A preliminary investigation of diversity, abundance, and distributional patterns of chitons in intertidal boulder-fields of differing rock type in South Australia

35 36 The 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 41 Australia. 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 under50 boulder chitons, and to ensure comprehensive marine reserve planning for protecting rare 51 invertebrates in rocky intertidal reefs. 52 53 54 Introduction 55 56 Environmental features within species habitats are often key in setting patterns of distribution 57 in populations. For rocky benthic assemblages, the physical composition and structure of the 58 rock substratum is often important. Varying surface heterogeneity can influence the choices 59 for settlement sites of invertebrate larvae, and the subsequent recruitment of juveniles and 60 adults (Le Tourneux and Bourget 1988; Kohler et al. 1999; Berntsson et al. 2000). Post61 recruitment processes can also be affected by the structure of the substratum, such as 62 movement (Chapman and Underwood 1994; Underwood 2004), mortality (Herbert and 63 Hawkins 2006), and growth (Giesel 1969; Gosselin and Bourget 1989). The rock type of the 64 substratum is one factor that can affect numerous benthic taxa, including barnacles (e.g. 65 Herbert and Hawkins 2006), tube worms (e.g. James and Underwood 1994), urchins (e.g. 66 Bavestrello et al. 2000), and algae (e.g., McGuinness and Underwood 1986). An early 67 descriptive account of the presence/absence of intertidal mollusc species over a large area in 68