Patterns of adult abundance in Chthamalus stellatus (Poli) and C. montagui Southward (Crustacea: Cirripedia) emerge during late recruitment

The aim of this study was to investigate when adult distribution patterns are established in the barnacles Chthamalus stellatus and C. montagui. Adult ‘zones’ were identified by analysing field counts of both species at mid and upper shore heights. Monthly collections of cyprids, < 1 month old metamorphs and recruits (all metamorphosed individuals older than approximately 1 month) were made for C. stellatus and C. montagui in natural barnacle beds at six shores in SW Ireland. This was carried out over one year in 1996/1997, using a hierarchical sampling design. Abundance of total recruits (0-3 months old) was compared between adult zones after the main settlement season had ended. In addition, scales of variability in 0-3 month recruitment into adult zones were compared between the species at two scales: shores (1000s of metres) and sites within shores (10s of metres). Older recruits of each species, up to 11 months of age, were also compared between adult zones.The majority of settlement (measured as attached cyprids) occurred between August and October 1996. In October, there was no effect of adult zone on the abundance of total (0-3 month) recruitment up to that point in either species. Despite this homogeneity in recruitment between adult zones, significant spatial variation was found in 0-3 month recruits of both species at both of the scales examined. In C. stellatus the amount of variation associated with the larger scale (shore) was more than twice that of sites or of the residual variation (replicates within sites). 0-3 month recruitment in C. montagui was also most variable at the scale of shores but the residual variability (between replicates within site) was of similar magnitude to that of shores. Variability in 0-3 month C. montagui recruitment was relatively low at the scale of sites.There was a small but consistent input of recruits to adult zones over 9 months of the year, complicating the assessment of when adult patterns were set-up in these species. By June 1997, characteristic patterns of adult dominance had been established at all shores. Settlement had completely ceased by this time and individual barnacles were potentially 11 months old. Neither settlement nor early recruitment are significant in determining adult zonation patterns in these species. Instead, differential mortality patterns in individuals up to the age of 11 months are implicated in determining patterns of distribution of both species.     

Spatial variation in the recruitment of the intertidal barnacles Chthamalus montagui Southward and Chthamalus stellatus (Poli) (Crustacea: Cirripedia) over an European scale

Spatial variation in the recruitment of the intertidal barnacles Chthamalus montagui and Chthamalus stellatus was examined over an European scale. The study was carried out using standardised protocols at a series of locations. The five locations chosen (SW Ireland, NW Spain, SW Portugal and NW and NE Italy) span a large part of the range of these species in Europe. The spatial scales were location (hundreds of kilometres) and shore (thousands of metres).Estimates of total cumulative recruitment (cyprids and metamorphs) summed over the year (April 1997 to March 1998) showed substantial variation between locations which was dependent on the species. Recruitment was highest in SW Portugal for C. montagui and in SW Ireland and NW Spain for C. stellatus. Overall recruitment of C. montagui was higher than that of C. stellatus at all locations except SW Ireland, where recruitment of the two species was not significantly different. There were significant differences among shores in each location.The recruitment period of both species varied with location, with recruitment beginning earlier further south. In general, recruitment of C. montagui and C. stellatus was recorded in 8 months in NW Spain and NE Italy, while only in 7 months in SW Ireland. Recruitment of C. montagui occurred in 10 months in SW Portugal, but no recruits of C. stellatus were found. In all locations there was at least one distinct peak of recruitment. In SW Ireland both species showed only one peak of recruitment, a month after initiation. At the more southerly Atlantic locations, as well as in the Mediterranean, two unequal peaks of recruitment were generally seen. During recruit census, the number of cyprids, in comparison to metamorphs, found at any location was very low. In SW Ireland and NW Spain cyprids of both species were found, while in SW Portugal and in the Mediterranean, only cyprids of C. montagui were found.     

Spatial variation in juvenile and adult Ecklonia radiata (Laminariales) sporophytes

Juvenile and adult sporophytes of Ecklonia radiata were counted on in-shore and off-shore reefs at Marmion (Perth, Western Australia), across spatial scales of kilometres, 10s of metres and metres. The position on in-shore reefs vs. off-shore reefs did not influence recruit and adult kelp density. There was considerable site-to-site (kilometres) variation in densities of recruits but not adults. The majority of variation in both recruit and adult densities was found between quadrats separated by a few metres. There was no relationship between abundance of recruits and abundance of adult sporophytes at any of the spatial levels of investigation, suggesting a decoupling of recruitment and canopy processes. The results emphasise the patchy nature of kelp canopy distribution and suggest that relatively small-scale processes, acting on scales of a few metres, are of primary importance in generating heterogeneity in the canopy of these kelps.     

