Patterns of distribution and abundance of chitons of the genus Ischnochiton in intertidal boulder fields

Abstract

Chitons of the genus Ischnochiton are found predominantly on the undersurfaces of boulders, compared with other intertidal or subtidal habitats. They therefore appear to be habitat-specialists at this scale. This, combined with the fact that boulder fields are relatively sparse compared with other intertidal habitats, makes these animals vulnerable to natural and anthropogenic disturbances. In addition, many species of Ischnochiton are relatively rare and appear to have very patchy abundances, making them likely to have very specific requirements for habitat. We need to understand the habitat requirements in order to manage, conserve and restore disturbed habitats. The present study was carried out at three intertidal boulder fields separated along approximately 200 km of the coast of New South Wales, Australia, centred around Sydney. The boulder fields were representative of those found in this region. The boulders were made of different materials: shale in the north, sandstone in Sydney and quartzitic sandstone in the south. Some boulders in each boulder field were covered by up to 0.4 m of water during low tide. The study showed that the seven species examined were overdispersed among boulders in each of these three intertidal boulder fields. Most boulders did not have associated chitons, but there were very large abundances on a very small number of boulders. Chitons were also overdispersed among boulders that they occupied. These patterns were consistent among shores and among species, even though patterns of abundance were extremely different among different species. These species appear therefore to show specific requirements for habitat at a small spatial scale, using only a small proportion of potential patches of habitat (i.e. boulders) in any place. Extremely patchy patterns of dispersion can be caused by variation in patterns of recruitment, mortality or behavioural responses to habitat or other species. Before performing experiments to investigate such processes, it is useful to test hypotheses of association with habitat using mensurative experiments to identify environmental correlates that might explain the observed patterns. In the present study, sizes of boulders and the associated sessile and mobile assemblages were proposed as mechanisms that could affect dispersion of chitons among boulders. None of these factors, however, showed strong associations with abundances of chitons. The lack of support of these models rules out some features of habitat to which species of Ischnochiton might respond, thereby precluding manipulative experiments involving these features, which are unlikely to be involved in the very patchy patterns of dispersion of species of Ischnochiton .

Small-scale variability in the dispersion of the sea urchin Heliocidaris erythrogramma among boulders

“…Similar small-scale patterns of dispersion have also been documented for other taxa living under intertidal and shallow subtidal boulders (see , Chapman 2005. For example, Grayson & Chapman (2004) found that all chitons of the genus Ischnochiton were extremely overdispersed among boulders, such that most of the boulders available were not occupied by chitons, but some boulders had very large abundances of individuals.…”

“…1). The boulder field at Cape Banks is sheltered from direct wave-action except during rough seas and has been described previously by and Grayson & Chapman (2004). The area used in this study was similar to that described in and Grayson & Chapman (2004).…”

“…The boulder field at Clovelly Bay is sheltered from direct wave-action, except during rough seas. The boulders are composed mostly of sandstone and, like those at Cape Banks, rest on bedrock, overlaid by shell grit, sand or small patches of algal turf (Grayson & Chapman 2004). The headland of Long Reef is surrounded by an extensive rock platform with small, sandy beaches and boulder fields (Smith & Otway 1997, Chapman 2005.…”

“…We evaluated the model that the sea urchin Heliocidaris erythrogramma (hereafter Heliocidaris) has an aggregated distribution under boulders. Most studies on the dispersion of mobile organisms in boulder fields have been about molluscs (Smith & Otway 1997, Chapman 2002a,b, 2005, Grayson & Chapman 2004, although Chapman (2002a) found that the sea star Patiriella exigua was very patchily distributed among boulders. In addition, we used manipulative field experiments to examine models that may explain the dispersion of Heliocidaris, specifically investigating: (1) patterns of recruitment and (2) behavioural responses of post-recruit urchins to different types of boulders.…”

“…These species are often extremely patchy at a hierarchy of spatial scales, from among different boulder fields to among individual boulders, with most variation at the scale of individual boulders within a site (Smith & Otway 1997, Chapman 2002a, 2005, Grayson & Chapman 2004.…”