Intertidal Boulder Field Research Articles

Population Dynamics of Three Polyplacophora Species from the Aegean Sea (Eastern Mediterranean)

The present study is the first to examine spatio-temporal variations in the densities and morphometrics of three shallow water Polyplacophora species (Rhyssoplax olivacea, Acanthochitona fascicularis and Lepidopleurus cajetanus), native to the eastern Mediterranean, while also estimating several growth parameters. Two intertidal boulder fields located in the Pagasitigos gulf (central Aegean) were sampled monthly with SCUBA diving using quadrant sampling, to compare the spatial and temporal (month, season) effects on their size, population density and dispersion pattern. Region was the most significant factor influencing the abundance and size for all three species, while the temporal scales affected mostly Rhyssoplax olivacea. The effect of a boulder under the surface was only significant for the density of Lepidopleurus cajetanus. The standardized major axis method showed that the three species exhibited different allometric relationships between length, width and weight, while a slope comparison between regions yielded significant, in most cases, results. Using the standardized Morisita index for dispersion, a clustered pattern was observed for all species seasonally, with the exception of Acanthochitona fascicularis in Plakes in autumn and winter. To estimate the growth parameters, a bootstrapped Electronic Frequency Analysis (ELEFAN) utilizing a genetic algorithm was employed on pooled populations. L∞ and K varied among the three species with A. fascicularis exhibiting the highest L∞ and L. cajetanus the lowest K value.

Identifying benthic macrofaunal assemblages and indicator taxa of intertidal boulder fields in the south of the Bay of Biscay (northern Basque coast). A framework for future monitoring

The French southern marine subregion of the Bay of Biscay presents local environmental features explaining the presence of specific communities in remarkable habitats. The aim of the study is, for the first time, to assess benthic macrofaunal assemblages and to identify indicator species of each assemblage within intertidal boulder field habitats in this marine subregion.Mobile boulders were sampled with a stratified random sampling design in upper and lower midlittoral zones in March–June 2015. Sessile (in percentage cover) and mobile macrofauna (in abundance) communities were identified and counted within 0.1 m2 quadrats. Ecological function and precision between the two groups were found dissimilar thus species richness, abundance, macrofauna distribution and indicator species/taxa of sessile and mobile macrofauna were analyzed separately. 78 species/taxa of macrofauna were recorded. A restricted list of 12 singletons (8 for mobile and 4 for sessile macrofauna) and 33 combinations of species/taxa were identified as significant indicators of each assemblage. Species ecological features (food webs, signs of disturbance, alien species, biogeographical range limit) were also considered as additional selection criteria. Therefore, species with a high ecological interest but not considered statistically as valid indicators were also highlighted.Thus, this work constitutes a framework for future monitoring of the Basque intertidal rocky shore and could be used as an alternative to sampling the entire biodiversity. It also meets the MSFD requirements (e.g. take into account marine subregion specificities; integrate the metric “macrofauna” and supports several descriptors as the D1 “Biodiversity” and D6 “Seafloor integrity”).

Correction: First evidence of biogenic habitat from tubeworms providing a near-absolute habitat requirement for high-intertidal Ulva macroalgae.

[This corrects the article DOI: 10.1371/journal.pone.0176952.].

Disturbance-mediated facilitation by an intertidal ecosystem engineer.

Ecosystem engineers facilitate communities by providing a structural habitat that reduces abiotic stress or predation pressure for associated species. However, disturbance may damage or move the engineer to a more stressful environment, possibly increasing the importance of facilitation for associated communities. In this study, we determined how disturbance to intertidal boulders (i.e., flipping) and the subsequent movement of a structural ecosystem engineer, the tube-forming serpulid worm Galeolaria caespitosa, from the bottom (natural state, low abiotic stress) to the top (disturbed state, high abiotic stress) surface of boulders influenced the importance of facilitation for intertidal communities across two intertidal zones. Theory predicts stronger relative facilitation should occur in the harsher environments of the top of boulders and the high intertidal zone. To test this prediction, we experimentally positioned boulders with the serpulids either face up or face down for 12months in low and high zones in an intertidal boulder field. There were very different communities associated with the different boulders and serpulids had the strongest facilitative effects on the more stressful top surface of boulders with approximately double the species richness compared to boulders lacking serpulids. Moreover, within the serpulid matrix itself there was also approximately double the species richness (both zones) and abundance (high zone only) of small invertebrates on the top of boulders compared to the bottom. The high relative facilitation on the top of boulders reflected a large reduction in temperature by the serpulid matrix on that surface (up to 10°C) highlighting a key role for modification of the abiotic environment in determining the community-wide facilitation. This study has demonstrated that disturbance and subsequent movement of an ecosystem engineer to a more stressful environment increased the importance of facilitation and allowed species to persist that would otherwise be unable to survive in that environment.

