilia — the team found numerous thalli referable to Lemanea and Paralemanea. The river flows eastwards between the Nebrodi and Peloritani mountains on its northern side and the volcano Etna on its southern side, and over millennia it has slowly eroded Etna’s hard lava rocks, creating rapids, waterfalls and narrow deep canyons alternating with typical riffle-run stretches. Samples were taken to the Laboratory of Phycology at the University of Catania, fixed in four percent formaldehyde or preserved in water and ethanol, and examined under an Olympus BX 40 microscope for the vegetative and reproductive characters needed to separate genera and species. Representative thalli were dried and deposited in the CAT herbarium, algae section, of the University of Catania.

The specimens collected at Monte San Nicola matched Lemanea fucina, a species with no previous reports anywhere in Italy. Its thalli formed light to dark green tufts about 10 to 13 centimetres long, stalked and gradually narrowing toward the base, with many branches and often secondary ones, while false branches appeared rarely where the thallus had broken off. Nodes and internodes were easily distinguished. The nodal basal portion measured 524 to 714 micrometres, the nodal median portion 643 to 809 micrometres and the nodal apical portion 619 to 786 micrometres, while the internodal portions ranged from 405 to 524 micrometres basally, 476 to 524 micrometres medially and 430 to 476 micrometres apically, giving a nodal-internodal ratio of 1.4 to 1.6. The central axial cell lacked a cortex and bore T-shaped ray cells, visible in both cross and longitudinal sections. The thalli were monoecious, with spermatangial papillae distributed in circular patches on the nodes and carposporophytes borne on the inner portion of the cortex. Carposporangia arranged in chains measured 35 to 40 micrometres long with a diameter of 20 to 30 micrometres. The Chantransia stage was not found.

Ecologically, L. fucina was found exclusively at Monte San Nicola, a natural site with rapids formed by erosive processes, where it colonised the surfaces of volcanic rocks within microhabitats such as concavities shaped by high-energy turbulent flow, which provided optimal substrates for thallus attachment. Only a few thalli were observed within a community dominated by P. catenata. Associated taxa included the cyanobacterium Nostoc and the macroalgae Cladophora and Vaucheria, which were abundant in areas with lower hydrodynamic stress and during the summer, when reduced river discharge produced less turbulent conditions and the subsequent desiccation of L. fucina.

Paralemanea catenata, by contrast, turned up at both sites. Its thalli formed turfs arising from a perennial basal crust, roughly 2 to 17 centimetres tall, with uniaxial, pseudoparenchymatous, unstalked, dark green and slightly curved upright axes lacking true branches, though young branches developed where axes were broken, forming the so-called false branches. The nodal basal portion varied between 381 and 520 micrometres, the nodal median between 596 and 820 micrometres and the nodal apical between 320 and 636 micrometres, while internodal portions measured 212 to 380, 437 to 540 and 400 to 485 micrometres respectively, with a nodal-internodal ratio of 1.3 to 1.4. The central axis was composed of numerous cortical filaments with radial cells in two layers, the proximal one not touching the outer cortex and the distal one Y-shaped and connected to it. The monoecious thalli bore spermatangial sori in narrow, sometimes interrupted nodal rings, and carposporangia in chains of 25 to 30 micrometres in length and 20 to 25 micrometres in diameter. Crucially, the reddish to brownish Chantransia stage was observed in several thalli, growing near the base of the gametophyte with alternating or unilateral branching, and in one case was seen producing juvenile gametophytes from its filaments.

At Monte San Nicola, P. catenata shared its habitat with L. fucina, while at Passo Mojo, an artificial water bridle, it colonised vertical surfaces beneath a small waterfall at the end of the concrete bridle. In early July, as river capacity decreases and rock surfaces dry, the thalli desiccate but the species persists as a perennial crust on the substrates. At both sites it formed extensive populations covering several square metres, becoming the physiognomic dominant, especially in areas more exposed to the flow. During summer, Cladophora and Vaucheria formed mosaic-like assemblages on surfaces colonised by P. catenata, growing between or above its thalli, and in some instances completely overgrew the basal crusts following the senescence of the gametophytes.

The findings extend the known European ranges of both species further south than previously reported, with P. catenata’s only prior Italian record coming from the Listi stream in Aspromonte, southern Italy. Interestingly, the carposporangia of both Sicilian species were smaller than literature values. The study also underscores how taxonomic ambiguity in the Batrachospermaceae — where diagnostic characters can vary with thallus age and maturity, and repeated revisions have left the same species described under different synonyms, as with L. fucina and L. mamillosa — complicates identification. Ecologically, the contrast is telling: L. fucina was restricted to the natural, low-anthropisation site, while P. catenata tolerated the more human-affected Passo Mojo, suggesting a broader ecological tolerance. The presence of carposporangia in both species may indicate healthy populations in non-anthropised stretches of the Alcantara. Because Lemanea and Paralemanea require turbulent water, climate change scenarios predicting reduced precipitation and river flow could favour competitively advantaged taxa such as Cladophora and Vaucheria at their expense, making sensitive freshwater red algae valuable early-warning indicators for the conservation and monitoring of river habitats under growing anthropogenic pressure.

Subject of Research: Botany

Article Title: First record of Lemanea fucina Bory in Italy and Paralemanea catenata (Kützing) Vis & Sheath ( Batrachospermaceae , Rhodophyta ) in Sicily (Italy)

Article References: Pagana, I., Di Stefano, M., Alongi, G., Cristaudo, A., & Emma, G. (2026). First record of Lemanea fucina Bory in Italy and Paralemanea catenata (Kützing) Vis & Sheath (Batrachospermaceae, Rhodophyta) in Sicily (Italy). Italian Botanist, 22, 51-63. https://doi.org/10.3897/italianbotanist.22.196629

Image Credits: AI Generated

DOI: 10.3897/italianbotanist.22.196629

Keywords: freshwater red algae, Batrachospermaceae, Lemanea fucina, Paralemanea catenata, Alcantara River, Sicily, biogeography, First, record, Lemanea, fucina, Bory