Small-scale spatial variability in intertidal and subtidal turfing algal assemblages and the temporal generality of these patterns

Spatial and temporal variation in patterns of distribution and abundance of algal assemblages is large and often occurs at extremely small spatial and temporal scales. Despite this, few studies investigate interactions between these scales, that is, how patterns of spatial variation change through time. This study investigated a number of scales of spatial variation (from tens of centimetres to kilometres) in assemblages of intertidal and subtidal turfing algae. Significant differences were found in the composition and abundances of species in assemblages of turf at all spatial scales tested. Much of the variation among assemblages could, however, be explained at the scale of quadrats (tens of centimetres apart) (27±1.4 (SE)% of dissimilarity) with an additional 7±1.2% explained at the scale of sites (tens of metres apart) and 10±1.5% at the scale of locations (kilometres apart). Although the greatest dissimilarity in assemblages occurred at the scale of habitats, this accounted for a relatively small proportion of the overall variation in assemblages. These patterns were consistent through time, that is, at each sampling time the spatial scale explaining the greatest proportion of variation in assemblages was replicate quadrats separated by tens of centimetres. These patterns appear to be due to small-scale variation in patterns of distribution and abundances of the individual species that comprise turfing algal assemblages. The results of this experiment suggest that large scale processes have less effect on patterns of variability of algal assemblages than those occurring on relatively smaller spatial scales and that small-scale spatial variation should not be considered as simply “noise”.     

Spatial and temporal variation in distribution of Gelidium canariensis (Rhodophyta) from natural populations of the Canary Islands

This study was designed to investigate spatial and temporal variation in Gelidium canariensis populations at two shores in northern Gran Canaria during two years. Spatial scales ranged from some hundred meters (distance between shores), 10 to 30 m (distance between plots) to less than 3 m (distance between quadrats). Gelidium individuals were defined as distinct Gelidium clumps. The results show a significant difference in size of clumps between shores, but not on the smaller spatial scales. No significant temporal variation was found. There was no significant temporal or spatial variation in standing crop or density (counts made in quadrats where Gelidium was present, rather than counts for the total shore). Sporophytic and gametophytic clumps were also distinguished by identifying reproductive structures in the field. The total proportion of sporophytes was larger than the proportion of gametophytes, but at a smaller scale there could be a shift in dominance. The survival rate of clumps was similar between shores with a mean survival rate of 85%, but there was a significant difference in recruitment between shores. The results indicate a stable population structure. This revised version was published online in June 2006 with corrections to the Cover Date.     

Population affinities of the asiatic cobra (Naja naja) species complex in south-east Asia: reliability and random resampling

The population systematics of the cobras of the genus Naja in southern Thailand, Malaysia and Indonesia are investigated, using multivariate analysis of a large number of morphological characters. These populations are found to constitute three distinct groups: a northern form, which occurs in Thailand and northern Peninsular Malaysia; an equatorial form, which occurs in southern Thailand, Peninsular Malaysia, Sumatra and Borneo; and a southern form, which occurs on Java and the Lesser Sunda Islands. The first two forms are sympatric in northern Peninsular Malaysia and southern Thailand, and therefore constitute separate species. This is of importance for the treatment of snakebite in the region. The distribution of the three forms can be related both to present ecological conditions and to Pleistocene geological and climatic events. The reliability of the results is demonstrated by the relationship between character number and congruence of patterns of geographic variation, investigated by random resampling. The pattern of geographic variation within two of the three main forms is investigated and related to current ecological conditions and Pleistocene events.     

