The Evolution of Thallus Form and Survival Strategies in Benthic Marine Macroalgae: Field and Laboratory Tests of a Functional Form Model

Abstract

A synthetic "functional form" paradigm concerning hypothetically important adaptive features of algal structure and function was developed and tested by a costs/benefits strategic approach. Successional manipulations were performed by disturbing mature, environmentally constant intertidal communities; from the array of colonizing macroalgae, Ulva sp. was chosen as an opportunistic representative of pioneer seral stages, Egregia menziesii and Gelidium purpurascens/robustum as intermediate to late seral species, and Pelvetia fastigiata and Corallina officinalis as characteristic of more mature climax communities. The ranking from high to low primary producers (Ulva > Gelidium > Egregia > Pelvetia > Corallina) indicates that selection in fluctuating environments has favored opportunistic species having high net productivity, while those species able to persist in benign predictable habitats do so at the cost of lower photosynthetic rates and presumably slower growth. The kilocalories per ash-free gram dry weight from highest to lowest species were Ulva (5.44), Gelidium (4.62), Pelvetia (4.17), Egregia (4.10), and Corallina (3.86). With the exception of Pelvetia, which occurs high in the intertidal, these calorific values suggest that selection has tended to reduce nutritive content in climax algal forms. The results for percent thallus loss to urchin grazing over a 48 h period generally support the hypothesis that selection has differentially favored the evolution of antipredator defenses in environmentally constant macrophyte communities; Ulva lost considerably more of its blotted wet weight (43.1%) compared to Pelvetia (36.2%), Gelidium (22.9%), Egregia (22.6%), and the late seral form Corallina (19.6%). Ulva contained the lowest percentage of nonpigmented (structural) components (0.6%), followed by Gelidium (46.2%), Pelvetia (57.3%), Corallina (81.7% CaCO3), and Egregia 82.9%) which indicates that selection in mature communities has tended to increase allocation of materials to nonpigmented supportive structure at the expense of photosynthetic tissue. Later successional macroalgae showed greater toughness as well as greater resistance to wave-shearing forces than the opportunistic species, thus implicating selection for persistence as opposed to rapid growth in climax communities. Data on succession, productivity, and toughness indicate that E. menziesii shifts from an opportunistic strategy during its juvenile stages to a highly differentiated complex form able to persist in mature successional stages. The co-occurrence of different morphological phases of the same species (e.g., Scytosiphon lomentaria, hollow sheet; Petalonia fascia, sheet; and their Ralfsia-like forms, crusts), along with information contrasting productivity, palatability, and longevity between Gigartina papillata and its crustose alternate Petrocelis middendorffii, suggests that some algae have the capacity to concurrently adopt strategies of both opportunistic and climax algal forms.