# Yaukthwa

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Genus of mussels

Yaukthwa Shell morphology of Yaukthwa species in Myanmar, photograph taken by Ekaterina S. Konoplev Scientific classification Kingdom: Animalia Phylum: Mollusca Class: Bivalvia Order: Unionida Family: Unionidae Subfamily: Rectidentinae Tribe: Contradentini Genus: Yaukthwa Konopleva et al., 2019

***Yaukthwa*** is a genus of freshwater mussels in the family [Unionidae](/source/Unionidae), subfamily [Rectidentinae](https://en.wikipedia.org/w/index.php?title=Rectidentinae&action=edit&redlink=1), tribe [Contradentini](https://en.wikipedia.org/w/index.php?title=Contradentini&action=edit&redlink=1). The genus is [endemic](/source/Endemism) to the Western Indochina Subregion, with species distributed across the [Ayeyarwady](/source/Ayeyarwady_River), [Sittaung](/source/Sittaung_River), [Bago](/source/Bago_River), and [Salween](/source/Salween_River) river drainages in [Myanmar](/source/Myanmar) and northwestern [Thailand](/source/Thailand).[1]

The genus name derives from the Burmese word *yaukthwa*, meaning "freshwater bivalve".

## Taxonomy

*Yaukthwa* was described by Konopleva *et al*. in 2019 following an integrative revision of the genus *[Trapezoideus](https://en.wikipedia.org/w/index.php?title=Trapezoideus&action=edit&redlink=1)* Simpson, 1900 [1]. Molecular phylogenetic analyses based on mitochondrial (*COI*, *16S rRNA*) and nuclear (*28S rRNA*) gene fragments demonstrated that *Trapezoideus* as previously circumscribed was non-monophyletic. Several of its western Indochina species formed a well-supported distinct clade, which was described as *Yaukthwa* **gen. nov.**, with *Yaukthwa nesemanni* designated as the type species [1].

*Yaukthwa* is placed within the tribe Contradentini of the subfamily Rectidentinae, and is phylogenetically distant from related genera such as *Trapezoideus*, *Physunio*, and *Contradens*, which are distributed east of the Salween–Mekong drainage divide. [2]

The genus has had the following synonymised or transferred names:

Former name Authority Notes Trapezoideus nesemanni Konopleva, Vikhrev & Bolotov, 2017 [3] Transferred to Yaukthwa as type species [1] Trapezoideus panhai Konopleva, Bolotov & Kondakov, 2017 [3] Transferred to Yaukthwa [1] Trapezoideus dallianus Frierson, 1913 [4] Transferred to Yaukthwa [1] Trapezoideus zayleymanensis Preston, 1912 [5] Transferred to Yaukthwa [1] Trapezoideus peguensis Anthony, 1865 [6] Transferred to Yaukthwa [1] Indonaia rectangularis Tapparone-Canefri, 1889 [7] Transferred to Yaukthwa [8] Balwantia baniensis Bolotov et al., 2020 [9] Transferred to Yaukthwa [8]

The recognised species (including all transferred species and newly found species) are:

1. *Yaukthwa nesemanni* (Konopleva, Vikhrev & Bolotov, 2017)

1. *Yaukthwa panhai* (Konopleva, Bolotov & Kondakov, 2017)

1. *Yaukthwa dallianus* (Frierson, 1913)

1. *Yaukthwa zayleymanensis* (Preston, 1912)

1. *Yaukthwa peguensis* (Anthony, 1865)

1. *Yaukthwa paiensis* (Konopleva *et al.*, 2019) [1]

1. *Yaukthwa inlenensis* (Konopleva *et al.*, 2019) [1]

1. *Yaukthwa elongatula* (Konopleva *et al.*, 2019) [10]

1. *Yaukthwa avaensis* (Bolotov et al., 2020) [9]

1. *Yaukthwa rectangularis* (Tapparone-Canefri, 1889)

1. *Yaukthwa baniensis* (Bolotov et al., 2020)

