Ecological, physiological, and biomechanical differences between gametophytes and sporophytes of Chondrus ocellatus (Gigartinales, Rhodophyta)1

Although variation among habitats in the ratio of gametophytes to sporophytes has been reported in various gigartinacean species, factors controlling the phase ratio remain poorly understood. Over 18 months, we examined the phase ratio of Chondrus ocellatus at three sites: a sheltered intertidal site, Hiruga A; an exposed intertidal site, Hiruga B; and a subtidal site, Shikimi. The mean proportion of gametophytes at Hiruga A (73.1%) was significantly higher than that at Shikimi (51.2%) and Hiruga B (44.7%). Due to a significantly higher water retention ability of the gametophytes, it was expected that the gametophytes would exhibit higher desiccation tolerance. After dehydration treatments, however, neither the photosynthetic rate of vegetative blades nor the survival rate of spores was significantly different between the phases. Measurements of blade strength indicated that the sporophytic blades were less stiff and more flexible, and a culture experiment revealed that the sporophytic germlings showed a significantly higher growth rate. Flexible blades and fast-growing germlings are considered advantageous for colonizing wave-swept intertidal habitats, so these properties may have caused the different fluctuation pattern of phase ratio among the sites. The present data demonstrate that biomechanical and physiological differences between the two phases of C. ocellatus make one phase advantageous in certain environmental conditions, and that these differences likely cause an unequal ratio of isomorphic phases.