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

Abstract

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.

Temperature requirements ofScytosiphon lomentaria (Scytosiphonales, Phaeophyta) from the Gulf of Thessaloniki, Greece, in relation to geographic distribution

“…1A). This is probably in accordance with the stenothermal character of macrothalli, as confirmed by this laboratory study and other field studies (Dethier, 1981). Germlings of Scytosiphon grew better at higher temperatures than the macrothalli (Fig.…”

A bank of microscopic forms on disturbed boulders and stones in tide pools

“…Crusts on the boulders are generally small (less than 2 cm2 diameter) and tend to persist in small irregularities or crevices. These findings are consistent with the capacity of crustose algal morphologies to survive disturbances such as boulder overturning, grazing or sand abrasion (Slocurn 1980, Dethier 1981, Littler & Littler 1983. The frequency of crusts in banks and their persistence in refuges resembles the situation described for mussel beds (Santelices & Martinez 19881, and stresses the importance of the banks of microscopic forms for survival of crusts in disturbed habitats.…”

Responses of algae with different life histories to temporal and spatial variability of disturbance in subtidal reefs

“…Contrary to other species that are not reproductive during the winter, crusts were reproductive throughout the year, as also observed in other studies (Adey & Vassar 1975, Dethier 1981, Davis & Wilce 1987, Kaehler & Williams 1997. Early rates of recruitment of crusts, however, were significantly lower during winter and were spatially very variable.…”

“…Crusts were always among the first colonisers of bare rock (see also Adey & Vassar 1975, Dethier 1981, Kendrick 1991, Figueiredo 1997, thus colonising primary space before lateral invasion by turf. Specific long-term experiments on the ecology of crusts have shown that, in contrast to erect algae, crusts were able to grow and colonize additional space even after they became covered by turf (Airoldi 2000).…”