Survival Strategies and Temperature Responses of Seaweeds Belonging to Different Biogeographic Distribution Groups

The relationship between distributional boundaries and temperature responses of six western European endemic and amphiatlantic rhodophytes were experimentally determined under varying regimes of temperature, light and daylength. The algae fall within two phytogeographic groups: the warm-temperate Mediterranean Atlantic group (Calliblepharis ciliata, Polyneura hilliae, Lomentaria articulata and Cryptopleura ramosa) and the amphiatlantic-temperate group (Callithamnion tetragonum and Lomentaria orcadensis). In the former group, four species appear to have their northern and southern boundaries delimited by winter and summer lethal temperatures, respectively. Growth and reproduction proceeded from 10—20 °C and survival potential at temperatures < 5 °C and > 20 °C was limited, making their occurrence in northeast America unlikely. The gametophytes of P. hilliae have a narrower survival ränge at low temperatures than the tetrasporophytes thereby restricting its distribution. For the two representatives of the amphiatlantic-temperate group evidence points to the occurrence of ecotypic differentiation with regard to temperature tolerance for populations from the same and from either side of the Atlantic. As yet, the available evidence is insufficient to explain the nature of their geographic boundaries completely. Partly, this may be due to the taxonomic uncertainties that exist in these species