The distribution of the criistose red alga I.ithophvllum inriuJtanJ in the British Isles is described. Three populations are analysed demographically, two having age class distributions corresponding to a type 111 sur\ivorship curve characteristic of poptilations in which the greatest mortality occurs in the youngest age classes. The more southern of these two populations has a longer generation time (14.2 years) than the more northern (8.5 years). The third and most southern population has an age class distribution dominated by the 6-and 7-year-old categories. There were few sexual thalli (around 3 " , , ) but they appeared to put more effort into reproduction than the asexual thalli.

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Phenotypic variation of Fucus spiralis was examined in terms of morphology and chemical composition, as determined by pyrolysis mass spectrometry (PyMS). After discriminant analysis both techniques revealed the existence of two morphotypes which were subsequently designated as F. spiralis and F. spiralis forma nanus. The nanus plants differed from the nominate spiralis in that they were shorter, narrower and had fewer branches, but had more units of growth from a single holdfast. F. spiralis f. nanus was consistently higher on the shore than F. spiralis, but the two morphotypes overlapped in terms of shore exposure level. We suggest that the formae exist within a mosaic of stable phenotypes representing populations specifically adapted to the local environment. PyMS data were in agreement with morphological descriptions showing that gross morphology can be correlated with chemical composition.

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