Hierarchical spatial structure in soft-bottom mussel beds

Mussels (Mytilus edulis L.) are unusual because they thrive in both rocky shore and soft-bottom habitats. Despite their ecological and economic importance, little is known about their spatial structure. Mussels do not generally recruit to bare soft substrate because larvae and postlarvae cannot attach to a bottom of small sediment particles. They attach to hard objects on the sediment surface (especially other mussels), so soft-bottom mussel beds may be spatially organized in ways that are fundamentally different from those on rocky shores. The purpose of our study was to characterize the scales of spatial variability for several mussel abundance parameters in soft-bottom, intertidal M. edulis beds in coastal Maine. We used a random factor nested-ANOVA design of 200 cm² Cores within 1 m² Quadrats within 6 m Transects within Positions within bed Sites along 70 km (euclidean distance) of the Maine coast. Based on the literature and our field observations, we hypothesized that Sites and Positions account for most of the spatial variance in soft-botttom mussel beds. We rejected this hypothesis. Sites and Positions were not important in explaining variation in total mussel density, density of new recruits, or density of larger mussels. Although most of the variance in surface silt-clay fraction did occur at these levels, most mussel variation occurred at smaller spatial scales, specifically at the Quadrat scale for new recruits and total mussels and at the Transect scale for larger mussels. Variance in mussel parameters was not closely linked to the silt-clay fraction of surface sediment or to Site rankings of wind exposure and tidal flow. Variance in total mussel density was due primarily to variance in recruitment. No single scale explained more than about half the mussel variance, and no single scale was best at explaining all the mussel parameters. Greater knowledge about mussel bed spatial variability would be useful because it can help direct scale-dependent sampling regimes, field experiments, and coastal management practices.     

Spatial analyses of intertidal assemblages on sheltered rocky shores

Abstract Understanding processes in complex assemblages depends on good understanding of spatial and temporal patterns of structure at various spatial scales. There has been little quantitative information about spatial patterns and natural temporal changes in intertidal assemblages on sheltered rocky shores in temperate Australia. Natural changes and responses to anthropogenic disturbances in these habitats cannot be accurately measured and assessed without quantitative data on patterns of natural variability in space and through time. This paper describes some suitable quantitative methods for examining spatial and temporal patterns of diversity and abundances of highshore, midshore and lowshore intertidal assemblages and the important component species for a number of shores in a bay that has not been severely altered by human disturbance. Despite a diverse flora and fauna on these shores, the midshore and lowshore assemblages on sheltered shores were characterized by a few species which were also the most important in discriminating among assemblages on a shore and, for each assemblage, among different shores. The same set of species was also important for measuring small-scale patchiness within each assemblage (i.e. between replicate sites on a shore). Therefore, these data provide a rationale for selecting species that are useful for measuring differences and changes in abundance among places and times at different scales and, hence, can be used in the more complex sampling designs necessary to detect environmental impacts. There was considerable spatial variability in all assemblages and all species (or taxa) examined at scales of metres, tens of metres and kilometres. There were no clear seasonal trends for most measures, with as much or more variability at intervals of 3 months as from year to year. Most interactions between spatial and temporal measures were at the smallest scale, with different sites on the same shore generally showing different changes from time to time. The cause(s) of this apparently idiosyncratic variability1 were not examined, but some potential causes are discussed. These data are appropriate for testing hypotheses about the applicability of these findings to other relatively undisturbed sheltered shores, about effects of different anthropogenic disturbances on sheltered intertidal assemblages and to test hypotheses about differences in intertidal assemblages on sheltered versus wave-exposed shores.     

Phylogeographic Pattern of the Striped Snakehead,Channa striata in Sundaland: Ancient River Connectivity,Geographical and Anthropogenic Singnatures

A phylogeographic study of an economically important freshwater fish, the striped snakehead, Channa striata in Sundaland was carried out using data from mtDNA ND5 gene target to elucidate genetic patterning. Templates obtained from a total of 280 individuals representing 24 sampling sites revealed 27 putative haplotypes. Three distinct genetic lineages were apparent; 1)northwest Peninsular Malaysia, 2)southern Peninsular, east Peninsular, Sumatra and SW (western Sarawak) and 3) central west Peninsular and Malaysian Borneo (except SW). Genetic structuring between lineages showed a significant signature of natural geographical barriers that have been acting as effective dividers between these populations. However, genetic propinquity between the SW and southern Peninsular and east Peninsular Malaysia populations was taken as evidence of ancient river connectivity between these regions during the Pleistocene epoch. Alternatively, close genetic relationship between central west Peninsular Malaysia and Malaysian Borneo populations implied anthropogenic activities. Further, haplotype sharing between the east Peninsular Malaysia and Sumatra populations revealed extraordinary migration ability of C. striata (>500 km) through ancient connectivity. These results provide interesting insights into the historical and contemporary landscape arrangement in shaping genetic patterns of freshwater species in Sundaland.     