## Distribution

Distribution ranges of Trapezoideus foliaceus and species in the genus Yaukthwa

*Yaukthwa* is endemic to the Western Indochina Subregion. Species have been recorded from the [Ayeyarwady](/source/Irrawaddy_River), [Sittaung](/source/Sittaung_River), [Bago](/source/Bago_River), and [Salween](/source/Salween_River) river drainages in [Myanmar](/source/Myanmar), and from the Salween Basin in [northwestern Thailand](/source/Northern_Thailand).[1] The two species described as new to science in the founding revision, *Yaukthwa paiensis* and *Y. inlenensis*, are both endemic to the Salween River basin, with *Y. paiensis* known from the [Khong River](https://en.wikipedia.org/w/index.php?title=Khong_River&action=edit&redlink=1) in northwestern Thailand and *Y. inlenensis* from the [Mway Stream](https://en.wikipedia.org/w/index.php?title=Mway_Stream&action=edit&redlink=1) tributary in Myanmar.[1]

As part of the broader Western Indochina Subregion, the drainages inhabited by *Yaukthwa*, particularly the [Ayeyarwady](/source/Irrawaddy_River), [Bago](/source/Bago_River), [Sittaung](/source/Sittaung_River), and [Salween](/source/Salween), form a distinct freshwater biogeographic unit separated from the Indian Subregion to the west by the [Naga Hills](/source/Naga_Hills), [Chin Hills](/source/Chin_Hills), and [Rakhine Yoma](/source/Rakhine_Yoma) mountain ranges, and from the [Sundaland](/source/Sundaland) Subregion to the south by the [Isthmus of Kra](/source/Isthmus_of_Kra).[9] The genus belongs to an endemic clade of Western Indochina, where it constitutes part of a larger radiation of freshwater bivalves characterised by high levels of species-level endemism.[10]

Many species are known only from their type localities, reflecting the limited extent of freshwater mussel sampling in the region and suggesting that further survey work may reveal additional populations or taxa.[1]

## Morphology

Shell of (a) *Yaukthwa" cf. "dalliana*, Nanuinhka Chaung River, Ayeyarwady Basin, Myanmar. (b) "Yaukthwa panhai", Kyan Hone River, Sittaung Basin, Myanmar. (c) "Yaukthwa nesemanni", Thauk Ye Kupt River, Sittaung Basin, Myanmar

Members of *Yaukthwa* have a medium-sized shell, transitioning from obovate in juvenile specimens to trapezoidal in adult individuals. The shell is inequilateral and rather compressed, and varies in thickness across specimens.[1] While the genus is morphologically similar to *[Contradens](https://en.wikipedia.org/w/index.php?title=Contradens&action=edit&redlink=1)* and *[Trapezoideus](https://en.wikipedia.org/w/index.php?title=Trapezoideus&action=edit&redlink=1)*, adult *Yaukthwa* can be reliably distinguished by a wider and more rounded anterior end, a straighter dorsal margin that lacks a developed wing, and a shallow posterior muscle scar.[1] The right valve bears one lateral tooth and two linear pseudocardinal teeth, and the left valve has two curved lateral teeth and one pseudocardinal tooth. In some specimens, the teeth may be reduced, typically to a single weak tubercle-like lateral tooth and one pseudocardinal tooth per valve.[1]

The anterior adductor muscle scar is well-developed and oval in shape. The posterior muscle scar is shallow. This is a feature that, combined with the shell outline and dental formula, forms the basis of the genus-level diagnosis.[1] Principal component analysis of shell shape across the genus has demonstrated that the shell outlines of most *Yaukthwa* species broadly overlap, with the notable exception of *Y. inlenensis*, whose specimens form a largely distinct morphological cluster, likely reflecting its adaptation to a lacustrine environment.[1]

## Behaviour and habitat

Type localities and habitats of the Yaukthwa species and habitat of Trapezoideus foliaceus

Species of *Yaukthwa* are predominantly found in rapidly flowing mountain streams and rivers with sandy, gravel, or clay substrates, mostly within upland portions of their respective drainages.[1] Type localities documented in the founding revision include swift rivers in the Sittaung and Ayeyarwady basins, such as the [Thauk Ye Kupt River](https://en.wikipedia.org/w/index.php?title=Thauk_Ye_Kupt_River&action=edit&redlink=1) and the [Nam Shu River](https://en.wikipedia.org/w/index.php?title=Nam_Shu_River&action=edit&redlink=1), consistent with a preference for lotic, high-gradient environments.[1]

An exception to this generalisation is *Yaukthwa inlenensis*, which inhabits rivers and streams with moderate current and clay substrate, including tributaries and the outlet of [Inle Lake](/source/Inle_Lake) in the Salween Basin. This species represents an intra-drainage endemic with a habitat preference distinct from the broader genus.[1] Similarly, *Yaukthwa elongatula*, described from the Chindwin Basin, represents a further example of intra-drainage endemism within the genus.[10]

