Density-dependent settlement and mortality structure the earliest life phases of a coral population

The local densities of heterospecifics and conspecifics are known to have profound effects on the dynamics of many benthic species, including rates of settlement and early post-settlement survivorship. We described the early life history of the Caribbean coral, Siderastrea radians by tracking the population dynamics from recently settled planulae to juveniles. Through three years of observation, settlement correlated with the abundance of other benthic organisms, principally turf algae (negatively) and crustose coralline algae (positively). In addition, adult density showed independent effects on coral settlement and early post-settlement survivorship. Settlement rates increased across low levels of adult cover and saturated at a maximum around 10% cover. Early post-settlement survivorship decreased with adult cover, revealing structuring density dependence in coral settlers. The earliest life stages of corals are defined by low survivorship, with survivorship increasing appreciably with colony size. However, recent settlers (one-polyp individuals, < 1-year-old) are more likely to grow into two-polyp juveniles than older single polyps (> 1-year-old) that were delayed in their development. The early benthic phase of corals is defined by a severe demographic bottleneck for S. radians, with appreciable density-dependent and density-independent effects on survivorship. For effective management and restoration of globally imperiled coral reefs, we must focus more attention on this little studied, but dynamic, early life history period of corals.     

Abundances of surgeonfishes on patch-reefs in Caribbean Panamá: due to settlement,or post-settlement events?

I examined whether patterns of settlement of planktonic juveniles determined the size and composition of adult populations of three Caribbean surgeonfishes (A canthurus bahianus, A. chirurgus and A. coeruleus) on six isolated patch-reefs in Panamá. I monitored settlement monthly on six reefs (0.14 to 0.67 ha) for 8 yr (1978–1986), and censused adults on those reefs annually. Year-to-year fluctuations in the absolute and relative abundances of settlers arriving on each reef were not extreme. Densities of adults in 1986 were not correlated with the densities of settlers that arrived between 1979 and 1984. Survival of settlers, although apparently unrelated to the density of adults, was negatively related to the density of settlers per unit of adult habitat (but not per unit of settlement habitat) for each species. Changes in adult populations of each species between 1978 and 1986 were not correlated with the abundance of settlers relative to the initial number of adults. The relative abundances of adults of each species in 1986 did not reflect the relative abundances of settlers (1979–1984); these abundances were due to both spatial variation and interspecific variation in survival. In some cases, relocation of adults between isolated patch-reefs (as much as 100 m from other reefs) substantially altered the size and composition of surgeonfish populations. Thus, either post-settlement mortality or relocation overrode effects of settlement patterns in determining adult abundances on the six reefs. Relocation is important in organizing reef-fish communities and must be further examined, particularly in large habitat mosaics.     

Balancing the edge effects budget: bay scallop settlement and loss along a seagrass edge

Edge effects are a dominant subject in landscape ecology literature, yet they are highly variable and poorly understood. Often, the literature suggests simple models for edge effects-positive (enhancement at the edge), negative (enhancement at the interior), or no effect (neutral)--on a variety of metrics, including abundance, diversity, and mortality. In the marine realm, much of this work has focused on fragmented seagrass habitats due to their importance for a variety of commercially important species. In this study, the settlement, recruitment, and survival of bay scallops was investigated across a variety of seagrass patch treatments. By simultaneously collecting settlers (those viable larvae available to settle and metamorphose) and recruits (those settlers that survive some period of time, in this case, 6 weeks) on the same collectors, we were able to demonstrate a "balance" between positive and negative edge effects, resulting in a net neutral effect. Scallop settlement was significantly enhanced along seagrass edges, regardless of patch type while survival was elevated within patch interiors. However, recruitment (the net result of settlement and post-settlement loss) did not vary significantly from edge to center, representing a neutral effect. Further, results suggest that post-settlement loss, most likely due to predation, appears to be the dominant mechanism structuring scallop abundance, not patterns in settlement. These data illustrate the complexity of edge effects, and suggest that the metric used to investigate the effect (be it abundance, survival, or other metrics) can often influence the magnitude and direction of the perceived effect. Traditionally, high predation along a habitat edge would have indicated an "ecological trap" for the species in question; however, this study demonstrates that, at the population level, an ecological trap may not exist.     