First evidence of biogenic habitat from tubeworms providing a near-absolute habitat requirement for high-intertidal Ulva macroalgae.

Disturbances in ecological systems can cause new resources to become available and can free the resources held by strongly competitive species. In intertidal boulder fields, wave-action causes disturbance by overturning boulders and freeing space for re-colonisation. In this study, mensurative experiments showed that boulder disturbance may also cause new biogenic-habitat resources to become available, if pre-disturbance boulders originally had tubeworm encrustations on their undersides. On the high-shore of a South Australian rocky coast, a small proportion of boulders had extensive encrustations of serpulid and spirorbid worm-tubes on their uppersides, and were likely to have recently been overturned, as spirorbid tubeworms are almost always only underneath boulders while living. Ulva macroalgae was absent from all boulders, except those with worm-tubes, where up to 61% Ulva cover was observed. Many boulders with tubes did not, however, have much algae, and this was likely caused by grazing. While limpets were seldom observed attached to tube encrustations, snails such as Nerita atramentosa and Bembicium nanum were equally abundant on and off tubes. N. atramentosa was likely the main grazer, as its densities were negatively correlated with Ulva cover. The mechanism causing association of Ulva and worm-tubes is unknown, but may be related to retention of moisture or algal spores within the complex topography of the tubes. Alternatively, some tubes may still have been living and providing nutrients for Ulva from excretory products. This study takes the first step towards understanding a very distinct habitat requirement which allows an important alga to persist in the hostile environment of the rocky-intertidal high shore.

Availability of microhabitats explains a widespread pattern and informs theory on ecological engineering of boulder reefs

The availability of suitable microhabitats has emerged as a key requirement for maximising species diversity gains from ecological engineering of coastal habitats. This includes intertidal boulder field habitat, which is threatened by increasing urbanisation. We examined faunal use of microhabitats offered by natural intertidal boulders on two continents, South Africa and Australia, and then used artificial boulders to test possible mechanisms driving the observed patterns. Gaps often occur between the undersides of boulders and the substratum on which they lie. Substrata underneath boulders are generally uneven, so gaps can occur anywhere along the boulder undersurface, but gaps are generally larger near the edges of naturally-occurring boulders. Boulder edges were found to provide a microhabitat that had greater densities of almost all macrofauna than closer to the centre of boulder undersides. To test the model that microhabitat use of boulder edges reflects their larger gap sizes (relative to boulder centres), artificial boulders were constructed with the underside surface either flat, or with a gap underneath mimicking the mean size of the gap normally found only under boulder edges (when their average shape is considered among numerous naturally-occurring boulders). Artificial boulders were deployed intertidally for seven weeks, and macrofaunal colonisation was compared (a) between flat boulders and those with gaps, and (b) between edges and centres. Five times more macrofauna colonised boulders with artificial gaps, while no effect of proximity to edges was found when the gap-width was controlled for. The size of gaps under boulders appears to be an important microhabitat feature that can explain a widespread distributional pattern in the diverse assemblages of boulder fields. Such gaps are clearly used by numerous species, including some that are rare or commercially important. Provision and augmentation of this microhabitat should be considered for any ecological engineering project that involves intertidal or subtidal boulders.