Like other freshwater mussels, *Yaukthwa* species are sensitive indicators of water quality and habitat condition. Freshwater mussels more broadly have been noted to exhibit the fastest rates of human-mediated global extinction among aquatic animals, a vulnerability rooted in their reliance on stable, high-quality lotic habitats and their complex life cycles, which typically involve parasitic larvae ([glochidia](/source/Glochidia)) on host fish.[10]

## Conservation

All *Yaukthwa* species have restricted distribution ranges, with several known only from single localities, making the entire genus vulnerable to localised disturbance.[1] The founding authors of the genus highlighted the need for dedicated conservation efforts by governments, local authorities, and international organisations in [Myanmar](/source/Myanmar) and [Thailand](/source/Thailand).[1]

Primary threats to *Yaukthwa* include dam construction, water pollution, and deforestation, which degrade the lotic freshwater habitats on which these mussels depend.[1] The broader context of rapid economic development and agricultural expansion in Myanmar has been identified as a driver of accelerating freshwater habitat degradation across the region, rendering a national conservation action plan for freshwater bivalves urgently necessary.[10] Bolotov *et al.* (2019) noted that current knowledge of freshwater mussels in Myanmar remains incomplete, with many lineages still awaiting description, and further taxonomic research is required before a comprehensive conservation strategy can be developed.[10]

The biogeographic research that underlies the circumscription of *Yaukthwa* has also highlighted Western Indochina more broadly as a freshwater biodiversity hotspot of worldwide significance, harbouring numerous narrowly endemic species across multiple aquatic invertebrate groups.[9] The authors of successive revisionary studies have consistently underscored that the freshwater bivalve fauna of Southeast Asia is species-rich but poorly known, and that many local endemic lineages may already be on the brink of extinction due to the combined pressures of anthropogenic change and climate change impacts.[10]

## References

1. ^ [***a***](#cite_ref-eskMAR2019_1-0) [***b***](#cite_ref-eskMAR2019_1-1) [***c***](#cite_ref-eskMAR2019_1-2) [***d***](#cite_ref-eskMAR2019_1-3) [***e***](#cite_ref-eskMAR2019_1-4) [***f***](#cite_ref-eskMAR2019_1-5) [***g***](#cite_ref-eskMAR2019_1-6) [***h***](#cite_ref-eskMAR2019_1-7) [***i***](#cite_ref-eskMAR2019_1-8) [***j***](#cite_ref-eskMAR2019_1-9) [***k***](#cite_ref-eskMAR2019_1-10) [***l***](#cite_ref-eskMAR2019_1-11) [***m***](#cite_ref-eskMAR2019_1-12) [***n***](#cite_ref-eskMAR2019_1-13) [***o***](#cite_ref-eskMAR2019_1-14) [***p***](#cite_ref-eskMAR2019_1-15) [***q***](#cite_ref-eskMAR2019_1-16) [***r***](#cite_ref-eskMAR2019_1-17) [***s***](#cite_ref-eskMAR2019_1-18) [***t***](#cite_ref-eskMAR2019_1-19) [***u***](#cite_ref-eskMAR2019_1-20) [***v***](#cite_ref-eskMAR2019_1-21) [***w***](#cite_ref-eskMAR2019_1-22) [***x***](#cite_ref-eskMAR2019_1-23) Konopleva, Ekaterina S.; Pfeiffer, John M.; Vikhrev, Ilya V.; Kondakov, Alexander V.; Gofarov, Mikhail Yu.; Aksenova, Olga V.; Lunn, Zau; Chan, Nyein; Bolotov, Ivan N. (2019). ["A new genus and two new species of freshwater mussels (Unionidae) from western Indochina"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6411986). *Scientific Reports*. **9**: 4106. [doi](/source/Doi_(identifier)):[10.1038/s41598-019-39365-1](https://doi.org/10.1038%2Fs41598-019-39365-1). [PMC](/source/PMC_(identifier)) [6411986](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6411986).

1. **[^](#cite_ref-2)** ["MolluscaBase. AphiaID: 1334840"](https://www.molluscabase.org/aphia.php?p=taxdetails&id=1334840). *MolluscaBase*. Flanders Marine Institute (VLIZ). 2026. Retrieved 2026-04-09.

