Reproductive strategy of florideophycean red algae with an isomorphic life cycle

In florideophycean red algae with an isomorphic life cycle, the haploid gametophyte and diploid sporophyte exhibit similar gross morphology, distribution, and phenology, and the significance of this life cycle is not fully understood. Previous work has demonstrated that the phase ratio differs between wave-exposed and protected sites and that water content, strength of the thallus, and growth rates of germlings differ between the two phases. These characteristics suggest that such differences may be advantageous for adaptation to various coastal environments. In addition, independent sporophytes, which we refer to as “recycling successive sporophytes”, have been reported in various isomorphic red algae. Molecular phylogenetic analyses suggest multiple occurrences of independent sporophytes as distinct populations of Caloglossa leprieurii; in fact, independent sporophytes have been found in various red algal species with isomorphic life cycles. Furthermore, dominance of independent sporophytes has occasionally been observed within marginal habitats, as independent sporophytes can reproduce under wider temperature ranges than normal gametophytes/sporophytes, implying that independent sporophytes may help expand the species' distribution. Isomorphic life cycles, which involve ecophysiologically different phases and do not necessarily require sexual reproduction, may be effective for increasing fitness in various intertidal environments.