Several aspects of the ecophysiology were compared in six eulittoral species of the mangrove red algal genus Caloglossa. The growth pattern in relation to different salt and light treatments, photosynthesis-irradiance curves, as well as the influence of salinity on the intracellular D-mannitol concentrations were studied. All Caloglossa isolates can be characterised as halotolerant due to a broad growth response under a range of salinities (5.3-70‰). The D-mannitol content of all isolates increased significantly with hypersaline treatments and decreased significantly with hyposaline treatments. These data indicate the strong involvement of this hexitol in the osmotic acclimation of Caloglossa. The growth-irradiance pattern and the photosynthesis-light response curves demonstrate an adaptation to shaded habitats for all species. All these physiological capabilities allow the successful colonisation of Caloglossa on various intertidal substrates.