Predation and anthropogenic impact on community structure of boulder beaches

Predator impacts on intertidal community structure have been studied for rocky platforms, but intertidal boulder fields, a habitat with a greater extension and heterogeneity, have not yet been considered. Keeping in mind that disturbances are considered an important force in determining intertidal habitat diversity, the aims of this work were to describe and quantify boulder field community structure and to assess boulder field community dynamics by proposing possible food webs, taking into consideration predatory and anthropogenic impacts. These aims were achieved by installing predator-exclusion cages outfitted with rocks that were monitored monthly over one year in two study zones, a Management and Exploitation Area for Benthic Resources (MEABR, Playa Chica) and open-access area (OAA, Playa Grande). For both study zones, juveniles were the dominant observed ontogenetic state and invertebrate richness and density were higher inside exclusion cages. Furthermore, the MEABR had a differentiated impact on community structure and dynamics in comparison with the OAA. In conclusion, the roles played by boulder fields in intertidal diversity, especially in recruitment and as a nursery zone, are important to consider in management plans.

Asexual reproduction and somatic growth of the fissiparous sea star Allostichaster polyplax in New Zealand

ABSTRACTAllostichaster polyplax is a fissiparous (capable of reproducing asexually by fission) sea star found in intertidal and shallow subtidal habitats in New Zealand and Australia. A localized population of A. polyplax occurred at a density of 2.7 individuals m−2 in an intertidal boulder field at Otago Harbour, South Island, New Zealand. The percentage of sea stars undergoing fission, recorded from January 2010 to March 2011, peaked in January (midsummer) in both years. The frequency of fission decreased for larger sea stars (mean arm length, R > 25 mm), suggesting an ontogenic shift from asexual to sexual reproduction. Effects of fission (i.e. arm regeneration) and food availability (small gastropods) on growth, storage and sexual reproduction were examined in a factorial laboratory experiment. There was no effect of arm regeneration or food on total arm growth. Storage capacity, measured as pyloric caeca index (PCI), was greater in full-armed than regenerating sea stars, and greater in fed than starved individuals. Fed individuals had a similar PCI as field-collected ones. Gonads were observed in two individuals (males) in the full-armed and fed treatment. Fission did not occur during the experiment. Feeding rate on snails was greater for full-armed than regenerating sea stars. Water temperature had a weak positive effect on the feeding rate of regenerating sea stars. Our findings suggest that fission and subsequent growth of ramets can sustain demographically stable and geographically isolated populations of A. polyplax. The role of sexual reproduction in population dynamics and connectivity of this species (and fissiparous sea stars in general) remains equivocal and requires further research on genetic differentiation within and among populations.

Physical and biological effects of introduced oysters on biodiversity in an intertidal boulder field

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 482:119-132 (2013) - DOI: https://doi.org/10.3354/meps10241 Physical and biological effects of introduced oysters on biodiversity in an intertidal boulder field Dannielle S. Green*, Tasman P. Crowe Marine Biodiversity, Ecology and Evolution group, School of Biology and Environmental Sciences, University College Dublin, Dublin 4, Ireland *Email: danniellesgreen@gmail.com ABSTRACT: Invasive species have been identified as a serious threat to biodiversity, particularly in protected habitats. The Pacific oyster Crassostrea gigas, which is invasive in many parts of the world, can form very dense populations affecting the abundance and distribution of native organisms. We experimentally separated the effects of the cover and state (physical structure or biological activities) of C. gigas on the development of invertebrate and macroalgal assemblages. Living (biologically active) and dead (physical structure only) oysters were glued in increasing cover to the tops of cleaned boulders and deployed within an intertidal boulder field. After 14 mo, diversity, evenness and assemblage structure of invertebrates and algae were affected by C. gigas, and results varied depending on the cover and state of oysters. Interestingly, boulders with the lowest cover of living C. gigas supported the most diverse assemblages. Differences in assemblage structure were driven by changes in the establishment of several key species including Fucus vesiculosus and Littorina littorea, which were facilitated by oysters and the honeycomb worm Sabellaria alveolata, which was inhibited by oysters. S. alveolata, an ecosystem engineer that can generate reefs protected by the European Union Habitats directive, mainly established on the underside of boulders but was nonetheless greatly reduced by increasing cover of dead or living oysters on the topside. This work highlights the importance of understanding the direct and indirect mechanisms by which invasive species alter biodiversity and how effects vary as invaders increase in abundance. KEY WORDS: Invasive species · Biodiversity · Protected habitats · Ecosystem engineer · Density dependent · Crassostrea gigas · Sabellaria alveolata Full text in pdf format PreviousNextCite this article as: Green DS, Crowe TP (2013) Physical and biological effects of introduced oysters on biodiversity in an intertidal boulder field. Mar Ecol Prog Ser 482:119-132. https://doi.org/10.3354/meps10241 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 482. Online publication date: May 22, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

