Species of the coralline algal genus Phymatolithon are the dominant algal calcifiers in the rocky intertidal and photic sublittoral zone of the Subarctic-Boreal transition that stretches across the North Atlantic from the Gulf of Maine and the southern Canadian Maritimes to southwestern Iceland and the Norwegian outer Coast. In this paper, we use extensive field and laboratory data on the biology, physiology, and ecology of Phymatolithon species, supported by statistical analysis and DNA sequence data, to develop a multiscale view of this key genus of the ecosystems of this region. We demonstrate that species of Phymatolithon that occur in the Boreal-Subarctic transition zone in the North Atlantic can be segregated systematically by a statistical/developmental analysis of their morpho-anatomical characters. We show these results to be congruent with DNA sequence based methods. Six species are recognized: P. laevigatum, P. rugulosum (P. lamii), P. squamulosum (P. lenormandii), P. investiens, P. borealis sp. nov. (P.polymorphum) and P. nantuckensis sp. nov.). Based upon paraffin section, compound microscope and EDS-SEM analysis, we show that coralline anatomy comprises a diversity of both tissue types and high magnesium carbonate wall structure. Variation in vegetative tissue morphology, particularly with respect to cell division and elongation patterns, as well as variation in conceptacle (reproductive structure) location and development are due to a complex of genetic and environmental factors. Some of these factors can be linked to adaptation to environmental and biogeographical niches, providing a basis for experimental analysis of the mechanisms of adaptation.--Provided by publisher.