The Ecological Role of a Common Seastar (Patiriella spp.) Within Intertidal Cobble Fields

Abstract

Intertidal cobble habitats are complex three-dimensional marine environments that are understudied despite having unique species assemblages and ecological patterns. New Zealand's common cushion star, Patiriella spp., is found in a wide range of coastal habitats, including intertidal cobble fields. This seastar is an omnivore that feeds predominantly on crustose coralline algae and micro-organisms, but also supplements its diet by scavenging on carrion. Study on the adult ecology of Patiriella spp. is limited and this thesis aims to expand on the knowledge of this species and its role in intertidal cobble communities. First, field surveys were conducted within intertidal cobble fields in Wellington Harbour and on the Wellington South Coast to determine density, size and feeding habits of Patiriella spp. and the density of associated organisms. Patiriella spp. were abundant at all sites, with no significant difference in density between Wellington Harbour and Wellington South Coast; however, feeding rates and sizes were significantly higher on the South Coast. Distribution of Patiriella spp. on cobbles was negatively correlated with the distribution of chitons, suggesting possible competition between these animals. Second, scavenging behaviour was examined in field and laboratory experiments. Patiriella spp. were readily attracted to mussel carrion bait in the field; however, whelks were the numerically dominant taxon attracted to bait and may therefore compete with Patiriella spp. for this resource. Laboratory results showed that movement towards carrion may be indicative of hunger and Patiriella spp. from Wellington Harbour and the Wellington South Coast reacted similarly to carrion, suggesting similar, limited levels, of carrion supply within these regions. The occurrence of interspecific feeding competition was tested in the laboratory by examining growth and mortality in response to varying densities of Patiriella spp. and a locally abundant chiton, and possible competitor, Chiton glaucus. Intraspecific competition was also tested in response to varying densities of Patiriella spp., with and without carrion supplementation and during spring and winter. No inter- or intraspecific competition was found for crustose coralline algae and micro-organisms and this food resource appears not to be limited. However, Patiriella spp. supplemented on mussel carrion had significantly higher growth rates than non-supplemented treatments and this was greater at lower densities, suggesting intraspecific competition for carrion. Increases in Patiriella spp. size and pyloric caeca weight were only observed for treatments supplemented with carrion. Therefore, carrion appears important for growth and reproduction and intraspecific competition for this resource may impact population sizes. Consequently, fluctuations in carrion supply have the potential to change the distribution and abundance of Patiriella spp., leading to changes in community dynamics. This study has provided baseline information on Patiriella spp. populations within intertidal cobble fields in Wellington Harbour and on the Wellington South Coast and also improved knowledge of the feeding behaviours and competitive interactions of this seastar; therefore, contributing to understanding of the ecological role of Patiriella spp. within intertidal cobble communities.