{{Short description|Species of lichen}} {{bots|deny=Citation bot}} {{Use dmy dates|date=April 2025}} {{Use British English|date=April 2025}} {{Speciesbox | image = | image_caption = | taxon = Porina rivalis | authority = [[Alan Orange|Orange]] (2015) }}

'''''Porina rivalis''''' is a rare species of semi-aquatic, [[saxicolous lichen|saxicolous]] (rock-dwelling), and [[crustose lichen]] in the family [[Porinaceae]]. First formally described in 2015, it had previously been misidentified as ''[[Porina lectissima]]''. The lichen features a thin, light orange-brown to grey-brown [[thallus]] and dark reproductive structures called [[perithecia]]. It specifically inhabits [[siliceous rock]]s in streams with moderate [[pH]] levels that are neither strongly acidic nor nutrient-rich, where it associates with other semi-aquatic lichens and [[bryophyte]]s. Classified as "Nationally Rare" in Britain, ''P. rivalis'' is best identified by its distinctive semi-aquatic habitat preference.

==Taxonomy==

The lichen was [[species description|scientifically described]] as a new species in 2015 by the lichenologist [[Alan Orange]]. The [[type (biology)|type specimen]] was collected in Nant Walch near [[Llanwrtyd Wells]] ([[Breconshire]]) at an altitude of {{convert|230|m|ft|abbr=on}}; here the lichen was found growing on stones submerged in a shaded stream. The species had been known previously from streams in Wales, but it had been incorrectly identified as ''[[Porina lectissima]]''. ''Porina rivalis'' has also been recorded from streams in South-west England and North England.<ref name="Orange 2015"/>

==Description==

''Porina rivalis'' has a thin, typically light orange-brown to grey-brown or dark grey [[thallus]] (main body). The orange tints in the thallus tend to fade when specimens are stored. The thallus measures between 20–70&nbsp;[[μm]] in thickness and appears either continuous or with occasional cracks throughout its surface. A very thin brown {{lichengloss|prothallus}} (initial fungal growth stage) has been observed rarely. The lichen contains a {{lichengloss|trentepohlioid}} photobiont, referring to the type of algae that lives in symbiosis with the fungal component. Its reproductive structures, called [[perithecia]], are prominent and measure 160–400&nbsp;μm in diameter. These perithecia are typically dark brown or black, though they can sometimes appear orange-brown or brown at their base.<ref name="Orange 2015"/>

The {{lichengloss|involucrellum}} (protective outer layer of the perithecium) consists of thick-walled cells of equal dimensions that enclose numerous photobiont cells without crystals. The inner portion of the involucrellum ranges from yellow to orange and produces an orange-red reaction when tested with [[potassium hydroxide]] solution ([[K test|K+]]). The upper surface area is dark grey to purplish red, turning dark grey or bluish grey when tested with potassium hydroxide. A small area near the [[ostiole]] (opening) often displays a dark dull violet colouration.<ref name="Orange 2015"/>

The centrum (inner mass of the perithecium) measures 185–295&nbsp;μm in diameter, with an {{lichengloss|exciple}} (inner wall) that is either colourless or yellow. The [[ascus]] (spore-producing structure) is more or less cylindrical with thin walls, does not react with iodine (I−), and features a truncate apex with a ring structure. The {{lichengloss|ascospores}} (reproductive spores) are narrowly [[ellipsoid]] with three [[septum|septa]] (internal divisions), measuring 13.0–17.5&nbsp;μm in length and 4.0–5.5&nbsp;μm in width, making them roughly 2.3–4.1 times longer than they are wide.<ref name="Orange 2015"/>

==Habitat and distribution==

''Porina rivalis'' is found on frequently inundated [[siliceous rock]]s alongside streams that are neither strongly acidified nor enriched with nutrients. [[Water chemistry]] studies conducted at two sites in Mid-Wales during 2011–2013 revealed that the species prefers in environments with [[pH]] values ranging from 5.1 to 7.2, [[Conductivity (electrolytic)|conductivity]] between 32–56&nbsp;[[μS]] per cm, and [[calcium]] concentrations of 1.7–4.6 milligrammes per litre.<ref name="Orange 2015"/>

This lichen species is typically associated with other semi-aquatic organisms, including lichens such as ''[[Dermatocarpon luridum]]'', ''[[Ephebe lanata]]'', ''[[Ionaspis lacustris]]'', and various ''[[Verrucaria]]'' species, as well as [[bryophyte]]s like ''[[Heterocladium heteropterum]]'', ''[[Hygrohypnum ochraceum]]'', and ''[[Scapania undulata]]''.<ref name="Orange 2015"/>

Geographically, ''P.&nbsp;rivalis'' has been documented in at least 10 streams across Mid and North Wales, with additional records from single streams in South-west England and North England. The species is best identified in the field by its distinctive semi-aquatic habitat preference, which distinguishes it from related species that may occasionally tolerate submersion but are not true members of semi-aquatic lichen communities.<ref name="Orange 2015"/> It is classified as "Nationally Rare" in Britain, meaning is has been recorded in 15 or fewer 10 km squares.<ref name="BLS"/>

==See also== * [[List of Porina species|List of ''Porina'' species]]

==References== {{Reflist|refs=

<ref name="BLS">{{cite web |title=''Porina rivalis'' | website=The British Lichen Society |date=2023-02-12 |url=https://britishlichensociety.org.uk/resources/species-accounts/porina-rivalis |access-date=2 April 2025}}</ref>

<ref name="Orange 2015">{{cite journal |last=Orange |first=Alan |title=A new freshwater ''Porina'' (Porinaceae, Ostropales) from Great Britain |journal=The Lichenologist |volume=47 |issue=6 |year=2015 |doi=10.1017/s0024282915000365 |pages=351–358}}</ref>

}}

{{Taxonbar|from=Q108282698}}

[[Category:Porina|rivalis]] [[Category:Lichen species]] [[Category:Lichens described in 2015]] [[Category:Lichens of Northern Europe]] [[Category:Taxa named by Alan Orange]]