Community recruitment: Settlement and juvenile survival of seven co-occurring species of sessile marine invertebrates

Recruitment of a group of co-occurring sessile invertebrates (a serpulid polychaete, an oyster, a bryozoan, and several colonial and one solitary ascidian) that encrust floating docks in Pearl Harbor, Hawaii, was studied in August 1985. Daily photographs were taken of acrylic settling plates over a 14 d period, and daily settlement and juvenile mortality were measured. Settlement rates were compared between species and days, while juvenile mortalities were compared between species, days, juvenile ages and densities of juveniles on the plates. Species differed in abundance of both settlers and juveniles, and in rate and pattern of juvenile mortality. Settlement intensity varied between days for some species. Significant juvenile mortality occurred during the 14 d for most species; it appeared to be caused by fish predation. For two species, mortality varied with juvenile age, with older individuals suffering higher mortality. Mortality was density-dependent for some species, increasing with numbers of juveniles on the plates. Mortality patterns varied even within the colonial species. Assessing larval settlement and juvenile mortality for a single species or type of organism in a community thus may not indicate that similar patterns exist for co-occurring species.     

Time and space: genetic structure of the cohorts of the common sea urchin <Emphasis Type="Italic">Paracentrotus</Emphasis><Emphasis Type="Italic">lividus</Emphasis> in Western Mediterranean

Spatio-temporal variability in settlement and recruitment, high mortality during the first life-history stages, and selection may determine the genetic structure of cohorts of long-lived marine invertebrates at small scales. We conducted a spatial and temporal analysis of the common Mediterranean Sea urchin Paracentrotus lividus to determine the genetic structure of cohorts at different scales. In Tossa de Mar (NW Mediterranean), recruitment was followed over 5 consecutive springs (2006–2010). In spring 2008, recruits and two-year-old individuals were collected at 6 locations along East and South Iberian coasts separated from 200 to over 1,100 km. All cohorts presented a high genetic diversity based on a fragment of mtCOI. Our results showed a marked genetic homogeneity in the temporal monitoring and a low degree of spatial structure in 2006. In 2008, coupled with an abnormality in the usual circulation patterns in the area, the genetic structure of the southern populations studied changed markedly, with arrival of many private haplotypes. This fact highlights the importance of point events in renewing the genetic makeup of populations, which can only be detected through analysis of the cohort structure coupling temporal and spatial perspectives.     

Spatial variability in habitat associations of pre- and post-settlement stages of coral reef fishes at Ishigaki Island,Japan

Successful settlement of pelagic fish larvae into benthic juvenile habitats may be enhanced by a shortened settlement period, since it limits larval exposure to predation in the new habitat. Because the spatial distribution of marine fish larvae immediately prior to settlement versus during settlement was unknown, field experiments were conducted at Ishigaki Island (Japan) using light trap sampling and underwater visual belt transect surveys to investigate the spatial distribution patterns of selected pre- and post-settlement fishes (Acanthuridae, Pomacentridae, Chaetodonidae and Lethrinidae) among four habitats (seagrass bed, coral rubble, branching coral and tabular coral). The results highlighted two patterns: patterns 1, pre- and post-settlement individuals showing a ubiquitous distribution among the four habitats (Acanthuridae) and pattern 2, pre-settlement individuals distributed in all habitats, but post-settlement individuals restricted to coral (most species of Pomacentridae and Chaetodontidae) or seagrass habitats (Lethrinidae). The first pattern minimizes the transition time between the larval pelagic stage and acquisition of a benthic reef habitat, the latter leading immediately to a juvenile lifestyle. In contrast, the second pattern is characterized by high settlement habitat selectivity by larvae and/or differential mortality immediately after settlement.     