Investigating patterns and processes of demographic variation: environmental correlates of pre-breeding survival in red-billed choughs Pyrrhocorax pyrrhocorax

1. Quantifying the pattern of temporal and spatial variation in demography, and identifying the factors that cause this variation, are essential steps towards understanding the structure and dynamics of any population. 2. One critical but understudied demographic rate is pre-breeding survival. We used long-term colour-ringing data to quantify temporal (among-year) and spatial (among-nest site) variation in pre-breeding survival in red-billed choughs (Pyrrhocorax pyrrhocorax) inhabiting Islay, Scotland, and identified environmental correlates of this variation. 3. Random-effects capture-mark-recapture models demonstrated substantial temporal and spatial process variance in first-year survival; survival from fledging to age 1 year varied markedly among choughs fledged in different years and fledged from different nest sites. Spatial variance exceeded temporal variance across choughs fledged from well-studied nest sites. 4. The best-supported models of temporal variation suggested that first-year survival was higher in years following high tipulid larvae abundance and when weather conditions favoured increased invertebrate productivity and/or availability to foraging choughs. These variables explained up to 80% of estimated temporal process variance. 5. The best-supported models of spatial variation suggested that first-year survival was higher in choughs fledged from nest sites that were further from exposed coasts and closer to flocking areas, and surrounded by better habitat and higher chough density. These variables explained up to 40% of estimated spatial process variance. 6. Importantly, spatio-temporal models indicated interactive effects of weather, tipulid abundance, local habitat and local chough density on first-year survival, suggesting that detrimental effects of poor weather and low tipulid abundance may be reduced in choughs fledged from nest sites surrounded by better foraging habitat and lower chough density. 7. These analyses demonstrate substantial temporal and small-scale spatial variation in pre-breeding survival, a key demographic rate, and indicate that this variation may reflect interactive effects of weather, prey abundance, habitat and geography. These patterns illustrate the value of holistic models of demographic variation, and indicate environmental factors that may limit the growth rate of Islay's protected chough population.     

Mangrove restoration without planting

Mangrove forests are disappearing at an alarming rate. Mangrove planting is the most common method of restoring these forests. However, this approach is not often successful, especially when the reasons for mangrove degradation were not removed prior to planting new seedlings or propagules. A successful mangrove restoration project may not necessarily include a planting phase. When the stressors are removed and suitable environmental conditions such as correct hydrology and calm area, particularly on exposed coasts, are provided, natural regeneration processes could recover mangroves from degradation. This paper describes an approach to mangrove restoration applied to an exposed shoreline on the west coast of Peninsular Malaysia. In this method a detached breakwater was erected seaward at the shoreline to shelter the restoration area from wave action, prevent the ongoing erosion and promote sediment deposition raising the elevation of the substrate to the target elevation. Survival of Avicennia marina seedlings transplanted in the sheltered area was significantly affected by sediment burial (P < 0.05). However, about eight months after construction of the breakwater, the sedimentation rate decreased and the substrate became stable. Subsequently, natural recruits appeared on the site. We postulate that waterborne seeds or propagules were available from adjacent stands in the study area but wave exposure reduced the possibility of natural recruitment. Thus the breakwater presence provided favourable conditions which could attract mangrove recruits, facilitating reestablishment and natural recovery of the mangrove ecosystem without planting.     

Effect of the substratum in the recruitment and survival of the introduced barnacle Balanus glandula (Darwin 1854) in Patagonia, Argentina

The barnacle Balanus glandula was introduced in Argentina in the 1970s, and today it dominates the high intertidal level in most Argentinean rocky shores. The aim of this work was to evaluate the effect of the type of substrata and intertidal height on a population of Balanus glandula by conducting field surveys and one-year field experiments in which we combined different substrata (hardness: hard and soft, and texture: smooth and rough) at two intertidal heights (mid and high). In natural populations, the highest density of adults and recruits occurred on soft-rough substratum and in the high intertidal. The different textures were important only on the soft substrata and high intertidal, and the density of barnacles of the soft-rough substrata was higher than soft and smooth ones. The most suitable experimental substratum was the soft-rough of the high intertidal, which had the highest recruitment, survival and final density of barnacles at the end of the experiment. In contrast, the hard and smooth of the high and middle intertidal were the least suitable in all cases. Although the recruitment of B. glandula occurred throughout the year, it was higher in the high intertidal, and it showed a recruitment peak in the winter and a second in the summer. While most studies on this barnacle investigated the effects of granite or other volcanic hard substrata, our study also focused on soft substrata. The effects of soft substrata are particularly important because soft sedimentary rocks characterise the southern Atlantic coast of South America and the presence of soft rocks appears to optimize the success of Balanus glandula.     

