{{Short description|Genus of mussels}} {{Automatic taxobox | image = Shell morphology of Yaukthwa species.png | image_caption = Shell morphology of Yaukthwa species in Myanmar, photograph taken by Ekaterina S. Konoplev | display_parents = 2 | taxon = Yaukthwa | authority = Konopleva ''et al.'', 2019 | subdivision_ranks = Species }}
'''''Yaukthwa''''' is a genus of freshwater mussels in the family [[Unionidae]], subfamily [[Rectidentinae]], tribe [[Contradentini]]. The genus is [[Endemism|endemic]] to the Western Indochina Subregion, with species distributed across the [[Ayeyarwady River|Ayeyarwady]], [[Sittaung River|Sittaung]], [[Bago River | Bago]], and [[Salween River|Salween]] river drainages in [[Myanmar]] and northwestern [[Thailand]].<ref name="eskMAR2019"/>
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]]'' Simpson, 1900 <ref name="eskMAR2019"/>. 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 <ref name="eskMAR2019"/>.
''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. <ref>{{cite web |year=2026 |title=MolluscaBase. AphiaID: 1334840 |url=https://www.molluscabase.org/aphia.php?p=taxdetails&id=1334840 |website=MolluscaBase |publisher=Flanders Marine Institute (VLIZ) |access-date=2026-04-09 }}</ref>
The genus has had the following synonymised or transferred names: {| class="wikitable" |+ !Former name !Authority !Notes |- |''Trapezoideus nesemanni''
|Konopleva, Vikhrev & Bolotov, 2017 <ref name="eskSEP2017">{{Cite journal |last1=Bolotov |first1=Ivan N. |last2=Vikhrev |first2=Ilya V. |last3=Kondakov |first3=Alexander |last4=Konopleva |first4=Ekaterina S. |last5=Gofarov |first5=Mikhail Yu. |last6=Aksenova |first6=Olga V. |last7=Tumpeesuwan |first7=Sakboworn|date=2017 |title=New taxa of freshwater mussels (Unionidae) from a species-rich but overlooked evolutionary hotspot in Southeast Asia |journal=Scientific Reports |volume=7 |pages=11573 |doi=10.1038/s41598-017-11957-9|pmc=5599626 }}</ref> |Transferred to ''Yaukthwa'' as type species <ref name="eskMAR2019"> {{Cite journal |last1=Konopleva |first1=Ekaterina S. |last2=Pfeiffer |first2=John M. |last3=Vikhrev |first3=Ilya V. |last4=Kondakov |first4=Alexander V. |last5=Gofarov |first5=Mikhail Yu. |last6=Aksenova |first6=Olga V. |last7=Lunn |first7=Zau |last8=Chan |first8=Nyein |last9=Bolotov |first9=Ivan N. |date=2019 |title=A new genus and two new species of freshwater mussels (Unionidae) from western Indochina |journal=Scientific Reports |volume=9 |pages=4106 |doi=10.1038/s41598-019-39365-1|pmc=6411986 }}</ref> |- |''Trapezoideus panhai'' |Konopleva, Bolotov & Kondakov, 2017 <ref name="eskSEP2017">{{Cite journal |last1=Bolotov |first1=Ivan N. |last2=Vikhrev |first2=Ilya V. |last3=Kondakov |first3=Alexander |last4=Konopleva |first4=Ekaterina S. |last5=Gofarov |first5=Mikhail Yu. |last6=Aksenova |first6=Olga V. |last7=Tumpeesuwan |first7=Sakboworn|date=2017 |title=New taxa of freshwater mussels (Unionidae) from a species-rich but overlooked evolutionary hotspot in Southeast Asia |journal=Scientific Reports |volume=7 |pages=11573 |doi=10.1038/s41598-017-11957-9|pmc=5599626 }}</ref> |Transferred to ''Yaukthwa'' <ref name="eskMAR2019"/> |- |''Trapezoideus dallianus''
|Frierson, 1913 <ref>{{Cite journal |last=Frierson |first=L. S. |date=1913 |title=Some criticisms on Dr. F. Haas' monograph of the Unionidæ |journal=Nautilus |volume=26 |pages=141–142}}</ref> |Transferred to ''Yaukthwa'' <ref name="eskMAR2019"/> |- |''Trapezoideus zayleymanensis''
