Grateloupia doryphora (Halymeniaceae, Rhodophyta) in Rhode Island waters (USA): Geographical expansion, morphological variations and associated algae

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Within a five-year period (1994-1999), the population of the invasive seaweed Grateloupia doryphora expanded from its initial location on a central, southern coastline in Rhode Island, USA. It spread north, east and west in Narragansett Bay, and south along the open coast of Rhode Island Sound. Large numbers of G. doryphora thalli were found in the lower intertidal (below +0.2 m mean low water [MLW]), the subtidal (down to -5.5 m MLW), and tide pools (as high as +1.5 m MLW). Because the species has survived six winters and summers, it seems firmly established in Rhode Island. Regression analyses at nine stations on a latitudinal gradient indicate strong relationships between location in the bay and average maximum blade length (r = 0.91), width (r = 0.92), and polymorphism (r = 0.85). We also found significant differences in blade size and shape among the stations and in relation to depth. Near its exposed southern reaches, at its upper vertical limits, and in tide pools, this species was frequently dissected (> 71% of observed individuals) and polymorphic. In the sheltered northern regions of this estuary and towards the lower vertical limit, down to -5 m MLW, blades were predominantly simple (74% of observed individuals) and either broadly linear or lanceolate. These included specimens as long as 175 cm, with a surface area over 3000 cm2, making this alga one of the largest Florideophyceae reported in the western North Atlantic. Throughout the study, 95-100% of blades were reproductive. No relationship was observed between the percentage of reproductive blades and the latitudinal position in the bay or vertical gradients. At the nine stations studied, 40 associated algal species were recorded within I m of G. doryphora thalli, and, of these, 33% were epiphytic. One endoepiphyte, Laminariocolax tomentosoides (Phaeophyta) appeared on no other host in Rhode Island waters. Data from this study may be incorporated in a model to examine expansion patterns of new invasions of G. doryphora and other introduced species, such as escapees from shipping or aquaculture.





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