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

Abstract

Densities of many organisms, particularly invertebrates, are inherently patchy from place to place, with much variability at small spatial scales. Intertidal and shallow subtidal boulder fields in New South Wales, Australia, support a diverse suite of species, many of which have very patchy distributions among boulders. In this study, the purple urchin Heliocidaris erythrogramma was very patchily distributed at small spatial scales, being abundant under some boulders and absent from others. This pattern was found consistently at different times and in boulder fields tens of kilometres apart. Experiments were done to identify mechanism(s) that may establish and maintain the observed distribution. Small-scale variability in distribution was established early in the animals' life-cycle, because Heliocidaris recruits were more abundant on boulders with adult urchins than on boulders without adult urchins. Manipulative experiments indicated that the pattern of dispersion was also maintained by the behavioural responses of juvenile and adult urchins to either feature(s) of boulders or to cues associated with conspecifics. When Heliocidaris were transplanted to boulders originally without urchins, they left at a greater rate than did urchins on boulders that originally had urchins but from which urchins had been removed. In addition, juvenile and adult urchins colonised more boulders with urchins than boulders without urchins. Behavioural responses to unidentified but specific requirements of habitat provided by some boulders may thus be important in maintaining non-random densities of urchins among boulders.

Habitat selection, facilitation, and biotic settlement cues affect distribution and performance of coral recruits in French Polynesia

“…For example, in the first approach, recruit abundances are correlated with habitat characteristics (e.g., urchin recruits were correlated with characteristics of intertidal boulders; Smoothey and Chapman 2007), while in the second, settlers are provided with a choice of different habitat types in a controlled experiment (Botero and Atema 1982). Commonly, individuals are monitored through time following settlement to relate subsequent performance to habitat selection.…”

Low Rates of Bindin Codon Evolution in Lecithotrophic Heliocidaris Sea Urchins

“…A second conclusion is that some life‐history trait of H. bajulus other than the evolution of large egg size and potentially high risk or cost of polyspermy (which it shares in common with H. erythrogramma and their common ancestor) must account for the significantly lower rate of nonsynonymous bindin nucleotide substitution that evolved secondarily in H. bajulus . That specific life‐history difference is not known, but a plausible candidate difference is the very high spatial density of adult H. erythrogramma (Wright et al 2005; Smoothey and Chapman 2007) relative to both H. bajulus and H . tuberculata (Cole et al 1992), leading to sperm competition and sexual conflict (Denny and Shibata 1989; Levitan and Young 1995; Lauzon‐Guay and Scheibling 2007), and relatively high rates of nonsynonymous bindin substitutions at a few sites in H .…”

“…Heliocidaris bajulus (Dartnall 1972) is endemic to the eastern shores of Tasmania, where its total known geographic range is <500 km (Dartnall 1972; Rowe and Gates 1995). There are no quantitative analyses of H. bajulus distribution and abundance, but qualitative surveys by one of us (R. Emlet) at several intertidal sites suggest that adults live under rocks at relatively low densities (<1 m −2 ) similar to H. tuberculata (Cole et al 1992), but much lower than adult H. erythrogramma densities (about 10 m −2 , up to 100 m −2 in dense feeding aggregations; Wright et al 2005; Smoothey and Chapman 2007). Males are broadcast spawners with typical‐sized testes like other Heliocidaris species.…”

Spatial and genetic investigation of aggregation inIschnochiton(Polyplacophora; Neoloricata; Ischnochitonina; Ischnochitonidae; Ischnochitoninae) species with different larval development

“…Smoothey and Chapman (2007) showed that, like these Ischnochiton species, this urchin is aggregated on certain boulders and there appears to be some effect of conspecifics contributing to this pattern. The urchin Heliocidaris erythrogramma also inhabits boulder fields in NSW.…”