Coralline red algae are calcified encrusting organisms that are universal on hard marine bottoms from the Arctic and Antarctic to the tropics. Widely considered to be ecosystem engineers, organisms that support entire communities, corallines build calcified structures and are reef builders, separately or with corals in the tropics. Even in colder waters, corallines are potentially builders of ecosystem-supporting and millennial age corstromes. 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 zone that stretches across the North Atlantic from the Gulf of Maine and the southern Canadian Maritimes to southwestern Iceland and the Norwegian outer Coast.

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This study integrates extensive field and laboratory data on the biology, physiology, and ecology of Phymatolithon species, with statistical analysis and DNA sequencing, to develop a comprehensive understanding of this key genus of the ecosystems of the region. We demonstrate that species of Phymatolithon that occur in the Boreal-Subarctic transition zone in the North Atlantic can be securely identified by a statistical/developmental analysis of their morphological and anatomical characters.  These results are corroborated by DNA sequencing.

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The key species recognized are 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 presents 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.

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We analyze reproductive structure development in Phymatolithon and demonstrate the linkage between genetic control and vegetative growth. These parameters interact to produce considerable variation in some characters of morphology but not in others. The results demonstrate that morpho-anatomical research in pan oceanic corallines requires greater biological sophistication, with utilization of quantitative population level data, to achieve success in DNA correlations.