Frustules cylindrical in girdle view, usually occurring singly or in short chains of 2–4 cells. Valves disc-shaped, 24–65 µm in diameter, mantle 11–13 µm deep. Valve surface flat, forming a nearly right angle with the mantle. Mantle striae uniseriate, 18–21/ 10 µm. Areolae on the valve face small, round, irregularly-scattered and rarely forming straight continuous striae restricted to the outer 2/3 of the valve face with the remaining surface forming an hyaline area pitted and containing carinoportulae. Carinoportulae relatively large, numbering 1–3. Marginal spines are usually straight but can but forked. Spines number 2–4/ 10 µm. No caverns or internal undulations present. Internal valves wanting. Copulae open, numbering 5–7/cell.
Type: — Pua Po’o lave tube, Volcanoes National Park, Hawai’i, slide no. BISH-755092 (Holotype, Bishop Museum of Natural History, Honolulu), ANSP-20040 (Isotype Academy of Natural Sciences, JPK-COLO 4195 (Isotype Kociolek Collection, University of Colorado, Boulder).
Etymology:— the specific epithet johansenii refers to our friend and colleague Dr. Jeffrey Johansen, John Carroll University (Cleveland, USA).
Observations:— In the SEM, the valve face is flat (Fig. 42) to weakly domed/concave (Fig. 43). Hyaline central area smooth (Figs 42, 43) and may be reduced due to irregularly-placed areolae (Fig. 42). Areolae in defined striae near the margin becoming irregularly spaced towards the valve centre (Figs 42, 45), small and poroid, sometimes surrounded by a slightly raised rim (Figs 44, 45). No difference in areolae structure or size between valve face and mantle (Fig. 42). Areolae on the mantle always arranged in parallel striae (Figs 45, 46). External marginal pore fields between spines absent (Fig. 45). Carinoportulae distinct in the central area, often surrounded by raised rosettes or fine ridges (Fig. 44), each with a pronounced (sometimes even conical) silica collar (Fig. 44). Valve face/mantle junction abrupt with rounded edge (Fig. 45). Ring of large linking spines may be present (Figs 45, 46), but in this species more frequently encountered are stout siliceous thickenings (Figs 45, 46). Linked spines plate-like, bifurcated or with a quite complex shape (Figs 45, 46). On the mantle small, irregular thickenings visible between the areolae (Fig. 45). Internally, valve face flat, smooth (Figs 48, 49). Areolae appearing as small rounded poroids (Fig. 48), with a velum (Figs 50, 51). Carinoportulae unoccluded, with small, simple and rounded internal openings (Figs48, 49, 52). No other processes are present on the valve interior. Copulae open, with very small poroids scattered or organized into one straight row and others scattered, parallel to the pervalvar axis (Figs 47). Sometimes copulae ornamentation is but a few poroids (Fig. 47).
Ecology:— The internal surface of this lava tube was damp, with soil, bryophytes and exposed lava representing the primary microhabitats. Present along with O. johansenii were Nupela sp., Melosira sp. Kobayasiella sp. Diadesmis biceps G.A.Arnott ex Grunow in Van Heurck 1880: 14 /31b, Nitzschia hantzschiana Rabenh. (1860: 40) and Pinnularia divergentissima (Grunow in Van Heurck 1880: 6 /32) Cleve 1895: 77. Of these diatoms, O. johansenii was the least abundant taxon.