Freshwater mussel shape variation in the Amazon

A paper from Liliane Sousa da Mata’s M.Sc. dissertation, which she defended in June 2015, went online on 12 February 2019 in the Journal of Molluscan Studies.

Photo (top): The iconic hill and sand spit at Alter do Chão, Santarém, Pará, Brazil, a popular tourist destination on the Tapajós river. October 2014. Tourists bathing in the river beaches there have complained of cuts to the feet due to the strong, sharp posterodorsal projections of freshwater mussels living in the sand.

Liliane Sousa da Mata, Claudia Helena Tagliaro, Diego Simeone, Colin Robert Beasley; Shell shape variation in Amazonian freshwater mussels (Unionida: Hyriidae: Hyriini), Journal of Molluscan Studies, eyz001, doi:

Empty Triplodon corrugatus shells exposed during low water in the Piriá river, Pará, Brazil. October 2012.

Quick summary

Unionid freshwater mussel shells are highly variable in shape. Such variation, often associated with hydrodynamics, makes it difficult to identify species and their conservation needs. This paper used geometric morphometrics to look at shell shape in populations of Triplodon corrugatus, Paxyodon syrmatophorus and Prisodon obliquus from six eastern Amazon rivers in Brazil, classified as either small or large drainages. Shell shape was related to drainage size in all three species. Mussels from the large Tapajós, Amazon and Tocantins rivers were triangular in shape with prominent posterodorsal and anterior winged processes. On the other hand, shells from the small Trombetas, Irituia and Piriá rivers were rounded, and lacked or had reduced dorsal winged processes. Where all three species co-occurred, shell shape converged to that associated with the drainage size. Paxyodon syrmatophorus from the Tapajós was particularly distinct, however, with very pronounced dorsal processes and a reduced shell disc. In all three species, most of the variation occurred in the posterodorsal winged process, posterior and ventral margins, and to a lesser degree, in the anterior winged process. This variation is probably ecophenotypic in origin as the small versus large river shapes occur in all three species and previous studies indicate little or no significant genetic structuring among freshwater mussel populations in the region.

Paxyodon syrmatophorus, from the Tapajós river, with the strong posterodorsal projection. The antero-posterior length of the shell disc is approximately 5 cm. Photo: Liliane Sousa da Mata.

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