Mussel of the Month

Indochinella is a new genus described just this year, and I. pugio is a player in the recent revolution of new genera and family-group level taxa that have erupted onto the scene over the last couple years. The story of Indochinella actually starts with the genus Oxynaia.

Oxynaia was introduced by Haas back in 1911 in the captions to three plates — just pictures and names. An actual description and type designation followed two years later — Haas’s contributions to Systematisches Conchylien-Cabinet von Martini und Chemnitz coming in installments. The original plate captions included other species in Oxynaia that by 1913 were transferred to Ensidens in revised plate captions. This story is already off to a complicated start.

There is little to say about Oxynaia over the subsequent 100 or so years. During that period the genus came to house 5 species: O. jourdyi (the type species), O. micheloti, O. diespiter, and O. gladiator from northern Vietnam, and O. pugio from Myanmar. Oxynaia got classified, of course, in various summaries of freshwater mussel taxonomy, but in the pre-cladistic era, everyone was just guessing and flinging their authority around. In 2007, when we did our global checklist, we had enough information to know that we didn’t know where Oxynaia should go. From the anatomy the genus was clearly representative of the family Unionidae but otherwise incertae sedis (Graf & Cummings, 2006, 2007).

In 2010, Bieler et al. proposed a bold classification of the Bivalvia including the Unionoida (or Unionida — that’s where that alternative spelling originated) down to the tribe level. No new data or phylogenetic analyses had become available since we decided we didn’t know enough to precisely classify Oxynaia three years earlier, and Bieler et al. (2010) for some reason located the tribe Oxynaiini among the North American Ambleminae (with a question mark). There was a lot that was novel in the Bieler et al. classification, and that classification provided a more reasonable starting place for the phylogenetic work that would follow than did the clearly flawed arrangements proposed by Modell (1964), Haas (1969), Starobogatov (1970), and others.

The first phylogenetic study to incorporate Oxynaia was Whelan et al. (2011). Perhaps unsurprisingly, O. pugio from Myanmar was not found to be a member of the Ambleminae. Instead, the tribe Oxynaiini (represented by O. pugio) was discovered to be a lineage of the subfamily Parreysiinae, along with other genera from southern Asia and Africa. Based on that study, we updated the classification of freshwater mussel subfamilies, moved the five species of the genus Oxynaia to the Parreysiinae, and wrote about Oxynaia as Mussel of the Month.

But this year, the heretofore untested monophyly of Oxynaia threw a wrench into the works. Bolotov et al. (2018) demonstrated that while indeed O. pugio of Myanmar belongs to the Parryesiinae, the type of the genus, O. jourdyi, and the other three species from Vietnam really belong with the genus Nodularia in the subfamily Unioninae. Oxynaia became a junior synonym of Nodularia, and O. pugio was left without a genus. Bolotov et al. (2018) gave us the name Indochinella for I. pugio and the tribe formerly called the Oxynaiini became the Indochinellini.

Further advances to the classification of southern and southeast Asian mussels have precipitated from a follow-up article by Pfeiffer et al. (2018) wherein Scabies, Harmandia, and Unionetta finally found a tribe in the Indochinellini. That research expanded our understanding of the taxonomic and geographic extent of the Parreysiinae from the Indus east to the Mekong River.

So many fuzzy areas of freshwater mussel phylogeny are coming into focus!