Antithamnion hubbsii E.Y. Dawson 1962

Burel, Thomas, Helias, Mathieu & Duff, Michel Le, 2023, Range expansion of some non-indigenous seaweeds along the coasts of Brittany - English Channel, Botanica Marina (Warsaw, Poland) 66 (3), pp. 151-164 : 153

3.1 Antithamnion hubbsii E.Y. Dawson 1962

The filamentous Antithamnion hubbsii is a Ceramiaceae originating from the Pacific Ocean. This species has been the subject of nomenclatural and typification problems and the type locality is still discussed between Japan and Baja California, Mexico ( Athanasiadis 2016; Cho et al. 2005). In Europe, A. hubbsii was first found in the Mediterranean Sea in 1988 in the Thau Lagoon, France, as Antithamnion nipponicum Yamada et Inagaki , and then in 1994 in Venice Lagoon, Italy, as A. pectinatum (Montagne) Brauner in Athanasiadis et Tittley ( Curiel et al. 1996). It was probably introduced via oysters transfers, as already reported for many seaweed species in both lagoons ( Sfriso and Curiel 2007; Verlaque 2001). It was then reported as A. nipponicum in the Atlantic Ocean, firstly in 2004 in the South Basque country ( Secilla et al. 2007), and secondly in 2007 in Norway ( Rueness et al. 2007). The first specimen on the coasts of France was found in 2011 in the North Basque country, probably arriving via marginal colonization ( Bárbara and DÍaz-Tapia 2012). Its finding on pontoons in the Gulf of Morbihan, Brittany in 2017 ( Le Roux 2018), then in the marinas of Brest in 2018, of Bénodet and Concarneau in 2020 and of Perros-Guirec in 2022 (this study), rather suggests a mode of propagation via maritime traffic. Fouling has also been suspected to explain the progression of the species in the English Channel and its probable spread northward ( Baldock 2020). Specimens sampled during this study were only found attached to floating pontoons mainly during spring and summer, or epiphytic on various red macroalgae, Fucus spp. and Ulva spp. Like other Antithamnion species, thalli of A. hubbsii are characterized by uniseriate, ecorticate filaments with typical opposite ramifications and secondary ramification ( Figure 3c ). The larger specimens were up to 2.5 cm high ( Figure 3a ), but usually around 1 cm ( Figure 3b ). Specimens grow characteristic refringent glandular cells on basal cells of the lateral branches ( Figure 3e ). Fertile thalli were observed in the Marina of Brest, developing tetrasporangia replacing adaxial branchlets (55–70 µm × 40–55 µm, Figure 3d ). As it can be easily confused with other Antithamnion spp. , a diagram of the branching patterns of the five Antithamnion species is shown in Figure 4.

Figures

Figure 3: Antithamnion hubbsii from Brittany: (a) dried specimen from Brest marina (herbarium of the European Institute for Marine Studies); (b) general habit; (c) characteristic segment showing a cell of the main axis, the opposite distichous branching and the opposite branchlets bearing glandular cells; (d) portion of axis with tetrasporangia; (e) branches and branchlets bearing glandular cells (arrowheads). Scale bars: (a) 5 mm; (b) 2 mm; (c–e) 50 µm.