Morphometrics: Quagga Mussel
Investigating shell shape variation across a full depth gradient in Lake Constance, asking whether two visually distinct morphs are the product of genes or environment.
Overview
Quagga mussels in Lake Constance come in two visually distinct forms. Individuals from shallow water (above ~30 m) tend to have pale, inflated shells with a rounded profile, the so-called shallow morph. Those from deeper water are darker, flatter, and more elongated, the deep morph. The difference is striking enough that the two forms have sometimes been mistaken for separate species or populations.
They are not. Genomic data indicates that shallow and deep individuals are genetically indistinguishable: they belong to the same population, connected by gene flow across the depth gradient.
This raises a straightforward but ecologically important question: if the two morphs share the same genome, why do they look so different?
Phenotypic Plasticity
The most likely explanation is phenotypic plasticity: the ability of a single genotype to produce different physical forms depending on the environment it grows up in. Think of it like identical twins raised in different climates: same DNA, different outcomes. In the case of Quagga mussels, depth-related gradients in light, temperature, food availability, and pressure may all influence how the shell grows, even when the underlying genetics are the same.
This matters because it means the two morphs are not fixed types. They are responses, and understanding what drives them tells us something fundamental about how this species adapts to its environment.
This Project
To test whether shell shape changes gradually with depth or shows a discrete transition, mussels are being sampled across a full depth gradient from 1 m to 200 m in Lake Constance, using diving at shallow depths and dredging below 30 m. Shell morphology (elongation, inflation, width-to-length ratio, colour) will be measured and analysed as a function of depth, after accounting for shell size.
If the transition is gradual, it suggests a continuous environmental gradient drives shell shape. If it is abrupt, it points to a threshold, perhaps a light or temperature boundary, below which shell development switches modes.
Fieldwork: May 2026.
This project is part of a broader research initiative on Quagga mussel morphology led by Joana L. Santos (Eawag).