|Preston, 1912 <ref>{{Cite journal |last=Preston |first=H. B. |date=1912 |title=A catalogue of the Asiatic naiades in the collection of the Indian Museum, Calcutta, with descriptions of new species |journal=Records of the Indian Museum |volume=7 |pages=279–308}}</ref> |Transferred to ''Yaukthwa'' <ref name="eskMAR2019"/> |- |''Trapezoideus peguensis''
|Anthony, 1865 <ref>{{Cite journal |last=Anthony |first=J. G. |date=1865 |title=Descriptions of new species of shells |journal=American Journal of Conchology |volume=1 |pages=351}}</ref> |Transferred to ''Yaukthwa'' <ref name="eskMAR2019"/> |- |''Indonaia rectangularis''
|Tapparone-Canefri, 1889 <ref>{{cite journal |last=Tapparone Canefri |first=C. |year=1889 |title=Viaggio de Leonardo Fea in Birmania e regioni vicine. XVIII. Molluschi terrestri e d'acqua dolce |journal=Annali del Museo Civico di Storia Naturale di Genova |series=2 |volume=7 |page=354 |url=https://www.biodiversitylibrary.org/page/35995965 }}</ref> |Transferred to ''Yaukthwa'' <ref name="pfeJUN2021">{{cite journal |last1=Pfeiffer |first1=J.M. |last2=Graf |first2=D.L. |last3=Cummings |first3=K.S. |last4=Page |first4=L.M. |title=Taxonomic revision of a radiation of South-east Asian freshwater mussels (Unionidae : Gonideinae : Contradentini+Rectidentini) |journal=Invertebrate Systematics |volume=35 |pages=394–470 |year=2021 |doi=10.1071/IS20044 |doi-access=free }}</ref> |- |''Balwantia baniensis'' |Bolotov et al., 2020 <ref name="bolAPR2020"/> |Transferred to ''Yaukthwa'' <ref name="pfeJUN2021"/> |}
The recognised species (including all transferred species and newly found species) are: # ''Yaukthwa nesemanni'' (Konopleva, Vikhrev & Bolotov, 2017) # ''Yaukthwa panhai'' (Konopleva, Bolotov & Kondakov, 2017) # ''Yaukthwa dallianus'' (Frierson, 1913) # ''Yaukthwa zayleymanensis'' (Preston, 1912) # ''Yaukthwa peguensis'' (Anthony, 1865) # ''Yaukthwa paiensis'' (Konopleva ''et al.'', 2019) <ref name="eskMAR2019"/> # ''Yaukthwa inlenensis'' (Konopleva ''et al.'', 2019) <ref name="eskMAR2019"/> # ''Yaukthwa elongatula'' (Konopleva ''et al.'', 2019) <ref name="eskAUG2019">{{cite journal |last1=Bolotov |first1=Ivan |last2=Konopleva |first2=Ekaterina |last3=Vikhrev |first3=Ilya |last4=Lopes-Lima |first4=Manuel |last5=Bogan |first5=Arthur |last6=Lunn |first6=Zau |last7=Chan |first7=Nyein |last8=Win |first8=Than |last9=Aksenova |first9=Olga |last10=Gofarov |first10=Mikhail |last11=Tomilova |first11=Alena |last12=Kondakov |first12=Alexander |date=2019 |title=Eight new freshwater mussels (Unionidae) from tropical Asia |journal=Scientific Reports |volume=9 |pages=1–15 |doi=10.1038/s41598-019-48528-z|pmc=6700347 }}</ref> # ''Yaukthwa avaensis'' (Bolotov et al., 2020) <ref name="bolAPR2020">{{cite journal |last1=Bolotov |first1=I.N. |last2=Konopleva |first2=E.S. |last3=Vikhrev |first3=I. |others=et al. |title=New freshwater mussel taxa discoveries clarify biogeographic division of Southeast Asia |journal=Scientific Reports |volume=10 |pages=6616 |year=2020 |doi=10.1038/s41598-020-63612-5 |pmc=7171101 }}</ref> # ''Yaukthwa rectangularis'' (Tapparone-Canefri, 1889) # ''Yaukthwa baniensis'' (Bolotov et al., 2020)
== Distribution ==
[[File:Distribution ranges of Trapezoideus foliaceus and species in the genus Yaukthwa.png|thumb|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 [[Irrawaddy River|Ayeyarwady]], [[Sittaung River|Sittaung]], [[Bago River|Bago]], and [[Salween River|Salween]] river drainages in [[Myanmar]], and from the Salween Basin in [[Northern Thailand|northwestern Thailand]].<ref name="eskMAR2019"/> 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]] in northwestern Thailand and ''Y. inlenensis'' from the [[Mway Stream]] tributary in Myanmar.<ref name="eskMAR2019"/>
