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35 36The rock type of hard-substrata marine habitats can affect numerous benthic invertebrates, but 37 little is known of the effects on molluscan assemblages, for example, the chitons often found 38 under intertidal boulders. We compared chiton assemblage composition, abundance, species 39 richness, and patterns of frequency distribution in ten boulder-fields containing either hard 40 metamorphic/igneous boulders or soft limestone boulders, in two geographical areas in South 41Australia. Similar species richness occurred in both types of boulder-fields, but hard rock 42 boulder-fields had greater overall abundances, because of particularly large abundances of 43 some common species. Differences in abundances of common species also resulted in 44 significantly different assemblages occurring between the boulder-field types. Some species 45 appeared aggregated among boulders, but this pattern was variable between boulders in 46 differing areas and of differing rock type. In one area, a common species had variable 47 aggregation that caused frequency distributions to differ significantly between boulders of 48 different rock types. These results indicate that rock type needs to be considered when 49 investigating ecological patterns and processes involving specialist molluscs such as under-50 boulder chitons, and to ensure comprehensive marine reserve planning for protecting rare 51 invertebrates in rocky intertidal reefs. 52 53 54 Introduction 55

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