The Role of Alternate Life-history Stages of Macroalga: a Seed Bank Analogue?

Many organisms occurring in temporally variable environments have evolved life-history traits that enable their populations to persist during unfavorable environmental conditions. Numerous terrestrial plants, insects, and marine invertebrates, for example, rely on resting stages that disperse their propagules in time. Although widely observed among many taxa, few examples exist for marine macroalgae, at least in part because of the methodology involved in studying them. Here, I determined that microscopic life stages of the annual marine macroalga Desmarestia ligulata overwinter during periods when the macroscopic thalli are absent, thereby allowing this species to persist in temporally variable environments. Examination of field-grown microscopic stages with fluorescence microscopy identified these stages as gametophytes. Holdfast tagging experiments determined that recruitment of the macroscopic stages was not enhanced by regrowth of perennial thalli as observed in other macroalgae, suggesting that overwintering gametophytes were the sole source of sporophyte recruitment. In contrast to true resting stages, Desmarestia gametophytes were not dormant, but rather were metabolically active, sensitive to small differences in environmental quality, and highly subject to physical damage. Gametophyte photosynthetic rates were greater under higher irradiance, and growth rates were greater under longer photoperiods and higher irradiance. Although their survival appeared to be reduced by grazing from large (>1 cm) invertebrates and sedimentation, gametophytes were able to survive in the field for at least 15 mo and thereby enhance sporophyte recruitment more than a year after settlement. I suggest that Desmarestia gametophytes be regarded as alternate life-history stages that simply maintain populations under a different set of environmental conditions than the macroscopic sporophytes do, rather than as "dormant" or