Sampling Quagga Mussels Across the Depth Gradient in Lake Constance

Lake Constance (Bodensee) is where the Quagga mussel (Dreissena rostriformis bugensis) first established itself in Switzerland. It is also where two visually distinct shell forms coexist: a pale, rounded shallow morph and a darker, flatter deep morph. Sampling across the full depth gradient — from 30 m down to 200 m — is the foundation of the Morphometrics: Quagga Mussel project, which asks whether this shape difference is driven by genes, environment, or both.

Sampling approach

Below 30 m, where diving is no longer practical, mussels are collected using Ponar grabs deployed from the Eawag boat “Calypso”. A Ponar grab closes on contact with the sediment, capturing a fixed volume of the top layer. The contents — mud, shells, and anything living in between — are then washed on deck and sorted.

I will post about scientific diving soon — stay posted!

Density varies strongly with depth

The number of grabs needed per station depended heavily on depth. At 80 m, a single Ponar grab yielded enough mussels — densities were high enough that one sample was sufficient. At 200 m, densities were far lower, and three grabs per station were required to collect a comparable number of individuals.

This gradient in density is itself ecologically informative. The deep zone of Lake Constance is cold, dark, and food-limited, conditions that appear to support smaller populations — and, as this project will test, a distinctly different shell shape.

Fieldwork carried out May 2026. This project is part of a broader research initiative on Quagga mussel morphology led by Joana L. Santos (Eawag).