SMURFs: standard monitoring units for the recruitment of temperate reef fishes

I evaluated a standard monitoring unit for the recruitment of reef fishes (SMURF) as a tool for ascertaining spatial and temporal patterns of reef fish recruitment in central California, USA. SMURFs consisted of a 1.0×0.35 m dia. cylinder of fine mesh plastic grid that contained a folded section of larger mesh plastic grid. SMURFs collected new recruits of 20 species of fish with 92% of the individuals collected from 10 species, mostly rockfish (genus Sebastes). An experiment varying depth of SMURFs in the water column (surface, mid-depth, or bottom) showed that surface SMURFs collected the greatest diversity of species and significantly greater abundance for eight species, with two species having significantly greater abundance on mid-depth SMURFs and three species having significantly greater abundance on bottom SMURFs. A comparison of cumulated recruitment from SMURFs that varied in sampling frequency (removal of new recruits every 1-3, 7, or 28 days) suggested that increasing the time between sampling caused a significant decrease in recruitment estimates for some species but not for others. To determine how well temporal patterns of recruitment to SMURFs reflected patterns to nearby reefs, I compared within season temporal patterns of recruitment to SMURFs with that at nearby reefs, estimated by visual transect surveys conducted on scuba. Temporal patterns of recruitment to SMURFs were significantly and positively related to early recruitment on reefs for one group of benthic-algal associated rockfish species when diver surveys were lagged by 30 days (r=0.87) and for another group of canopy-algal associated rockfish species when lagged by 5 days (r=0.72). SMURFs appeared to be an effective and efficient method for indexing relative rates of delivery of competent juveniles for many temperate nearshore reef fishes.     

Combining Traits and Density to Model Recruitment of Sessile Organisms

We propose an integrative approach that explains patterns of recruitment to adult populations in sessile organisms by considering the numbers of individuals and their body size. A recruitment model, based on a small number of parameters, was developed for sessile organisms and tested using the barnacle Semibalanus balanoides, a marine invertebrate inhabiting North Atlantic intertidal shores. Incorporating barnacle body size improved model fit beyond that based on density alone, showing that growth played an important role in how resource limitation affected survival. Our approach uncovered the following: First, changes in the shape of the recruitment curve resulted from the balance between individual growth and mortality. Second, recruitment was limited by the least plastic trait used to characterise body size, operculum area. Basal area, a trait that responded to increases in barnacle density, did not contribute significantly to explain patterns of recruitment. Third, some temporal variation is explained by changes in the amount of space occupied by shells of dead barnacles: at high cover barnacles are densely packed and these shells remain long after death. Fourth, seasonal variation and spatial variation in survival can be separated from that resulting from resource limitation; survival was predicted for two different shores and four sampling times using a single recruitment model. We conclude that applying this integrative approach to recruitment will lead to a considerable advance in understanding patterns of mortality of early stages of sessile organisms.     

Genetic differentiation in the mud crab Scylla serrata (Decapoda: Portunidae) within the Indian Ocean

Scylla serrata (Decapoda: Portunidae) is a swimming crab that is widespread in the Indo-Pacific region and commonly found in estuarine and mangrove waters. An extended planktonic larval phase suggests high dispersal potential and the possibility of extensive gene flow between conspecific populations at least on a geographic mesoscale (tens to hundreds of kilometres).Intraspecific variation of the mitochondrial DNA cytochrome oxidase subunit I (mtDNA COI) gene was investigated in 77 individuals from four representative mangrove swamps of the African tropics (Kenya and Zanzibar) by means of DNA sequencing. We examined 535 base pairs (bp) and identified 24 different haplotypes. Each population sample is characterised by a single most frequent haplotype, shared among all four populations, and a small number of rare ones, typically present in only one or two individuals and representative of a specific population.Analysis of molecular variance (AMOVA), F_ST statistics and χ² contingency analysis of spatial distribution of mtDNA haplotype frequencies revealed in toto a significant genetic differentiation among populations. These results could indicate that gene flow might be reduced, even between geographically close sites, despite the high potential for dispersal; anyway, at the recorded level of divergence and owing to the abundance of rare haplotypes and singletons in our data set, repeated sampling over time is necessary to establish whether the recorded pattern of genetic differentiation is stable and biologically significant.Finally, integration of our data with those reported by Gopurenko et al. [Mar. Biol. 134 (1999) 227] on S. serrata from South Africa, Red Sea and Mauritius Islands allowed to infer S. serrata population structure within a larger area of the Indian Ocean region.     

