Sea urchins provide habitat for rare chitons in intertidal boulder-fields

Sea urchins provide habitat for many other invertebrates and fish which live in association with the spines or tests of the urchins. In boulder-fields in New South Wales, Australia, there is a large suite of chitons of the genus Ischnochiton, most of which show very overdispersed patterns of abundance among boulders, with many individuals aggregated onto few boulders. Most species are too sparse to manipulate experimentally to test hypotheses about their strong patterns of association with particular boulders, but the most common species, Ischnochiton australis, is strongly associated with the urchin, Heliocidaris erythrogramma, at two spatial scales. The two species occurred together on boulders more than expected by chance and, on boulders occupied by both species, abundances were positively correlated. In addition, I. australis were predominantly found under the spines and tests of the urchins. These patterns were general in time and space. Despite the relative rarity of the chiton, experiments were done to test hypotheses about the presence of each species influencing dispersal among boulders of the other species. Sample size was necessarily small because the chitons are not widespread among the boulders and not all experimental treatments and procedural controls could be included due to limited habitat containing chitons and urchins. Nevertheless, replicate small experiments showed that chitons decreased in abundance under boulders from which urchins had been removed, more than under boulders with chitons, although not all tests were significant. Urchins, in contrast, showed no effects of removal of chitons. Thus, it is likely that urchins provide habitat for chitons, rather than the reverse. Therefore, removal of urchins from these habitats by harvesting could have large negative effects on abundances of this relatively rare intertidal chiton, which appears to be a habitat-specialist. These experiments should offer insight into responses to habitat-needs for other rare molluscs and provide guidance on the value of experimental tests of habitat-specificity for rare species, even when rarity itself reduces the scope of the experimental treatments that can be used.