Historical invasions of the intertidal zone of Atlantic North America associated with distinctive patterns of trade and emigration

Abstract

Early invasions of the North American shore occurred mainly via deposition of ballast rock, which effectively transported pieces of the intertidal zone across the Atlantic. From 1773-1861, >880 European ships entered Pictou Harbor, Nova Scotia, as a result of emigration and trade from Europe. The rockweed Fucus serratus (1868) and the snail Littorina littorea ( approximately 1840) were found in Pictou during this same period. With shipping records (a proxy for propagule pressure) to guide sampling, we used F. serratus as a model to examine the introductions because of its relatively low genetic diversity and dispersal capability. Microsatellite markers and assignment tests revealed 2 introductions of the rockweed into Nova Scotia: 1 from Galway (Ireland) to Pictou and the other from Greenock (Scotland) to western Cape Breton Island. To examine whether a high-diversity, high-dispersing species might have similar pathways of introduction, we analyzed L. littorea, using cytochrome b haplotypes. Eight of the 9 Pictou haplotypes were found in snails collected from Ireland and Scotland. Our results contribute to a broader understanding of marine communities, because these 2 conspicuous species are likely to be the tip of an "invasion iceberg" to the NW Atlantic from Great Britain and Ireland in the 19th Century.

Fig. 1.

Distribution of the European seaweed Fucus serratus in 1903 (19; blue and green border) and today (Inset, gray border) in the Canadian Maritimes. In 1903, F. serratus occurred from Pugwash to Mulgrave (M) on the Strait of Canso (arrow) between Cape Breton Island (CBI) and mainland Nova Scotia (NS), the tip of Prince Edward Island (PEI), and southward from Cheticamp to an area near Port Hawkesbury (PH). Our molecular results indicate that this distribution represents a convergence at the Strait of Canso of 2 different introductions. The shore near the French Fortress (1713–1758) of Louisbourg (L) remains uncolonized. Secondary (20th Century) colonization sites in mainland NS are shown in Inset.

Fig. 2.

Neighbor-joining tree based on microsatellite analysis supports 2 distinct introductions of F. serratus into Nova Scotia, 1 from western Ireland and 1 from the Clyde area (Greenock) of Scotland. Nova Scotian sites are in bold type.