Biodiversity of stream insects in the Malaysian Peninsula: spatial patterns and environmental constraints

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Theloderma licin McLeod and Ahmad, 2007 （Lissamphibia： Anura： Rhacophoridae）, a New Frog Record for Borneo

A new record of the rhacophorid frog, Theloderma licin is reported from Gunung Serambu, in the Bau region of western Sarawak, East Malaysia, which comprises the first published record of the species from the Sundaic island of Borneo. The species was previously known from the Malay Peninsula （southern Peninsular Thailand and Peninsular Malaysia）. Species of Theloderma are widespread in mainland and insular Southeast Asia, their arboreal habits perhaps contributing to our perception of rarity.     

An experimental test of the effects of variation in recruitment intensity on intertidal community composition

The rocky intertidal has been a model system for experimentally testing hypotheses regarding the factors that structure natural communities. Many ecologists have proposed that changes in the intensity of recruitment of individuals into a community should influence community structure. Past work investigating this hypothesis has primarily been surveys of recruitment and community structure across large spatial scales. Surprisingly, no researchers to date have manipulated recruitment into a rocky intertidal community to assess the response of interactions of the whole community to variation in recruitment intensity. We manipulated the densities of Balanus glandula and Chthamalus spp. recruits across a four-fold difference at two sites spanning the Monterey Bay, California, USA, to experimentally test if differences in recruitment intensity influenced initial changes in community composition and if these changes persisted through time. The results of this work indicate that differences in recruitment influence community composition initially, but that these changes can be short-lived.     

Temporal changes in the strength of density-dependent mortality and growth in intertidal barnacles

1. In demographically open marine systems, the extent to which density-dependent processes in the benthic adult phase are required for population persistence is unclear. At one extreme, represented by the recruitment limitation hypothesis, larval supply may be insufficient for the total population size to reach a carrying capacity and density-independent mortality predominates. At the opposite extreme, populations are saturated and density-dependent mortality is sufficiently strong to reshape patterns established at settlement. 2. We examined temporal variation in the way density-independent and density-dependent mortality interact in a typical sessile marine benthic invertebrate, the acorn barnacle Semibalanus balanoides (L.), over a 2-year period. 3. Recruitment was manipulated at two high recruitment sites in north Wales, UK to produce recruit densities covering the range naturally found in this species. Following manipulation, fixed quadrats were monitored using digital photography and temporal changes in mortality and growth rate were examined. 4. Over a 2-year period there was a clear, spatially consistent, over-compensatory relationship between the density of recruits and adult abundance indicating strong density-dependent mortality. The strength of density dependence intensified with increasing recruitment. 5. Density-dependent mortality did not operate consistently over the study period. It only operated in the early part of the benthic phase, but the pattern of adult abundance generated was maintained throughout the whole 2-year period. Thus, early life-history processes dictated adult population abundance and dynamics. 6. Examination of the natural recruitment regime in the area of study indicated that both positive and negative effects of recruitment will occur over scales varying from kilometres to metres.     

Influence of sediment on burrowing by the soldier crab Mictyris longicarpus Latreille

This study analyses the spatial distribution of burrowing by the soldier crab Mictyris longicarpus (Mictyridae) in relation to the topography of intertidal sandflats. Four different locations were sampled in Port Jackson and Botany Bay (Sydney, New South Wales, Australia). In each location, abundances of crabs and the proportion of juvenile crabs varied among sites 50 m² and tens of metres apart, but not between high- and lowshore levels, nor among shores. Topographic formations, referred to as humps and depressions, were common on the flats and were of the same order of size and spacing as the sites, which showed variation in numbers/sizes of crabs. Manipulative experiments were done in one of the locations to distinguish between the hypotheses that the spatial distribution of the crabs varied according to (1) topography, or (2) the composition of sediment in the different topographic features. In the first experiment, small plots (0.09 m² and 20 cm deep) of sediment were transplanted between humps and depressions, including appropriate controls for disturbance and translocation. The numbers and sizes of crabs colonizing the experimental plots were then measured. Differences in abundance among plots were found higher on the shore. Therefore, the experiment was repeated at highshore levels with increased replication. There were more crabs in depressions than in humps. The proportion of juveniles varied according to the type of sediment, but only in depressions. Heterogeneity and sediment were, therefore, important for explaining some of the spatial variability of the abundances of the crabs.