As part of the broader Western Indochina Subregion, the drainages inhabited by ''Yaukthwa'', particularly the [[Irrawaddy River|Ayeyarwady]], [[Bago River|Bago]], [[Sittaung River|Sittaung]], and [[Salween]], form a distinct freshwater biogeographic unit separated from the Indian Subregion to the west by the [[Naga Hills]], [[Chin Hills]], and [[Rakhine Yoma]] mountain ranges, and from the [[Sundaland]] Subregion to the south by the [[Isthmus of Kra]].<ref name="bolAPR2020"/> 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.<ref name="eskAUG2019"/>
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.<ref name="eskMAR2019"/>
== Morphology ==
[[File:Shell morphology of Yaukthwa species.png|thumb|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.<ref name="eskMAR2019"/> While the genus is morphologically similar to ''[[Contradens]]'' and ''[[Trapezoideus]]'', 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.<ref name="eskMAR2019"/> 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.<ref name="eskMAR2019"/>
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.<ref name="eskMAR2019"/> 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.<ref name="eskMAR2019"/>
== Behaviour and habitat == [[File:Type localities and habitats of the Yaukthwa species and habitat of Trapezoideus foliaceus.png|thumb|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.<ref name="eskMAR2019"/> Type localities documented in the founding revision include swift rivers in the Sittaung and Ayeyarwady basins, such as the [[Thauk Ye Kupt River]] and the [[Nam Shu River]], consistent with a preference for lotic, high-gradient environments.<ref name="eskMAR2019"/>
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]] in the Salween Basin. This species represents an intra-drainage endemic with a habitat preference distinct from the broader genus.<ref name="eskMAR2019"/> Similarly, ''Yaukthwa elongatula'', described from the Chindwin Basin, represents a further example of intra-drainage endemism within the genus.<ref name="eskAUG2019"/>
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]]) on host fish.<ref name="eskAUG2019"/>
== Conservation == All ''Yaukthwa'' species have restricted distribution ranges, with several known only from single localities, making the entire genus vulnerable to localised disturbance.<ref name="eskMAR2019"/> The founding authors of the genus highlighted the need for dedicated conservation efforts by governments, local authorities, and international organisations in [[Myanmar]] and [[Thailand]].<ref name="eskMAR2019"/>
Primary threats to ''Yaukthwa'' include dam construction, water pollution, and deforestation, which degrade the lotic freshwater habitats on which these mussels depend.<ref name="eskMAR2019"/> 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.<ref name="eskAUG2019"/> 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.<ref name="eskAUG2019"/>
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.<ref name="bolAPR2020"/> 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.<ref name="eskAUG2019"/>
== References == {{Reflist}}
{{Taxonbar|from=Q139032977}}
[[Category:Unionidae]] [[Category:Bivalve genera]]