Conspicuous shell shape plasticity across lake-stream habitats in a freshwater mussel (Pyganodon grandis)

Abstract

2.AbstractHydrodynamic forces and their absence appears to exert differential selection pressure between lake and stream populations, creating a tight fit between organismal phenotypes and their environments. Ecophenotypic variants may be the result from isolated genetic evolution or phenotypic plasticity, where a widespread genotype can produce multiple phenotypes dependent on the environment. Freshwater mussels possess a wide degree of morphological variation that frequently covaries with the environment, making them a good system to understand the mechanisms of ecophenotypic variation across hydrological conditions. We designed a two-year experiment where individuals from the samePyganodon grandismaternal brood (half-full siblings) were reared at a controlled site and four natural sites involving one lake and three streams. At the end of the experiment, shell shape was quantified for recaptured (N=70), wild (N=206), and zoo-reared (N=305) mussels. Analysis of covariance found significant differences in shell shape between rearing sites, particularly between stream and lake habitats, but no shape differences were detected across the three stream sites. At two of the four sites, shell shape of recaptured individuals matched the morphology of wild populations. Genomic sequencing and parentage analysis identified 27 different fathers among recaptured individuals. Yet, no genetic differences were present between stream and lake habitats and there was no effect of parentage on shell shape. Taken together, phenotypic plasticity, over genetic differentiation, is identified as the primary mechanism of shell shape ecophenotypy. Plasticity may be a key adaptation for freshwater mussels and possibly provide a buffer against their imperilment in degraded habitats.1.Summary statementStreamlined and obese freshwater mussel shell shapes in stream-lake environments are developed from the same maternal brood and may reflect adaptations to the presence and absence of streamflow.

Variations in shell morphology of freshwater mussels Anodonta anatina and Unio tumidus (Bivalvia: Unionidae) from Lake Polvijärvi, Finland

“…Generally, shell morphology of freshwater mussels is species-and habitat dependent ( Keogh et al, 2023). Indeed, the current study revealed interesting patterns of shell morphology in A. anatina and U. tumidus, particularly in the width-length relationship.…”

“…Several studies observed variations in the shell thickness, shell shape, shell sculpture, growth rate, and overall development of freshwater mussels between the same species across different lake-stream habitats, such as A. anatina and U. tumidus in Lake Polvijärvi (the current study) have different shell morphology in comparison to the same species at Lake Viinijärvi (Abdelsaleheen, 2023), and National Nature Reserve, Wicken Lode, United Kingdom (Aldridge, 1999); or between different species at the same habitat, as Pyganodon grandis and Lampsilis radiata siliquoidea in USA ( Keogh et al, 2023); Anodonta cygnea, Pseudanodonta complanate, Anodonta anatina, Unio pictorum and Unio tumidus in National Nature Reserve, Wicken Lode, U. K. (Aldridge, 1999). Likewise, the b value (0.72) of the width-height relationship was higher for A. anatina, the present study, compared to previously studied mussel species, such as A. anatina, P. complanate, U. tumidus, P. corrugate, and P. favidens, and lower than A. cygnea and U. pictorum (Aldridge 1999;Ramesha & Thippeswamy, 2009;Thippeswamy et al, 2014).…”