The influence of phenotypic variation in a habitat-forming seaweed on intertidal gastropod communities

Canopy-forming seaweed are ecosystem engineers and play a central role in structuring entire communities through the amelioration of biotic and abiotic conditions and the provision of habitat. Their effects are particularly important in high-stress environments such as the intertidal zone where they provide the foundation for entire communities that would otherwise not occur there. Gastropods are among the most common group of invertebrates on intertidal rocky shores. Many species colonize seaweed canopies, or occur on the rock beneath the canopies, which reduce abiotic stress from strong wave action, high temperature, low humidity and provide a refuge from predation. While the importance of canopy-forming seaweeds as ecosystem engineers is recognised, the influence of morphological variation within intertidal seaweed species in traits such as thallus length, number and shape of structural elements on their engineering capacity and gastropod communities is not well understood. Hormosira banksii (Turner) Descaisne (Fucales, Phaeophyceae) is one of the most abundant canopy-forming species on intertidal rocky shores in southern Australasia. As with many other intertidal seaweeds, H. banksii shows a large degree of morphological variation and several morphologies are recognized across coastal and estuarine environments. While this morphological variation is likely to reflect an adaptation to differences in the tidal regime and wave action at sites, whether morphology differentially influences engineering of abiotic factors and associated gastropod communities is not well understood. This thesis examines variation in the structure of gastropod communities associated with distinct H. banksii morphologies and experimentally examines the evidence for a link between H. banksii morphology, amelioration of abiotic stress and gastropod abundance. Chapter 2 is a large-scale descriptive survey of morphological variation in H. banksii and associated gastropod communities among multiple sites occurring in different habitats (coasts and estuaries), regions (east and north) and times (February-March and October-November), in Tasmania. Three seaweed morphs were identified on the north coast (small individuals), east coast (intermediate sized individuals) and in estuaries (large individuals). Each Hormosira banksii morph supported distinct gastropod communities with a small overlap in species and two dominant species were found in the different habitats. A slight correlation occurred between H. banksii morphology and gastropod abundance, and thallus length, number and length of vesicles were the best predictors of the observed pattern. Chapter 3 investigates variation of shell morphology of the two commonest gastropod species found within the canopies of H. banksii and the link between gastropod shell size and seaweed morphology. Large and small-scale differences occurred in shell morphology for both Lunella undulata (Lightfoot, 1786) and Chlorodiloma odontis (W. Wood, 1828). A different correlation occurred between the morphology of H. banksii and the shell morphology of gastropods, suggesting a species-specific influence of seaweed morphological traits, specifically thallus length and vesicle size, on the observed patterns. Chapter 4 used two field experiments to explore the extent to which canopy cover and morphological variation of H. banksii influences engineering of key abiotic factors (temperature and irradiance) and colonisation by gastropod species. In the first experiment, canopy cover was manipulated to three levels (full [unmanipulated], half and quarter cover) and temperature, irradiance and gastropod abundance monitored. Despite any significative effect of canopy density on temperature and light levels, the full canopies supported the highest abundance of gastropods, with a species-specific colonization of the different sized canopies. In the second manipulative experiment, large (east coast) and small (north coast) morphs were transplanted to a single site and temperature, irradiance and gastropod abundance monitored for three months. The small morph of H. banksii lowered the temperature and irradiance under the canopies and supported the highest abundance of gastropods. Overall, there were weak correlations between temperature and light levels and gastropod abundance. Chapter 5 used lab experiments to examine the habitat choice of the gastropod Lunella undulata and Chlorodiloma odontis and H. banksii for different morphologies (habitat quality) and biomasses (habitat quantity) at different temperatures. There was strong variation in species behaviour, highlighting the importance of H. banksii morphology and cover in buffering temperature and providing different microhabitats to intertidal gastropods. Overall, this thesis has confirmed the importance of H. banksii as ecosystem engineer in the intertidal zone, supporting gastropod communities that differ between seaweed morphologies. However, the experimental tests of gastropod colonisation in the field and habitat choice in the lab suggest the presence of small-scale and species-specific effects of H. banksii morphology and cover on gastropod abundance, highlighting the importance of H. banksii morphological traits in determining the bioengineer potential of this species across different habitat with different environmental conditions.