Spatial patterns of abundance and shell morphology of two gastropod species associated with different morphologies of an intertidal seaweed

Patterns of abundance and shell morphology of intertidal gastropods are typically thought to depend on environmental conditions, but ecosystem engineers such as canopy-forming seaweeds can also influence these traits. The intertidal seaweed Hormosira banksii (H. banksii) is an abundant species and important ecosystem engineer on rocky shores and estuaries in south-east Australia, where it creates thick canopies colonized by a range of invertebrates, particularly gastropods. In Tasmania (southern Australia), H. banksii shows large variability in thallus length and structure, with different algal morphologies on coasts and in estuaries. In this study, individuals of two gastropod species were sampled from different habitats (north coast, east coast and northern estuaries) to: (1) describe the spatial variability in their abundance and shell morphology, and (2) understand if any relationship exists between H. banksii morphology and gastropod shell morphology. Gastropod abundance and morphology were variable, but multivariate analysis suggested a marginal influence of H. banksii morphological traits, particularly vesicle size, on the patterns observed. As the two gastropod species use habitat differently and that different H. banksii morphs occur in different locations, our results suggest that both algal morphology and environmental conditions may affect gastropod abundance and shell morphology across the different habitats. Hence, from these results a model can be developed and tested in subsequent experimental work to tease apart the effects of seaweed morphology from environmental conditions.