Are North Atlantic Alaria esculenta and A-grandifolia (Alariaceae, Phaeophyceae) conspecific?

Alaria (Alariaceae, Phaeophyceae) is a common genus of kelps in the norther n hemisphere. Fourteen species are currently recognized, of which three, Al aria esculenta (L.) Greville, A. pylaii (Bory de Saint-Vincent) Greville an d A. grandifolia J. Agardh, are reported for the cold-temperate North Atlan tic Ocean. Alaria esculenta, the type species described originally from the North Atlantic, exhibits a range of biogeographically correlated morphotyp es suggesting the possibility of multiple species, subspecies or hybrids. I n Ireland we discovered an A. esculenta population with unusually long stip es resembling the type specimen of A. grandifolia described from Spitsberge n by J. Agardh in 1872. These and other plants of A. esculenta h om Ireland were compared with plants from Spitsbergen fitting the description of A. g randifolia, using sexual hybridization relative growth rate measurements an d DNA sequence comparisons. Complete interfertility was observed between th e different isolates. Three nucleotide substitutions (0.37%) were found in the rbcL and RuBisCo spacer of A. grandifolia, and two in the partial 18S r RNA gene and ITS1 sequences. The relative growth rate at 10 OC of an Irish self-cross was significantly lower than those of all the other crosses. Com parison of RuBisCo spacer sequences of the Spitsbergen A. grandifolia and s ix A. esculenta isolates showed that A. grandifolia was identical to A. esc ulenta from Halifax, Canada. The partial 18S rRNA gene and ITS1 sequence of A. grandifolia was identical to that of A. praelonga from Japan and differ ed by a single substitution from A. esculenta from Scotland and by two nucl eotide substitutions from the isolate from Ireland. The intraspecific diffe rences in A. esculenta, together with the hybridization and morphometric re sults, suggest that A. grandifolia is to be considered conspecific with A. esculenta, and that A. grandifolia is most probably a large deep-water morp hological variant subspecies or ecotype of A. esculenta.