Caloplaca mereschkowskiana S. Y. Kondr. & Kärnefelt
Mycobank MB 517747
Thallus 0.5–2 cm wide, usually aggregated into larger colonies, appearing dull orange or brownish orange due to the numerous apothecia, thallus areoles yellowish white to greenish yellow-white or brownish, very thin, immersed and indistinct in the peripheral zone to rather thick and areolate in the centre, areoles immersed and almost indiscernible at the periphery to distinct in the centre, (0.2–)0.3–0.8(–1.3) mm wide, 0.3–0.4 mm thick. Apothecia 0.2–0.5(–0.7) mm diam., 0.15–0.22 mm thick, numerous, immersed at first, becoming sessile, 1–2 per areole, aggregated but regularly rounded, rarely crowded; disc plane to subconvex, dull orange, brownish yellow to brownish orange or somewhat orange-pink, ± with sparse whitish pruina; biatorine in section with a slightly paler proper margin, 0.03–0.05 mm wide, becoming excluded or very rarely zeorine; thalline exciple developed only at the base, 70–80 µm thick, cortical layer 10–15 µm thick; hymenium 50–60 µm high, hyaline, epihymenium brownish orange; subhymenium 80–100 µm thick; paraphyses swollen towards the tips to 4-6(–7) µm diam., usually with oil droplets 1–2(–4) µm diam., very rarely with oil cells 5–6(–7) µm diam. present; asci (1–2–4–6–)8-spored, bipolar and simple spores seen in the same ascus; ascospores distinctly widened at the septum, with rounded ends, varying from elongate-ellipsoid to almost spherical, (9–)10–12(–15) x 6–7 µm in water and 10–13(–14) x (5–)6–7(–8) µm in K (often with wrinkled surface in K), and rather narrow septa, 3–4(–6) µm wide in water and (4–)5–7 in K. Secondary chemistry: thallus K+ purple, epihymenium K+ purple or blackish purple in places, then dark cherry blossom pink to dark cherry blossom red; containing parietin (major), parietinic acid (minor), fallacinal (trace), teloschistin (trace).
Caloplaca mereschkowskiana belongs to the Caloplaca lactea group, characterized by a whitish thallus and yellow apothecia with small ascospores. It is similar to C. lactea (A. Massal.) Zahlbr. from which it differs by wider ascospore septa and absence of a well-developed thalline margin of the apothecia (Kondratyuk et al., 2004). The whitish thallus of C. mereschkowskiana resembles C. brachyspora Mereschk., a species that differs in having less aggregated apothecia, in lacking oil droplets and oil cells in the upper portion of paraphyses and in having narrower ascospore septa and the presence of a dark grey hypothallus (Kondratyuk et al., 2004). Further, C. mereshkowskiana differs from C. marmorata (Bagl.) Jatta and C. lacteoides Nav.-Ros. & Hladun, two species growing on limestone outcrops with reddish apothecia, by its smaller ascospores (Kondratyuk et al., 2004). Caloplaca yorkensis often grows together with C. mereschkowskiana but differs in having narrower ascospores and a more immersed thallus and apothecia without the pink orange-pink tinge observed in C. mereschkowskiana.
This species is named after the Russian biologist, Konstantin Sergejewicz Mereschkowsky (1855–1921), known for his hypothesis on symbioses, who described C. brachyspora belonging to this species group.
Common on limestone outcrops in Western Australia, South Australia, Victoria and Tasmania.
Type:—AUSTRALIA: Western Australia: North of Northampton, road to Horrocks, Bower River Road, on calcareous rocks; 28°24’S, 114°27’E; Jan 2004, Kärnefelt et al. 20041503 (holotype PERTH; isotypes CANB, LD, KW).
Additional specimens examined (paratypes):—AUSTRALIA: Western Australia: E. Wallabi, N end of airstrip WA, Apr 1980, Green s.n. (PERTH). Nullarbor Plain, Eyre Highway, near Eucla Pass, 1 km W of Eucla; Curnow 4753 (CANB). N of Northampton, road to Horrocks, Bower River Road; Kärnefelt 20041502 (LD). 0.25 km E of mouth of Bower River, S of Horrocks Beach, Kondratyuk et al. 20421 (KW). South Australia: Salt Creek; Bratt 67/138 (HO 64728). 15 km W of Peake along Highway 12, Elix 8727 (MEL). Eyre Peninsula, 28.3 km E of Cowell on the Lincoln Highway, Filson 11795 (MEL). Nullarbor Homestead; Jan 1952, Kempsey s.n. (MEL). Victoria: Buchan-Gelantipy road, 6 km NNE of Buchan; Streimann 39787 (CANB). Tasmania: Bass Strait, Hogan Group, Hogan Island; 4 m; Dec 1973, Whinray s.n. (MEL).
Caloplacae brachysporae similis, sed apotheciis aggregatis, paraphysibus supra cum cellulis et guttulis oleosis, ascis saepe 1–2–4–6-sporis, ascosporis latius septatis (3–4(–6) µm), et hypothallo obscure cinereo deficiente differt.