• aggregation;
  • boulder;
  • intertidal;
  • marine;
  • mode of development;
  • spatial pattern.


Many species are aggregated at some spatial scale but for some species, due to habitat or behaviour, aggregation can be difficult to assess quantitatively. Here, aggregation of chiton species (Polyplacophora; Neoloricata; Ischnochitonina; Ischnochitonidae; Ischnochitoninae; Ischnochiton) living under boulders in intertidal boulder fields was investigated at several relatively small spatial scales. Closely related species were found to be aggregated at the level of individual boulders, but evidence for grouping at the larger scale of patches of boulders varied. When organisms are mobile, dispersal can have an important influence on spatial patterns. Some organisms, such as marine invertebrates, have a highly dispersive larval phase that can influence spatial patterns of adults, particularly where the adult is sessile or relatively sedentary. Aggregation was compared in two species to test whether different modes of larval development influence spatial pattern. There was some evidence that species with a planktonic larva were even more aggregated than expected, in contrast to predictions based on this mode of development. Both types of larval development (planktonic and non-planktonic) produce larvae with short development times in these species, so one possible explanation for the grouping habit of these chitons is that they do not disperse, at all, from their natal boulder. The complexity of the boulder field habitat and the cryptic behaviour of these chitons may also contribute to a lack of dispersal. A simple application of a genetic method indicated, however, that philopatry at this scale is unlikely.