The effect of conspecifics on habitat selection in territorial species

Summary Despite widespread interest in habitat selection, many of the assumptions about how territorial animals choose habitats have not been tested. This study of juvenile Anolis aeneus lizards focuses on the relationship between the number of previous settlers in a habitat and the subsequent behavior of new arrivals at that habitat. Clearings containing the types of microhabitat preferred by juveniles were established in the field, several juvenile residents were allowed to establish territories in enclosures in the center of each clearing, and then naturally occurring immigrants were allowed access to the empty microhabitat surrounding the enclosures. Arrival rates and the probability of settlement were monitored on a daily basis from the day the first juveniles arrived until several days after the last juvenile had settled (=saturation). In each of seven trials, arrival rates were comparable early and late in the settlement process, and were unrelated to the degree of habitat saturation. Arrival rates did vary on a temporal basis, probably as a result of environmental factors affecting egg laying and hatching schedules, and habitats with high arrival rates saturated more quickly than those with lower arrival rates. All of the individuals arriving at the clearings did not settle, but settlers and non-settlers did not differ with respect to competitive ability, as measured by body size. The probability of settlement increased as settlement proceeded in each of seven trials, up to the day of saturation. These results refute the commonly held assumptions that prospective territory owners avoid entering relatively full habitats, and that they prefer to settle in relatively unsaturated habitats. The discussion considers why assumptions about the behavioral processes of habitat selection are so widely accepted, given the dearth of empirical information on the subject.     

Population dynamics of the barnacle <Emphasis Type="Italic">Chthamalus montagui</Emphasis> at two spatial and temporal scales in northern Spain

This study describes spatial and temporal patterns of variability in population parameters in the barnacle Chthamalus montagui Southward in three localities of northern Spain and evaluates whether density-dependent settlement may regulate population dynamics. The sampling design considered two spatial scales, localities and sites within localities, and two temporal scales, years and six month intervals. Density, amount of free space, mortality, growth rate and magnitude of settlement (both absolute and per unit of free space) were obtained from photographs of permanent quadrats and from direct counts in the field. The number of settlers in scraped and untouched quadrats was used to estimate the importance of the presence of conspecifics in settlement. Significant variation at the two spatial and temporal scales was found for most parameters. Large spatial and temporal variations in adult mortality rate, density, and settlement were observed. Patterns of mortality were not consistent with differences in density among localities. Differences in settlement among localities were maintained through time. We suggest that magnitude of settlement is regulated by persistent features such as topography or local water circulation. We assume that early post-settlement mortality does not differ among localities. In the absence of differential mortality, settlement determines average population density within localities. Within localities, settlement was independent of density and free space. No consistent evidence was found on preferential settlement at the vicinity of conspecifics. The main conclusion is that density-dependent settlement is not relevant for the regulation of the populations of C. montagui in the northern Spain. Regulation might occur by density-dependent processes within the adult fraction of the population and/or the larval phase before settlement.     

Settlement, recruitment and potential predators and competitors of juvenile echinoderms in the rocky subtidal zone

Recruitment patterns of marine invertebrates are affected both by settlement and early post-settlement events. This study examined the settlement and recruitment patterns of echinoderms at three sites in the rocky subtidal zone of Bocabec Cove, Bay of Fundy, Canada using artificial turf collectors and quadrats on the natural substrate. Potential predators were quantified at two of the sites along transects and in 1-m² quadrats. Both potential predators and competitors were quantified in 0.0625-m² quadrats. Settlement varied across sites (1.5–3 km apart) and two years of sampling (2004, 2005). The site of most potential settlement differed for the three groups of echinoderms: sea urchins (Strongylocentrotus droebachiensis), sea stars (Asterias spp.) and sea cucumber (Psolus fabricii). Settlement densities on the artificial turf collectors tended to be greater than the densities of settlers on the natural substrate. On the natural substrate, the only significant difference between densities of juveniles over time was that newly settled sea stars were found in July and were not found the following October. Large lobsters and carnivorous worms were potential predators with densities that varied between sites. Potential competitors that differed in abundance between sites were herbivorous gastropods and conspecifics for sea urchins; and carnivorous worms for sea stars. This study suggests that patterns of recruitment are either set up by patterns of settlement or by events during the first few weeks/months on the benthic substrate for these echinoderms.     

