Distribution, morphology, and genetic affinities of dwarf embedded Fucus populations from the Northwest Atlantic Ocean

Dwarf embedded Fucus populations in the Northwest Atlantic Ocean are restricted to the upper intertidal zone in sandy salt marsh environments; they lack holdfasts and are from attached parental populations of F. spiralis or F. spiralis × F. vesiculosus hybrids after breakage and entanglement with halophytic marsh grasses. Dwarf forms are dichotomously branched, flat, and have a mean overall length and width of 20.3 and 1.3 mm, respectively. Thus, they are longer than Irish (mean 9.3 mm) and Alaskan (mean 15.0 mm) populations identified as F. cottonii . Reciprocal transplants of different Fucus taxa in a Maine salt marsh confirm that F. spiralis can become transformed into dwarf embedded thalli within the high intertidal zone, while the latter can grow into F. s . ecad lutarius within the mid intertidal zone. Thus, vertical transplantation can modify fucoid morphology and result in varying ecads. Microsatellite markers indicate that attached F. spiralis and F. vesiculosus are genetically distinct, while dwarf forms may arise via hybridization between the two taxa. The ratio of intermediate to species-specific-genotypes decreased with larger thalli. Also, F. s. ecad lutarius consists of a mixture of intermediate and “pure” genotypes, while dwarf thalli show a greater frequency of hybrids.