1. ^ [***a***](#cite_ref-eskSEP2017_3-0) [***b***](#cite_ref-eskSEP2017_3-1) Bolotov, Ivan N.; Vikhrev, Ilya V.; Kondakov, Alexander; Konopleva, Ekaterina S.; Gofarov, Mikhail Yu.; Aksenova, Olga V.; Tumpeesuwan, Sakboworn (2017). ["New taxa of freshwater mussels (Unionidae) from a species-rich but overlooked evolutionary hotspot in Southeast Asia"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5599626). *Scientific Reports*. **7**: 11573. [doi](/source/Doi_(identifier)):[10.1038/s41598-017-11957-9](https://doi.org/10.1038%2Fs41598-017-11957-9). [PMC](/source/PMC_(identifier)) [5599626](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5599626).

1. **[^](#cite_ref-4)** Frierson, L. S. (1913). "Some criticisms on Dr. F. Haas' monograph of the Unionidæ". *Nautilus*. **26**: 141–142.

1. **[^](#cite_ref-5)** Preston, H. B. (1912). "A catalogue of the Asiatic naiades in the collection of the Indian Museum, Calcutta, with descriptions of new species". *Records of the Indian Museum*. **7**: 279–308.

1. **[^](#cite_ref-6)** Anthony, J. G. (1865). "Descriptions of new species of shells". *American Journal of Conchology*. **1**: 351.

1. **[^](#cite_ref-7)** Tapparone Canefri, C. (1889). ["Viaggio de Leonardo Fea in Birmania e regioni vicine. XVIII. Molluschi terrestri e d'acqua dolce"](https://www.biodiversitylibrary.org/page/35995965). *Annali del Museo Civico di Storia Naturale di Genova*. 2. **7**: 354.

1. ^ [***a***](#cite_ref-pfeJUN2021_8-0) [***b***](#cite_ref-pfeJUN2021_8-1) Pfeiffer, J.M.; Graf, D.L.; Cummings, K.S.; Page, L.M. (2021). ["Taxonomic revision of a radiation of South-east Asian freshwater mussels (Unionidae : Gonideinae : Contradentini+Rectidentini)"](https://doi.org/10.1071%2FIS20044). *Invertebrate Systematics*. **35**: 394–470. [doi](/source/Doi_(identifier)):[10.1071/IS20044](https://doi.org/10.1071%2FIS20044).

1. ^ [***a***](#cite_ref-bolAPR2020_9-0) [***b***](#cite_ref-bolAPR2020_9-1) [***c***](#cite_ref-bolAPR2020_9-2) [***d***](#cite_ref-bolAPR2020_9-3) Bolotov, I.N.; Konopleva, E.S.; Vikhrev, I. (2020). ["New freshwater mussel taxa discoveries clarify biogeographic division of Southeast Asia"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7171101). *Scientific Reports*. **10**. et al.: 6616. [doi](/source/Doi_(identifier)):[10.1038/s41598-020-63612-5](https://doi.org/10.1038%2Fs41598-020-63612-5). [PMC](/source/PMC_(identifier)) [7171101](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7171101).

1. ^ [***a***](#cite_ref-eskAUG2019_10-0) [***b***](#cite_ref-eskAUG2019_10-1) [***c***](#cite_ref-eskAUG2019_10-2) [***d***](#cite_ref-eskAUG2019_10-3) [***e***](#cite_ref-eskAUG2019_10-4) [***f***](#cite_ref-eskAUG2019_10-5) [***g***](#cite_ref-eskAUG2019_10-6) Bolotov, Ivan; Konopleva, Ekaterina; Vikhrev, Ilya; Lopes-Lima, Manuel; Bogan, Arthur; Lunn, Zau; Chan, Nyein; Win, Than; Aksenova, Olga; Gofarov, Mikhail; Tomilova, Alena; Kondakov, Alexander (2019). ["Eight new freshwater mussels (Unionidae) from tropical Asia"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6700347). *Scientific Reports*. **9**: 1–15. [doi](/source/Doi_(identifier)):[10.1038/s41598-019-48528-z](https://doi.org/10.1038%2Fs41598-019-48528-z). [PMC](/source/PMC_(identifier)) [6700347](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6700347).

Taxon identifiers Yaukthwa Wikidata: Q139032977 Wikispecies: Yaukthwa CoL: 7PL87 GBIF: 10283695 iNaturalist: 995681 WoRMS: 1334840

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