Polysiphonia morrowii Harvey 1857

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 : 157

3.5 Polysiphonia morrowii Harvey 1857

Polysiphonia morrowii was originally described from Japan, and its native range is the north-western Pacific ( Chihara 1970; Kudo and Masuda 1992). The species was introduced in several locations in the Pacific Ocean ( Kim et al. 2004; Raffo et al. 2014) and in the Atlantic Ocean ( Geoffroy et al. 2016). This filamentous species, belonging to the Tribe Polysiphonieae in the Rhodomelaceae can be difficult to identify. The occurrence of uncut rhizoids ( Figure 8e ), arising directly from periaxial cells, distinguishes P. morrowii from most other Polysiphonia s. l., but it was previously misidentified with the native Polysiphonia stricta (Mertens ex Dillwyn) Greville (see illustrations for P. stricta in Loiseaux-de Goër and Noailles 2008, pp. 118–119). It differs from the latter species by the shape of its apical cells, which are always rounded in P. stricta , but spiky in P. morrowii ( Figure 8b, c and f ). Plants of P. morrowii collected in Brittany can be found as epiliths in tidepools or on pontoons, growing up to 25–30 cm ( Figure 8a ), and as a floating mass of freeliving entangled filaments in marinas. The species was also found in intertidal maerl beds, growing on Lithothamnion corallioides ( Helias and Burel 2023) . In transverse section, plants of P. morrowii have a central cell surrounded by four periaxial cells ( Figure 8d ). Tetrasporangia develop on ultimate and auxiliary branchlets, in series of 3–5 mature tetrasporangia ( Figure 8g ). Some plants bearing urceolate cystocarps were observed. Intertidal plants living in tidepools are most fully developed during winter and early spring, disappear in summer and are absent in autumn.

Figures

Figure 8: Polysiphonia morrowii from Brittany:(a) dried specimen from Brest marina (Herbarium M.Helias);(b)general habit showing characteristic spike branches; (c) apex with secondary branches giving a spike aspect; (d) transverse section with four pericentral cells; (e) rhizoid with open section; (f) variation in branch tips; (g) tetrasporangia. Scale bars: (a) 5 cm; (b) 500 µm; (c) 100 µm; (d–g) 20 µm.