A preliminary investigation of diversity, abundance, and distributional patterns of chitons in intertidal boulder fields of differing rock type in South Australia

The rock type of hard substrata marine habitats can affect numerous benthic invertebrates, but little is known of the effects on molluscan assemblages, for example, the chitons often found under intertidal boulders. We compared chiton assemblage composition, abundance, species richness, and patterns of frequency distribution in 10 boulder fields containing either hard metamorphic/igneous boulders or soft limestone boulders in two geographical areas in South Australia. Similar species richness occurred in both types of boulder fields, but hard rock boulder fields had greater overall abundances, because of particularly large abundances of some common species. Differences in abundances of common species also resulted in significantly different assemblages occurring between the boulder field types. Some species appeared aggregated among boulders, but this pattern was variable between boulders in differing areas and of differing rock type. In one area, a common species had variable aggregation that caused frequency distributions to differ significantly between boulders of different rock types. These results indicate that rock type needs to be considered when investigating ecological patterns and processes involving specialist molluscs such as under-boulder chitons and to ensure comprehensive marine reserve planning for protecting rare invertebrates in rocky intertidal reefs.

Constructing replacement habitat for specialist and generalist molluscs—the effect of patch size

Urban shorelines are threatened by 'armouring' from anthropological constructions that are replacing the natural intertidal habitat at an increasing rate. In places, intertidal boulder fields are listed as threatened habitats in urbanized areas. On sheltered shores, these habitats support diverse assemblages, including specialist species, many of which are rare that predominantly or only live under boulders. Activities associated with urbanization threaten these habitats and this specialist biota. Thus, there is a need to learn how to restore these habitats for these assemblages. Most studies of intertidal habitat replacement have focused on habitats dominated by large plants, e.g. mangroves, or by animals that create habitat, e.g. oyster reefs, although previous work has shown that invertebrates will colonize newly created patches of quarried boulders. Here, colonization of newly created patches, either of 50 (50B patches) or 100 (100B patches) quarried boulders replicated in 2 intertidal sites in New South Wales, Australia, is described. Rare specialist and common widespread animals readily colonized these patches, with most of the species randomly distributed between the patch sizes. Of those that did show differences between patch sizes, some species were more abundant on the 100B patches, whereas other related species showed the opposite pattern. After a few weeks, most of the species were as abundant in these patches as on natural boulders. There was no consistent tendency for abundances or diversity to be smaller on the artificial patches than on natural boulders. It is clearly cheap and easy to build an intertidal boulder habitat which is rapidly used by many different common and rare animals. This may assist with the conservation of these fauna living in urbanized environments, where much of their natural habitat has been lost.

Intercontinental tests of the effects of habitat patch type on the distribution of chitons within and among patches in intertidal boulder field landscapes

Patchy distributions within landscapes may be caused by migration in response to different types of habitat patches. Intertidal boulder fields are landscapes in which boulders are discrete habitat patches, often with chitons attached to their under-surfaces. Chiton densities and associations with patch edges differed between boulders overlying coarse- versus fine-sediment types, with greater densities occurring over fine sediments. We tested whether adult migration caused between-boulder distributions by measuring immigration and emigration following experimental replacement of coarse sediments with fine sediment under boulders. We also assessed whether the manipulations altered chiton positions relative to patch edges, and large-scale generality was tested by including data from two continents. The manipulations did not influence the association of chitons with edges or amounts of emigration, but chitons did display positive density-dependent immigration that mirrored their distributional patterns, indicating the importance of immigration. Strikingly, all results were consistent between continents despite involving different species and even genera of chitons. By using boulder fields as a small-scale, easily manipulated landscape, we show that, even in sedentary organisms, patchy distributions within landscapes can be caused by migration alone, without the need to invoke mortality or larval recruitment.

