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The effect of small-scale variation in beach morphology has been largely ignored in studies of beach macrofauna. This study examined the distribution and abundance of the large bivalve, Donax deltoides (commonly known as the 'pipi') in relation to beach cusps (rhythmically spaced undulations of the beachface consisting of scalloped bays and flanking horns on a scale of tens of metres). No differences were found in the abundance of pipis downshore of cusp horns versus cusp bays at any of three sites. In contrast, previous work on a relatively low-energy (reflective) beach reported greater abundances of bivalves below cusp bays than below cusp horns. Results suggest that the swash climate on the higher-energy (more dissipative) beach studied here produces weaker cusping and weaker patterns in the active or passive redistribution of beach fauna by swash action. Wider more dissipative beaches also allow beach macrofauna to remain on flatter parts of the beach, further below cusps than on reflective beaches. The more rigorous study design used here may also contribute to the difference in results between this study and previous studies. However, populations of pipis below cusp bays did contain a greater proportion of small individuals (≤20 mm length) than populations on cusp horns, suggesting some effect related to beach cusps. Small pipis live closer to the beach surface as they have shorter siphons and are subject to larger rates of dislodgement and movement from cusp horns to cusp bays by swash action than are large pipis. Future ecological studies of beaches with cusps should take beach cusps and the processes producing them into account as they may affect the distribution, abundance and the size distribution of beach invertebrates.

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