Population structure and range-limit dynamics of the ribbed mussel (Geukensia demissa) from southern Maine (USA) to northern New Brunswick (Canada)

Ribbed mussels are conspicuous inhabitants of salt marshes along the east coast of North America, but abundance and distributional data at their northern range limit are generally lacking. We determined the distribution, density, and size structure of ribbed mussel populations at 64 sites along more than 1500 km of coastline (∼4° latitude) from southern Maine, USA, to northern New Brunswick, Canada. Ribbed mussel densities were patchy throughout their northern range. Many sites (35/64) had no ribbed mussels; sites with mussels in our sampling quadrats (26/64) averaged 0.8–120 mussels m -2 . Sites with higher mussel densities (≥ 10 individuals m -2 ) were mainly concentrated in the central Northumberland Strait in the southern Gulf of St. Lawrence (7/11 of the high-density sites), and exhibited a mixed-size structure (juveniles and adults of all sizes). Low-density sites had proportionally more large adults, likely reflecting recruitment limitation. Abundance data (ours and published, covering the full species range) were then used to show (via AIC-based model selection) that ribbed mussels conform to the Abundant Centre Hypothesis (ACH), whereby a species is most abundant at the centre of its range and declines toward its range limits. Ribbed mussels peak in density in southern New England and New York before rapidly declining toward their equatorial and poleward range limits (best fitted by an inverse quadratic distribution). This compilation (using our and other studies) provides a robust and geographically widespread dataset to test the ACH as well as a useful baseline to compare future population dynamics, including range shifts. • Ribbed mussel distribution conforms to Abundant Centre Hypothesis • Densities peak in southern New England-New York area (USA) • Populations very patchy near northern range limit (Maritime Canada) • Northern populations likely recruitment and ice-disturbance limited • Provides geographically widespread baseline to assess future population dynamics