Actinotaenium spinosporum Coesel et Van Westen, 2014

Actinotaenium spinosporum Coesel et Van Westen , spec. nov. ( Figs. 11–19 View FIGURES 8–12 View FIGURES 13–21 )

Cells in frontal view cylindric with a very slight median constriction and broadly rounded apices. Cells in apical view circular. Cell wall smooth. Chloroplast lobostelloid. Each semicell with seven or eight cell wall pores: one in the centre of the apex and six or seven arranged in a ring about half-way the length of the semicell. Zygospores globose, furnished with simple, rather long spines. Dimensions: cell length 8.8–9.5 µm, cell breadth 4.8–6.6 µm. Diameter of zygospores exclusive of spines 9.0– 10.7 µm, including spines 14.3–18.1 µm.

Type:— THE NETHERLANDS. Drenthe: pool ‘Baggelhuizen’ near Assen , 53.106° N, 6.648° E, among submerged mosses on sandy soil of the bank, pH 7.2, conductivity 50 µS cm -1, Van Westen, 25 May 2010 (holotype L! Hugo de Vries Lab 2013.01, preserved as a fixed natural sample) GoogleMaps .

Differential diagnosis: —Vegetative cells of A. spinosporum ( Figs. 11 View FIGURES 8–12 , 13, 14, 17, 18 View FIGURES 13–21 ) may be confused with those of some other, small-sized Actinotaenium species , in particular A. subsparsepunctatum ( Grönblad 1921: 38) Coesel (2002: 76) and A. subtile (W. et G.S. West 1897: 479) Teiling (1954: 410) which have a comparable cell shape and chloroplast. However, A. subsparsepunctatum in the original description ( Grönblad 1921, as Cosmarium subtile var. subsparsepunctatum ) has a cell size that is about twice as large and a cell wall with evenly scattered pores, whereas the zygospores are supplied with short, bifurcate, blunt spines ( Coesel 2002). Actinotaenium subtile , in the original description by West & West (1897, as Penium subtile ) adopted in their British desmid flora ( West & West 1904), is characterized by cells with a subtruncate apex and evenly scattered cell wall pores. Moreover, cell dimensions are also about twice as large as those in our A. spinosporum .

As its name indicates, one of the main characteristics of A. spinosporum is the spiny zygospore ( Figs. 12 View FIGURES 8–12 , 15, 16, 19 View FIGURES 13–21 ). According to various floras (e.g. Prescott et al. 1981, Růžička 1981), there are but very few Actinotaenium species known to have such spinous zygospores, so it is a most relevant diagnostic feature. Presumably, because of their extremely small cell dimensions, vegetative populations of A. spinosporum are easily overlooked or misidentified. For instance, A. perminutum (G.S. West 1914: 1041) Teiling (1954: 410) as described in Szelag-Wasielewska & Tomaszewicz (2003) does not concern A. perminutum but most likely A. spinosporum .

Ecology: —In the type locality ‘Baggelhuizen’ an abundantly sporulating population of A. spinosporum was encountered in spring 2010. Later on, sporulating populations of this species were also found in a shallow pool in a former agricultural land (53.106° N, 6.648° E, pH 6.4, conductivity 58 µS cm -1) and in small pools on the site ‘ Hondstongen’ (53.106° N, 6.570° E; pH 5.4, conductivity 47 µS cm -1). Bijkerk & Bultstra (2012) appear to have found a stable population of this species in the ‘ Besthmenerven’ for many years (52.495° N, 6.444° E, pH 4.2–5.5, conductivity 21–30 µS cm -1). This   GoogleMaps highly acidic, oligotrophic site differs in habitat from our oligo-mesotrophic and less acidic Drenthe sites. Maybe   GoogleMaps this species has a rather wide ecological range.