Morphology and reproduction of the adventive Mediterranean rhodophyte Polysiphonia setacea

Abstract

The red alga Polysiphonia setacea Hollenberg (=Womersleyella setacea (Hollenberg) R. Norris), described originally from the Hawaiian Islands and later reported for other tropical localities, has recently become widespread in the Mediterranean. In several localities it forms a dense, almost monospecific turf but, despite its abundance, it appears to reproduce only vegetatively; neither sporangia nor sexual reproductive structures have thus far been found. In order to elucidate its life history, plants were cultured in a variety of conditions of temperature, daylength and photon irradiance, and the upper thermal limit was also determined. Isolates of P. setacea grew well in culture, but were more tufted and branched than wild plants. No reproduction by spores or gametes was observed and only an unusual form of vegetative regeneration was found. Some pericentral cells became darker and larger than the others and produced proliferations from which new plants arose. Plants grew best at 15 and 20 °C, and relatively poor growth was observed at 10 and 25 °C. At 20 °C growth was better in long days than in short days. The upper thermal limit was 28 °C, and plants were able to tolerate a temperature as low as 5 °C for 4 weeks without any damage. These observations show that P. setacea is well adapted to the environmental conditions of the western Mediterranean, and there is good agreement with the phenology of the species in the wild. Although this entity has been treated as an introduction from a tropical area, our results suggest that the Mediterranean entity would not be able to survive or grow in surface waters of tropical areas whence this species has been reported. However, the occurrence of thermal ecotypes in P. setacea is a possibility and further studies, based perhaps on molecular data, are necessary to assess the origin of the Mediterranean populations.