Temperature responses of tropical to warm-temperate Atlantic seaweeds .1. Absence of ecotypic differentiation in amphi-Atlantic tropical-Canary Islands species

Thirty-two isolates of four amphi-Atlantic seaweed species with a tropical to subtropical distribution (Dictyopteris delicatula, Ceratodictyon intricatum, Ernodesmis verticillata and Lophocladia trichoclados) have been investigated for their temperature responses. Long (8 week) incubation times at 15-18 degrees C caused damage or death in all isolates. Lower temperatures were tolerated only for shorter incubation times or caused severe damage. High temperatures in the range 30-35 degrees C were tolerated. For L. trichoclados growth response curves were also determined which showed growth at temperatures ranging from 18/20 to 30 degrees C, with optimum growth rates at 25-30 degrees C. Crossing experiments with isolates of L. trichoclados showed that no reproductive barrier exists between isolates from opposite sides of the Atlantic. Comparison of upper and lower tolerance limits and growth response curves showed no indication of ecotypic differentiation between isolates from different localities on eastern and western Atlantic coasts, despite different local seawater temperature regimes. These results indicate that (long-range) dispersal events are likely to have overruled older, vicariant patterns. This is further supported by Random Amplified Polymorphic DNA analysis in L. trichocalados, which showed that one of the Canary Islands isolates has an intermediate position between an isolate from the Caribbean and two other isolates from the Canary Islands. Based on palaeoclimatic evidence and temperature tolerance data, it is suggested that the populations on the Canary Islands became extinct during Pleistocene glaciations and that the islands were subsequently repopulated by means of dispersal from tropical donor populations.