![]() In unoccupied pits volume of all macroinvertebrates increased as available space increased. Generally, in occupied pits, small-sized faunas were more abundant than larger faunas, suggesting that sea urchin's body may function as a biogenic structure providing refugia for small-sized individuals. Chitons and limpets were found exclusively on unmodified substrate whereas relatively large-sized gastropods inhabited unoccupied pits, and occupied pits harbored small-sized crustacean and gastropod species. Although a few species were found in all habitat types, community compositions were different. Size structure of the most abundant mobile fauna, top shells Trochus radiatus Gmelin, 1791, were also analyzed to assess whether they exhibit ontogenetic changes in habitat use. Size structures of macroinvertebrates were compared between pits and analyses were performed to investigate whether the use of habitat depends on availability of space, or biological interactions between sea urchins and other macroinvertebrates. We investigated small-scale spatial variation in community of macroinvertebrates by comparing community composition of epilithic macroinvertebrates between those found on unmodified rocky substrate, inside pits occupied by rock-boring sea urchin Stomopneustes variolaris (Lamarck, 1816), and unoccupied pits, on an intertidal rocky shore in southern Thailand. These pits are not only used as their home but seem to harbor a distinct assemblage of organisms. Many sea urchin species excavate pits in sedimentary rock, transforming primary rocky substrates into sea urchins' pits. Rock-boring sea urchins are found on rocky shorelines in several regions worldwide, for example, Paracentrotus lividis was reported from Britain to Mediterranean Sea (Solovjev and Markov, 2013), Echinometra sp., in tropical zone in South Africa (Schoppe and Werding, 1996) Strongylocentrotus purpuratus, along Pacific coast of California, USA (Solovjev and Markov, 2013) Echinostrephus molaris, on the coast of southern Japan and Indian Ocean (Campbell et al., 1973 Kobayachi and Tokioka, 1976 Yamamori and Kato, 2017). (Asgaard and Bromley, 2008 Ayyagari and Kondamudi, 2014 Bronstein and Loya, 2014 Campbell et al., 1973 Carreiro -Silva and McClanahan, 2001 Davidson and Grupe, 2015 Dumont et al., 2013 Ganapati, 1972 James, 1995 Schoppe and Werding, 1996 Solovjev and Markov, 2013 Yamamori and Kato, 2017). These rock-boring sea urchins are considered to be important ecosystem engineers, as they transform natural rocky substrates into sea urchin pits harboring assemblages of other benthic organisms, for example, molluscs, crustaceans, echinoderms and sponges. purpuratus in addition to its biological activities alter community structure at spatial scales finer than those typically considered for sea urchins. ![]() These results suggest the habitats created by S. Herbivores, however, were densest in flat spaces, concordant with high algal coverage. The protective empty pit microhabitat harbored the highest densities of most trophic functional groups. Unlike primary space holders, mobile fauna exhibited higher diversity and richness in empty pits than in flat spaces and occupied pits, although results were not significant. The composition of these primary space holders in the microhabitats also varied. The Shannon diversity and richness of these sessile taxa were significantly higher in flat spaces and empty pits than in occupied pits. In tidepools, flat spaces harbored the highest percent cover of algae and sessile fauna, followed by empty pits and then occupied pits. We investigated how a temperate rocky intertidal community uses three small-scale (<100 cm2) microhabitats created by or adjacent to populations of the purple sea urchin (Strongylocentrotus purpuratus): pits occupied by urchins, unoccupied pits, and adjacent flat spaces. These microhabitats appear to harbor distinct assemblages of species. By eroding rocky substrata, urchins can create a mosaic of urchin-sized cavities or pits separated by exposed, often flat surfaces. Many sea urchins are important habitat modifiers in tropical and temperate systems. By creating novel habitats, habitat-modifying species can alter patterns of diversity and abundance in marine communities.
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