Catillaria typically grows as a thin, crust-like layer (crustose) on various surfaces. The fungal body (thallus) can appear in several forms – it may be barely visible, cracked, warty, or divided into small polygonal areas called areoles. These lichens display various colours including white, grey, green, brown, or black, though some species that grow on other lichens may lack a visible thallus entirely.
Like all lichens, Catillaria represents a symbiotic partnership between a fungus and photosynthetic algae (known as the photobiont). In this genus, the algal partners are green algae, specifically from genera such as Dictyochloropsis, Myrmecia, or Trebouxia. The fungal portion lacks a protective outer layer (cortex) or has only a rudimentary one.
The reproductive structures (apothecia) are black and typically lack a powdery coating (pruina). They have a distinctive microscopic structure, including specialised cells called paraphyses that have abruptly swollen tips capped with dark brown pigmentation. The spore-producing cells (asci) typically contain eight spores, though occasionally up to 16, and show a characteristic blue reaction when treated with iodine-based stains. The spores themselves are colourless and divided into two cells by a single wall (septum), without any surrounding gelatinous sheath.
The genus can be distinguished from similar lichens by its combination of asci that turn blue with iodine, paraphyses with dark-capped swollen tips, two-celled spores without a gelatinous coating, and spore-producing cells arranged in chains. While the similar genus Halecania shares many of these features, it differs in having thick-walled spores with a distinct gelatinous coating and is not closely related based on genetic analysis. When analyzed chemically using thin-layer chromatography, these lichens do not show evidence of specialised lichen products.
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↑Coppins, Brian J.; Aptroot, André (2008). "New species and combinations in The Lichens of the British Isles". The Lichenologist. 40 (5): 363–374. doi:10.1017/S0024282908008165.
↑Tretiach, M.; Hafellner, J. (1998). "A new species of Catillaria from coastal Mediterranean regions". The Lichenologist. 30 (3): 221–229. doi:10.1006/lich.1998.0126.
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