Encyonopsis minuta Krammer & Reichardt, 1997

Kennedy, Bryan, Buckley, Yvonne & Allott, Norman, 2019, Taxonomy, ecology and analysis of type material of some small Encyonopsis with description of new species in Ireland, Phytotaxa 395 (2), pp. 89-128 : 107-110

Encyonopsis minuta Krammer & Reichardt ( Figs 288–352 )

Description: Outline symmetric to weakly dorsiventral, lanceolate with weakly convex dorsal and ventral sides. Poles are capitate to subcapitate, always broadly rounded and at least slightly protracted even in diminutive valves. Typically lacking clear shoulders with a gently sloping outline. Axial area is very narrow and central area absent. Length 8.0– 17.0 μm; width 2.8–3.8 μm; maximum L:W ratio 4.9. Striae parallel to weakly radiate and rarely with only a slightly shortened single stria at centre in smaller valves. Striae appearing quite uniform and easily visible over valve length towards poles. Maximum stria density in Irish populations higher than reported for type description (~ 30–32 in 10 μm). Raphe filiform, dorsally deflected at centre, with a distinctive short section recurved back towards centre on ventral side and lacking obvious expanded terminal raphe endings. Areolae always small and rounded.

Easily confused with E. microcephala in past studies but populations converge and may be difficult to separate in LM. The size reduction series ( Figs 320–352 ) are less clearly protracted and have a higher maximum stria density but retain the distinctive ultrastructure of more clearly capitate frustules. The general size, outline and broadly rounded poles in combination with relatively coarse sub-parallel striae are often sufficiently clear to permit identification in most cases.

Ecology: Described as cosmopolitan and well distributed in suitable habitats in temperate to boreal regions. Well distributed in this dataset occurring in 40 lakes, reaching a maximum relative abundance of 16.5%. Density is greatest at the upper end of the alkalinity gradient and a total phosphorous optimum near 10 μg/l-P suggests a diatom sensitivity score of S=1 for lake ecological assessment.

Figures

FIGURES 288–319. Encyonopsis minuta Krammer & Reichardt Figs 288–319. LM. Figs 288–311. Valves showing morphological range and size variation in populations with clearly capitate apices. Figs 312–319. SEM. Figs 312–315. External view. Figs 316–318. Internal view. Fig. 319. Internal view of raphe fissure with clear intermissio visible. Scale bar in Fig. 311 = 10 μm for all LM figures. Scale bar in all SEM micrographs = 1 μm.

FIGURES 320–352. Encyonopsis minuta Krammer & Reichardt Figs 320–345.LM. Figs 320–345.Illustrating morphology of populations with a size reduction series. Figs 346–352. SEM. Figs 346–350. External view with valves maintaining bluntly rounded poles. Figs 351– 352. Internal view showing distinctive intermissio at central area. Scale bar in Fig. 345 = 10 μm for all LM figures. Scale bar in all SEM micrographs = 1 μm.