Heteromorphic algal life histories: The seasonal pattern and response to herbivory of the brown crust, Ralfsia californica

Summary

Little is known about the natural history of crustose phases of heteromorphic algae. A qualitative model describing their life history strategies has hypothesized that the morphologies and seasonal occurrences of the two phases evolved largely in response to variable grazing pressure. Crustose phases are predicted to occur when grazing is intense, and erect phases when grazing is reduced. The brown algal crust Ralfsia calfornica, which is the non-obligatory alternate phase of Scytosiphon lomentaria and/or Petalonia fascia, was studied on San Juan Island, Washington, USA. Both phases inhabit pools in the midto upper-intertidal zone, with the crust reaching maximum abundance between 1.8 and 2.3 m. The crust undergoes radical and predictable seasonal fluctuations in abundance, with a maximum in March–April and minimum in August. The erect phases are winter annuals, with maxima in January–April. Thus the crustose and erect phases show high temporal overlap. Colonization studies on new substrata showed that: 1) the crust is ephemeral, with a lifespan of 7–12 months, 2) it reproduces throughout the year and its propagules rapidly colonize new substrata at almost any time, while 3) erect phases appear only during the winter. Manipulations of limpets indicated that they do not directly graze the crust and are not responsible for its seasonal fluctuations in abundance; however, they are crucial to its persistence because they remove potential competitors. The crust fits poorly into models relating morphology to successional status in that it is ephemeral and opportunistic yet is grazer-resistant and grazer-dependent. Furthermore, both the crust and its erect phases are most abundant in the winter, when grazing appears to be least intense. Thus this heteromorphic complex exhibits interesting characteristics not predicted by current models.