Effects of eutrophication and sedimentation on juvenile corals

Settlement of juvenile scleractinian corals was investigated from 1987 to 1990 on eutrophic and less eutrophic fringing reefs on the west coast of Barbados, West Indies. The number of coral recruits and number of recruiting coral species on cement blocks decreased with increasing eutrophication of the reefs. This may suggest lower settlement rates on eutrophic reefs, but could also liave resulted from higher post-settlement mortality, since blocks were examined only once after 3 yr of immersion. Coral settlement rates to artificial plates that were checked monthly were also lower on the more eutrophic reefs. This could result from lower local availability of larvae caused by fewer adult corals and/or lower reproductive rates of corals on eutrophic reefs. However, the ratio of coral recruits to adult coral abundance was considerably lower on eutrophic reefs, suggesting that local coral abundance alone can not explain lower settlement rates on eutrophic reefs. The lower rates on eutrophic reefs may result from a lower probability of coral larvae settlin when present, perhaps because of a limited availability of suitable settlement substrate. Colonization of settlement plates by non-coralline organisms was heavier on eutrophic reefs, and unoccupied space was lower, supporting the suggestion that suitable coral settlement substrate may be limiting on eutrophic reefs. Moreover, coralline algae, which facilitate metamorphosis and settlement of coral larvae, were less abundant on settlement plates on eutrophic reefs.     

Safety in numbers and the spatial scaling of density-dependent mortality in a coral reef fish

In coral reef fishes, density-dependent population regulation is commonly mediated via predation on juveniles that have recently settled from the plankton. All else being equal, strong density-dependent mortality should select against the formation of high-density aggregations, yet the juveniles of many reef fishes aggregate. In light of this apparent contradiction, we hypothesized that the form and intensity of density dependence vary with the spatial scale of measurement. Individual groups might enjoy safety in numbers, but predators could still produce density-dependent mortality at larger spatial scales. We investigated this possibility using recently settled juvenile bluehead wrasse, Thalassoma bifasciatum, a small, aggregating reef fish. An initial caging experiment demonstrated that juvenile bluehead wrasse settlers suffer high predation, and spatial settlement patterns indicated that bluehead wrasse juveniles preferentially settle in groups, although they are also found singly. We then monitored the mortality of recently settled juveniles at two spatial scales: microsites, occupied by individual fish or groups of fish and separated by centimeters, and sites, consisting of approximately 2400-m2 areas of reef and separated by kilometers. At the microsite scale, we measured group size and effective population density independently and found that per capita mortality decreased with group size but was not related to density. At the larger spatial scale, however, per capita mortality increased with settler density. This shift in the form of density dependence with spatial scale could reconcile the existence of small-scale aggregative behavior typical of many reef fishes with the population-scale density dependence that is essential to population stability and persistence.     