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

AbstractMany species are aggregated at some spatial scale but for some species, due to habitat or behaviour, aggregation can be difficult to assess quantitatively. Here, aggregation of chiton species (Polyplacophora; Neoloricata; Ischnochitonina; Ischnochitonidae; Ischnochitoninae; Ischnochiton) living under boulders in intertidal boulder fields was investigated at several relatively small spatial scales. Closely related species were found to be aggregated at the level of individual boulders, but evidence for grouping at the larger scale of patches of boulders varied. When organisms are mobile, dispersal can have an important influence on spatial patterns. Some organisms, such as marine invertebrates, have a highly dispersive larval phase that can influence spatial patterns of adults, particularly where the adult is sessile or relatively sedentary. Aggregation was compared in two species to test whether different modes of larval development influence spatial pattern. There was some evidence that species with a planktonic larva were even more aggregated than expected, in contrast to predictions based on this mode of development. Both types of larval development (planktonic and non‐planktonic) produce larvae with short development times in these species, so one possible explanation for the grouping habit of these chitons is that they do not disperse, at all, from their natal boulder. The complexity of the boulder field habitat and the cryptic behaviour of these chitons may also contribute to a lack of dispersal. A simple application of a genetic method indicated, however, that philopatry at this scale is unlikely.

Restoring Intertidal Boulder‐Fields as Habitat for “Specialist” and “Generalist” Animals

Restoration is important in urban areas where habitat destruction is greatest. It incorporates many levels of intervention, with creation of new habitat the most extreme form. Most research on habitat creation has been terrestrial, or in marine habitats dominated by large structuring biota, such as mangroves. Intertidal boulder‐fields in urban areas are vulnerable to disturbances and habitat loss, which adversely affect numerous habitat specialists. This study describes experiments in which quarried stones were used to create new habitat outside natural boulder‐fields as a practical approach to restoring habitat. Colonization by specialist fauna and by common algae and invertebrates was measured for a year after deployment. Despite sessile assemblages on new boulders differing from those on natural boulders, common and rare animals rapidly colonized the new habitat. There was no clear succession, but colonization was variable and patchy at all scales examined, although diversities and abundances of some species in this novel habitat matched those of natural boulders within a few months. Rare and common animals generally colonized the new habitat as adults moving in from surrounding areas. Creating new boulder‐fields using quarried rocks is a successful approach to restoration and conservation of fauna where natural boulder‐fields are threatened.

Microscale variations of food web functioning within a rocky shore invertebrate community

The assessment of relevant spatial scales at which ecological processes occur is of special importance for a thorough understanding of ecosystem functioning. In coastal ecosystems, the variability of trophic interactions has been studied at different spatial scales, but never at scales from centimetres to metres. In the present study, we investigated the link between habitat structure and small-scale variability of food web functioning on intertidal boulder field ecosystems. Two microhabitats, boulder-top and boulder-bottom, were considered, and the trophic ecology of invertebrate consumers was studied using stable isotope tracers. We found for two of the main suspension feeders of northern Atlantic rocky shores (the sponges Halichondria panicea and Hymeniacidon sanguinea) consistent 15N enrichment for individuals sampled under boulders, suggesting that these consumers relied on different trophic resource according to the microhabitat inhabited, at a centimetre scale. The high δ15N signatures found underneath boulders suggested higher use of highly decomposed organic matter in this microhabitat. The isotopic difference between the two microhabitats decreased in higher trophic level consumers, which likely foraged at a spatial scale including both microhabitats. Finally, our results reveal that in highly heterogeneous habitats such as boulder fields, trophic interactions are likely to vary strongly in space, which should be considered in future researches. The link between habitat physical structure and food web variability might also contribute to the high biological diversity characterizing heterogeneous ecosystems.

Colonization of novel habitat: Tests of generality of patterns in a diverse invertebrate assemblage

