Functional morphology of intertidal seaweeds; adaptive significance of aggregate vs. solitary forms

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Abstract
Many intertidal seaweeds [Corallina vancouveriensis, Lithothrix aspergillum, Gelidium coulteri, Rhodoglossum affine] show a tremendous gradient of morphological form ranging from spatially separated thalli, to thalli that are aggregated into dense turfs. Aggregation of seaweeds into turfs decreases productivity per gram organic weight due to crowding of thalli but increases resistance to desiccation. The intertidal distribution of the turf growth form is correlated with the intensity of desiccation stress. Also turfs transplanted into tide pools developed the non-turf morphology, while non-turfs transplanted to emergent substrate either developed the turf form or died. The turf growth form is energetically expensive; apparent productivity of turfs was 23-48% less than that of individuals. Increasing light and nutrients available to turfs by separating the thalli increased apparent productivity by 36-113%. The hypothesis that these turfs minimize energetic costs of this aggregated growth form through the spatial partitioning of photosynthetic and respirative activity was tested. The lower portions of turfs showed less apparent photosynthesis than the upper portions (reductions of 37-85%); however, this spatial partitioning was also found in the individual forms (37-63% reductions) of these relatively simple seaweeds. Spatial differentiation of respiration was similar in turfs and non-turfs. The seaweeds examined are able to alter the extent of thalli compaction in accordance with varying levels of environmental stress. This phenotypic plasticity allows seaweeds to adopt morphological features that maximize fitness in a wide variety of habitats without being developmentally committed.

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