Determinants of habitat association in a sympatric clade of marine fishes

Triplefin fishes reach their greatest diversity in New Zealand with 26 endemic species, and habitat diversification has been implicated as a key factor in the divergence of this group. Despite this, it is unknown whether species-specific habitat patterns in these sympatric fishes are established by passive processes (e.g. differential mortality) or by habitat selection during settlement. We investigate this question by comparing the habitat associations of new recruits with those of conspecific adults in five species. In addition, the amount of variation in habitat use of conspecific recruits and adults was calculated to identify ontogenetic shifts in habitat association. The results indicated that while there were some differences between recruit and adult habitats, these differences were small in magnitude and habitat use of new recruits was similar to that of adult conspecifics. This finding was further supported by the small difference in variation of habitat use between conspecific recruits and adults. The study suggests that new recruits are actively involved in the selection of habitats at settlement and maintain the use of these throughout demersal life. Habitat use in these territorial species has a large influence on mate choice, thus habitat selection by new recruits would provide a powerful mechanism for pre-zygotic isolation between individuals with different habitat preferences. Together these findings support the notion that habitat diversification has been a major component in the radiation of this sympatric group.     

Multiple-locus heterozygosity,mortality, and growth in a cohort of Mytilus edulis

A positive correlation between the degree of individual heterozygosity (H) at five polymorphic enzyme loci and shell length was reported for a cohort of Mytilus edulis L. sampled in September, 1983, two months after settlement (Koehn and Gaffney, 1984). In the present study, the same cohort was resampled four and eight months after settlement in November, 1983, and April, 1984, respectively. Among individuals four months of age, there was a small, but statistically significant, negative correlation between H and shell length. In this sample, heterozygote deficiency in the cohort was less than that of any size group in the September sample. The reversal of the H-size relationship between September and November suggests that differential mortality, particularly among small, homozygous individuals, occurred in this period. For individuals eight months of age, there was no correlation between H and shell length, there was no heterozygote deficiency in the cohort and the average heterozygosity was the same as that of the largest individuals in September. At the youngest age, there was a positive relationship, between H and growth rate; as aging occurred, differential mortality altered the relationship between H and growth rate. viability differences among heterozygosity classes obscured the original effect of H on growth rate. If there is a positive relationship between heterozygosity and size, it can most likely be detected from sampling a large outbreeding population before differential mortality occurs and before energy is allocated to reproduction.     

Spatiotemporal coupling/decoupling of planktonic larvae and benthic settlement in decapods in the Scottish east coast

Settlement patterns and the relationship between meroplanktonic larvae and settlement in decapods were studied on the Scottish east coast. Artificial settlement substrates (ASS), deployed at two locations (sandy vs. rocky sea substrates), were employed to collect megalopae and newly settled juveniles. Abundance of meroplanktonic larvae was used as an indicator of larval supply. The results showed a clear seasonality in settlement rates, and in some cases, significant differences between sites were detected. Nevertheless, the interference of the ASS with the surrounding habitat limits the study of spatial variability in settlement rates. Significant cross-correlation was found between the abundance of megalopae and juveniles in the collectors and planktonic larval abundance a month earlier. For individual species, this relationship was observed only in Pisidia longicornis. Complexities caused by the great variety of pre- and post-settlement processes, alongside effects of secondary dispersals of early juveniles may have obscured the relationship between meroplanktonic larvae and juveniles in other species.     

Trace metals in mussels (Mytilus edulis) from southwest Iceland

The temperate population of the tropical anemonefish Amphiprion clarkii has a seasonality of reproduction, larval settlement and growth. There was a considerable difference in size (15–47 mm) among 0-year olds in December, after the season of larval settlement and growth. This difference could be attributed mainly to a difference in the duration of the first growing season spent after settlement and a social inhibition of growth of the subordinate late settlers by frequent attacks by the dominant early settlers. The growth equation which was used to describe the growth of tagged individuals estimated that an anemonefish would get an opportunity to spawn within four of five years after settlement. Breeding adult pairs occupied almost all sea anemones essential to breeding within a 50×50 m study area, and all 17 new adults that matured from non-breeding juveniles to breeding adults between June 1983 and August 1985 were found in the territories from which either or both of the mated adults had disappeared. The new adults were large juveniles who had resided near those vacant territories before. These results suggest that the onset of breeding by an individual is not only determined by his age, but also by his ranking in the dominance hierarchy. Therefore, the difference in size among 0-year olds in December might give rise to the difference in age of the earliest breeding and might be a factor in making a female apply more reproductive effort at the beginning of the season.     