Understanding colonization and use of novel habitat by diverse assemblages is increasingly important as natural habitats are disturbed or replaced. How well novel habitats will be used by biota depends on (i) how well they mimic natural habitat and (ii) how adaptable are the fauna in utilizing different types of habitat. Much of our understanding of colonization of patchy habitats is derived from studies in which colonizing fauna respond strongly to biotic stimuli. Comparisons of patterns of colonization among patches, across landscapes, over time, etc., may thus be confounded by differences in the quality of the patches themselves, which may interact with other features of the broader landscape. Intertidal boulder-fields have many features that make them ideal for extending studies of colonization by diverse assemblages because the organisms living on boulders live in naturally patchy habitats, separated by a matrix which is differentially permeable to different components of this biota. This study used standardized, novel patches of habitat as surrogates of natural boulders in order to control the physical features of these patches. These units of habitat resemble quarried rock that is frequently added to nearshore environments in the form of groynes or breakwaters, or which is commonly disposed of in urbanized estuaries. Previous work has shown that they are readily colonized and used by a similar range of taxa as are natural boulders. Here, they were added to existing boulder-fields to test specific hypotheses regarding colonization of different types of patches across replicate sites, times, periods of deployment and levels of taxonomic grouping. All tests showed extremely variable colonization, with little support for more deterministic models that have been proposed to explain species–habitat relationships. Variability in colonization mimicked natural levels of variability in these fauna reported elsewhere. Colonization of novel habitats in this system seems therefore to be strongly influenced by stochastic variation in supply of colonists and their ability of colonists to utilize and move through he surrounding matrix, rather than on the quality of habitat itself. This needs careful consideration in attempts to manage or repair damaged marine habitats.

Latitudinal patterns of diversity and abundance in North Atlantic intertidal boulder-fields

In order to study taxon richness, biodiversity and abundance patterns in the North Atlantic from temperate latitudes through Arctic to high Arctic latitudes, we recorded the faunas (at ELWS level) colonizing 20 cobbles from three sites at each of seven boulder-field localities (south-west England, 50°N; Wales, 51°N; west Scotland, 56°N; Iceland, 64°N; Tromso, 70°N; Svalbard, 77°N, 79°N). Inverse correlations were found between latitude and all measures of richness (species, orders, and phyla numbers) and biodiversity (S–W, P). However, these correlations were driven mostly by the consistently impoverished Arctic sites; an even cline of decreasing diversity from south to north did not exist. Multidimensional scaling revealed two communities, temperate–subarctic species-rich and high arctic species-poor. Evenness as measured by Pielou’s index was similar across all latitudes. Abundance data exhibited a similar trend to the species richness and diversity data with a significant negative correlation with latitude, but when Arctic data were excluded the correlation vanished.

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.

The use of sandstone blocks to test hypotheses about colonization of intertidal boulders

This study tested the hypothesis that regularly-shaped sandstone blocks in intertidal boulder-fields are colonized by mobile macrofauna at a similar rate to colonization of natural boulders. In addition, the sessile component on these plates (three levels of sessile assemblages) and their position in the boulder-field (three different positions relative to other boulders) were varied to test hypotheses about effects of these features on colonization. The epibiota varied among the three sets of plates and the natural boulders at the start of the experiment. The boulders were very rapidly colonized by a suite of mobile animals, particularly chitons, gastropods and echinoderms. Colonization was sparse and patchy to start with, causing great variability among replicates. Nevertheless, it did not differ among any of the treatments, indicating that even at this early stage, colonization did not differ between natural boulders and blocks, nor according to the sessile assemblage, or the proximity of other boulders. By six months, there was little variability among replicates and the assemblages had converged. This indicates that sandstone plates/blocks can form a standardized unit of habitat which can be used to test models about spatial variation in this relatively specialized fauna.

Benthic assemblages in sediments associated with intertidal boulder-fields

Assemblages on top of and underneath rocks in intertidal boulder-fields have been investigated in several studies, although macrobenthic assemblages in soft-sediments below these rocks have generally been ignored. The model where the presence of a boulder changes the sediment below it and/or the composition of the soft-bottom benthic assemblages living in those sediments, in comparison to sediments without boulders, was evaluated in this study. Six boulder-fields in New South Wales (Australia), with different levels of wave-exposure (three exposed and three sheltered), were sampled at two times (August, 2001 and November, 2001). In each location, at each time, sediments below boulders and from similar-sized patches without overlying boulders ( n=6) were sampled using 12 cm diameter cores for analyses of granulometry, organic content and macrobenthic assemblages. Results showed that total organic content in the sediment was greater below boulders at all locations and sampling times. Macrobenthic assemblages and distribution of grain-sizes of the sediments were different below boulders from elsewhere in all sheltered places and in one exposed location at one sampling time. Where differences were observed, the sediments below boulders had a greater percentage of gravel than did sediments elsewhere. The taxa associated with most of the differences between assemblages living below boulders and those living in sediments without boulders differed across locations. Processes explaining these patterns probably differ among locations, even among those with a similar exposure to waves.