# Porina rivalis

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Species of lichen

Porina rivalis Scientific classification Kingdom: Fungi Division: Ascomycota Class: Lecanoromycetes Order: Gyalectales Family: Porinaceae Genus: Porina Species: P. rivalis Binomial name Porina rivalis Orange (2015)

***Porina rivalis*** is a rare species of semi-aquatic, [saxicolous](/source/Saxicolous_lichen) (rock-dwelling), and [crustose lichen](/source/Crustose_lichen) in the family [Porinaceae](/source/Porinaceae). First formally described in 2015, it had previously been misidentified as *[Porina lectissima](https://en.wikipedia.org/w/index.php?title=Porina_lectissima&action=edit&redlink=1)*. The lichen features a thin, light orange-brown to grey-brown [thallus](/source/Thallus) and dark reproductive structures called [perithecia](/source/Perithecia). It specifically inhabits [siliceous rocks](/source/Siliceous_rock) in streams with moderate [pH](/source/PH) levels that are neither strongly acidic nor nutrient-rich, where it associates with other semi-aquatic lichens and [bryophytes](/source/Bryophyte). Classified as "Nationally Rare" in Britain, *P. rivalis* is best identified by its distinctive semi-aquatic habitat preference.

## Taxonomy

The lichen was [scientifically described](/source/Species_description) as a new species in 2015 by the lichenologist [Alan Orange](/source/Alan_Orange). The [type specimen](/source/Type_(biology)) was collected in Nant Walch near [Llanwrtyd Wells](/source/Llanwrtyd_Wells) ([Breconshire](/source/Breconshire)) at an altitude of 230 m (750 ft); 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](https://en.wikipedia.org/w/index.php?title=Porina_lectissima&action=edit&redlink=1)*. *Porina rivalis* has also been recorded from streams in South-west England and North England.[1]

## Description

*Porina rivalis* has a thin, typically light orange-brown to grey-brown or dark grey [thallus](/source/Thallus) (main body). The orange tints in the thallus tend to fade when specimens are stored. The thallus measures between 20–70 [μm](/source/%CE%9Cm) in thickness and appears either continuous or with occasional cracks throughout its surface. A very thin brown [prothallus](/source/Glossary_of_lichen_terms#prothallus) (initial fungal growth stage) has been observed rarely. The lichen contains a [trentepohlioid](/source/Glossary_of_lichen_terms#trentepohlioid) photobiont, referring to the type of algae that lives in symbiosis with the fungal component. Its reproductive structures, called [perithecia](/source/Perithecia), are prominent and measure 160–400 μm in diameter. These perithecia are typically dark brown or black, though they can sometimes appear orange-brown or brown at their base.[1]

The [involucrellum](/source/Glossary_of_lichen_terms#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](/source/Potassium_hydroxide) solution ([K+](/source/K_test)). 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](/source/Ostiole) (opening) often displays a dark dull violet colouration.[1]

The centrum (inner mass of the perithecium) measures 185–295 μm in diameter, with an [exciple](/source/Glossary_of_lichen_terms#exciple) (inner wall) that is either colourless or yellow. The [ascus](/source/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 [ascospores](/source/Glossary_of_lichen_terms#ascospores) (reproductive spores) are narrowly [ellipsoid](/source/Ellipsoid) with three [septa](/source/Septum) (internal divisions), measuring 13.0–17.5 μm in length and 4.0–5.5 μm in width, making them roughly 2.3–4.1 times longer than they are wide.[1]

## Habitat and distribution

*Porina rivalis* is found on frequently inundated [siliceous rocks](/source/Siliceous_rock) alongside streams that are neither strongly acidified nor enriched with nutrients. [Water chemistry](/source/Water_chemistry) studies conducted at two sites in Mid-Wales during 2011–2013 revealed that the species prefers in environments with [pH](/source/PH) values ranging from 5.1 to 7.2, [conductivity](/source/Conductivity_(electrolytic)) between 32–56 [μS](/source/%CE%9CS) per cm, and [calcium](/source/Calcium) concentrations of 1.7–4.6 milligrammes per litre.[1]

This lichen species is typically associated with other semi-aquatic organisms, including lichens such as *[Dermatocarpon luridum](/source/Dermatocarpon_luridum)*, *[Ephebe lanata](/source/Ephebe_lanata)*, *[Ionaspis lacustris](https://en.wikipedia.org/w/index.php?title=Ionaspis_lacustris&action=edit&redlink=1)*, and various *[Verrucaria](/source/Verrucaria)* species, as well as [bryophytes](/source/Bryophyte) like *[Heterocladium heteropterum](https://en.wikipedia.org/w/index.php?title=Heterocladium_heteropterum&action=edit&redlink=1)*, *[Hygrohypnum ochraceum](https://en.wikipedia.org/w/index.php?title=Hygrohypnum_ochraceum&action=edit&redlink=1)*, and *[Scapania undulata](https://en.wikipedia.org/w/index.php?title=Scapania_undulata&action=edit&redlink=1)*.[1]

Geographically, *P. 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.[1] It is classified as "Nationally Rare" in Britain, meaning is has been recorded in 15 or fewer 10 km squares.[2]

## See also

- [List of *Porina* species](/source/List_of_Porina_species)

## References

1. ^ [***a***](#cite_ref-Orange_2015_1-0) [***b***](#cite_ref-Orange_2015_1-1) [***c***](#cite_ref-Orange_2015_1-2) [***d***](#cite_ref-Orange_2015_1-3) [***e***](#cite_ref-Orange_2015_1-4) [***f***](#cite_ref-Orange_2015_1-5) [***g***](#cite_ref-Orange_2015_1-6) Orange, Alan (2015). "A new freshwater *Porina* (Porinaceae, Ostropales) from Great Britain". *The Lichenologist*. **47** (6): 351–358. [doi](/source/Doi_(identifier)):[10.1017/s0024282915000365](https://doi.org/10.1017%2Fs0024282915000365).

1. **[^](#cite_ref-BLS_2-0)** ["*Porina rivalis*"](https://britishlichensociety.org.uk/resources/species-accounts/porina-rivalis). *The British Lichen Society*. 12 February 2023. Retrieved 2 April 2025.

Taxon identifiers Porina rivalis Wikidata: Q108282698 CoL: 4LXNB GBIF: 10741254 IndexFungorum: 814184 MycoBank: 814184 NBN: BMSSYS0000050016 SpeciesFungorum: 814184

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Adapted from the Wikipedia article [Porina rivalis](https://en.wikipedia.org/wiki/Porina_rivalis) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Porina_rivalis?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.