Reproduction and recruitment of scleractinian corals in a high-latitude coral community, Amakusa, southwestern Japan

Reproduction and recruitment in high-latitude coral populations in Japan have been little studied. A comprehensive study of the reproduction and early life history was conducted on nine common scleractinian coral species in Amakusa, southwestern Japan (32°N) from 2001 to 2003 including; (1) fecundity (the proportion of colonies with mature eggs), (2) timing and synchrony of spawning, (3) initial larval settlement pattern, (4) recruitment, (5) post-settlement mortality. The fecundity was high (76.7–100%) in six of seven species examined in 2002 and 2003. Annual spawning of the seven species occurred from mid July to August in 2001–2003, when seawater temperature was at the annual maximum. Spawning was highly synchronised among conspecific colonies and species in 2002 and 2003, with five species spawning five to nine nights after the full moon and another two spawning around the new moon. Temporal patterns of larval settlement of three spawning species during the first 10 days after spawning were similar to those of other spawning species from low latitudes. The number of scleractinian recruits on settlement plates, deployed from July to October (the major recruitment period at the study site), was low (2 recruits/m²) for the three consecutive years. Post-settlement mortality of 1–1.5 month old spat of five species ranged between 88 and 100% over 3–10 months in the field, similar to the values reported for both high and low latitude species (>94–99%). Among the key stages examined, the low recruitment rate may be the most important step in limiting successful reproduction and recruitment of these high-latitude scleractinian populations. The low recruitment rate may be attributable to (1) the reduced influx of larval supply from other coral populations, which are smaller and more isolated at high-latitudes and (2) the longer precompetent larval phase of broadcast-spawning corals which results in an increased chance of larvae being dispersed away from parent populations.     

Settlement and recruitment of sea urchins (Strongylocentrotus spp.) in a sea-urchin barren ground and a kelp bed: are populations regulated by settlement or post-settlement processes?

I sampled recruitment of very small sea urchins (Strongylocentrotus spp.) by using the anesthetic magnesium chloride to remove individuals from substrata collected in sea-urchin barren grounds (barrens) and kelp beds at Naples Reef near Santa Barbara, California, USA. Preliminary sampling found low numbers of newly settled individuals(<0.6 mm test diam) from April–July in 1984 and 1985, and in April, 1986. In early May, 1986, I found many newly settled seaurchins (0.3 to 0.6 mm, 5 to 17 d old), and I compared the densities of the cohort on several types of natural substrata in barrens and kelp-bed habitats. Newly settled individuals of both purple sea urchins (S. purpuratus) and red sea urchins (S. franciscanus) were present in similar, high densities (1 000 S. purpuratus m^-2) on foliose red algal turf, a dominant substratum ofthe kelp bed, and on crustose coralline algae, the dominant substratum of an adjacent barrens. Larvae of S. purpuratus reared and tested in the laboratory showed high rates of settlement on both red algal turf and on crustose coralline algae, but significantly lower rates on rock. Larvae also settled in response to a partiallypurified extract of coralline algae. The reduced settlement on natural rock surfaces relative to either algal treatment and the significant settlement in response to the extract of coralline algae indicate that larvae discriminate between natural substrata and probably respond to a settlement cue other than, or in addition to, a simple microbial (bacterial) film. The similar densities of young recruits of S. purpuratus on dominant substrata of barrens and kelp bed show that, at least in this case, differential settlement cannot explain the high densities of sea urchins in the barrens habitat. Movement between barrens and kelp bed is unlikely given the small sizes of the newly recruited sea urchins relative to the large distances often involved. Reduced post-settlement mortality of newly settled individuals in the barrens remains the most likely mechanism leading to the higher densities of sea urchins in barrens relative to kelp-bed habitats.     

