Distinctive morphological features, life-cycle phases and seasonal variations in subtropical populations of Dictyota dichotoma (Dictyotales, Phaeophyceae)
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Although it has been suggested that the distribution of Dictyota dichotoma is probably restricted to the European Atlantic Ocean and the Mediterranean Sea, its occurrence in the Canary Islands (new southernmost distribution limit) is confirmed by nuclear ribosomal sequence data (LSU rDNA). Even though D. dichotoma has been described and illustrated prominently in early studies of brown algae, the species remains difficult to characterize due to considerable morphological plasticity. An exhaustive analysis of several quantitative characters confirms significant morphological variation both seasonally and between life-cycle phases. The species may be characterized in the Canary Islands only by the following qualitative features: erect thallus attached by a single holdfast, subdichotomous branches always of similar width, straight terminal segments and smooth margins and an entirely unilayered medulla. In the Canary Islands, D. dichotoma is an aseasonal annual with at least three overlapping generations in which sporophytes and gametophytes grow simultaneously. Thallus life span seems to be less than 3 months. The species occurs throughout the year, but as cryptic microthalli in autumn. Two abundance peaks were detected with the maximum value in February (6.2 thalli m-2). The optimum reproductive stage (88.3% fertile specimens) occurred in winter and the maximum vegetative stage (maximum thallus length: 18.6 cm) in summer. Sporophytes outnumbered gametophytes throughout the year, with ratios decreasing from 13.2"1.1 in winter to 1.9"0.2 in summer. Gametophytes made up to 25% of the population and fertile thalli were always dominant. The populations from the Canaries exhibited a temporal displacement compared to northern populations with the favorable period in the coldest season (winter) and the resting period in the warmest season (autumn).