Barnacle larvae actively select flow environments supporting post-settlement growth and survival

Many marine dispersive propagules select specific settlement sites based on a range of environmental cues. However, the link between larval choice and post-settlement growth and survival is still poorly understood. Here we show that cypris larvae of the barnacle Balanus improvisus actively reject surfaces exposed to local flow speeds exceeding 5-10 cm/s. Field experiments show that post-settlement growth and survival decline in freestream flows above 15 cm/s. Moreover, studies in flume flow at local speeds exceeding 10 cm/s reveal that early juveniles show reduced feeding rates caused by deformation of the cirral fan, reduced retention efficiency, and a decrease in time spent feeding. We conclude that cypris larvae actively reject flow environments that will be suboptimal for suspension feeding in the early post-settlement phase. Our study suggests that larval choice can be adaptively connected to a specific part of the life cycle, in this case the very sensitive time after metamorphosis.     

Recruitment and ontogenetic habitat shifts of the yellow snapper (<Emphasis Type="Italic">Lutjanus argentiventris</Emphasis>) in the Gulf of California

We examined recruitment and ontogenetic habitat shifts of the yellow snapper Lutjanus argentiventris in the Gulf of California, by conducting surveys and collections in multiple mangrove sites and major marine coastal habitats from 1998 to 2007. Over 1,167 juvenile individuals were collected and 516 otoliths were aged to describe the temporal pattern of the settlement. L. argentiventris recruits in mangroves, where juveniles remain until they are approximately 100 mm in length or 300-days-old. Back-calculated settlement dates and underwater surveys indicated a major recruitment peak during September and October, around 8 days before and after the full moon. The majority of mangrove sites in the Gulf of California had a similar L. argentiventris average size at the beginning of the settlement season for the cohort of 2003; although there were significant differences in individual sizes at the end of the nursery stage. When sub-adults leave mangroves, they live in shallow rocky reefs and later become abundant in deeper rocky reefs. The density of migratory individuals (10–20 cm SL) decreased exponentially as the distance between a reef and a nearby mangrove site increased. This finding has important implications for local fishery regulations and coastal management plans.     

Supply-side ecology of the brown mussel,<Emphasis Type="Italic"> Perna perna</Emphasis>: an investigation of spatial and temporal variation in,and coupling between,gamete release and larval supply

Sampling of recruitment-associated variables of Perna perna was done approximately monthly for 14 months at intertidal locations 500 m apart, nested within sites 25 km apart. Paired with intertidal locations were nearshore locations, 600 m to sea. Sampling assessed spawning, densities of larvae in the water column and densities of late plantigrades and juveniles on the shore. Major events in each variable were synchronous over larger scales (10s of kilometres) while subsidiary events were synchronised at smaller scales, varying within sites (100s of metres) or even within locations (metres). This suggests that the processes driving major events operated over large scales while processes operating at much more local scales drove less intense, more localised events. A major spawning event occurred at all locations in May–June 1998. Weaker spawning events occurred at different times in different locations. Larvae were found on 80% of sampling occasions, densities peaking in January–March 1998 and 1999 at all locations. Plantigrades and juveniles showed less clear patterns, with considerable residual variation. There was no sign of strong coupling among variables with few significant direct or cross correlations. The major sources of variability shifted from time to space as one progressed from spawning, to plantigrade density to juvenile density. For spawning, time was the most important source (58%) of heterogeneity and space accounted for little (8%) of the total variance. For larvae and late plantigrades, time was still the most important source of variability (41% and 33%, respectively), but space was a much more substantial component. For juveniles, small-scale (residual) spatial variability dominated total variability (75%). This strongly suggests the importance of hydrography and its effects on variation in delivery of larvae to the intertidal from offshore. These findings also indicate greater spatial heterogeneity as recruits age, reflecting small-scale variations in larval delivery and the increasing importance of post-settlement mortality.Communicated by G.F. Humphrey, Sydney