Fine-scale spatial and temporal variation in settlement of the intertidal mussel Perna perna indicates differential hydrodynamic delivery of larvae to the shore

Scales of spatial and temporal variation in the settlement of early and late plantigrades of the mussel Perna perna to artificial substrata were measured on the south coast of South Africa. One study examined daily settlement over one month at three sites of similar tidal height and aspect. The sites were separated by 100′s of m with 5 replicated artificial settlement substrata approximately 1 m apart within each site. Early (< 0.5 mm) and large late plantigrades (1 - 3.5 mm) dominated samples with few individuals of 0.5-1 mm. Settler abundance showed clear peaks, synchronised among sites and apparently following spring tides, though the relationship between tide and settlement was not significant. Two-way ANOVA indicated significant spatial variation, which varied with time, at the 100 m-scale among sites. A second study investigated the local effects of substratum aspect. Seaward facing surfaces had consistently lower settlement (but higher adult cover) than horizontal and landward facing surfaces, this effect was significant for large late plantigrades. This suggests that either juveniles migrate after settlement, or that post-settlement mortality differs markedly among aspects. The high degree of temporal variation at the scale of days and weeks was expected, but the results also suggest a degree of pulsing in settlement, with maximum intensities around spring tide. A previous study of nearshore waters indicated a common pool of larvae at scales of 100 s m. Thus, consistent ranking of sites for all size classes of settlers indicates predictably differential hydrodynamic delivery of larvae to sites separated by 100 s m. The effect of aspect on settlement highlights the importance of smaller scale hydrodynamics, suggesting that the stronger turbulence experienced by seaward facing surfaces may affect larval settlement negatively.     

Field-based video observations of wild barnacle cyprid behaviour in response to textural and chemical settlement cues

Many marine invertebrate larvae respond behaviourally to environmental settlement cues, yet behaviours are often only inferred from settlement patterns or are limited to laboratory studies. The behaviour of wild cypris larvae of Semibalanus balanoides L. was filmed on settlement tiles in the field. Tiles were of five different textures with a nested treatment of crude conspecific adult extract (AE). The effects of texture and AE on eleven defined behaviours were analysed. Texture affected the gross and net exploratory distances, velocity, acceleration and time spent exploring. AE attracted more cyprids during the first minute of immersion and increased the time spent on surfaces. Relatively few arrivals that either travel far and fast, or exit the surface rapidly, may indicate a lower chance of settlement. An increase in time spent on a surface may increase the probability of being in contact with the surface when the sign stimulus to settle occurs.     

Seasonal variation in the effects of hard substratum biofilming on settlement of marine invertebrate larvae

The effect of season on "biofilming";, as a cue for the settlement of marine invertebrate larvae, was investigated in a long-term field study during the years 1992-1994. The series of settlement experiments was conducted in a tidal rapid on the west coast of Scotland, and involved manipulations of artificial panels. Biofilming of substrata, whilst excluding larval settlement, was achieved by the enclosure of panels within tight-fitting (but removable) mesh screens so that the number of settlers on filmed and unfilmed substrata were counted in the initial absence of other incumbent post-larvae. Depending on larval species, the effects of biofilming were found to be either facilitatory or inhibitory. Significant within- and between-species seasonal differences in the settlement responses were detected, and a reversal of the effect of biofilming on larval settlement response, from inhibitory to facilitatory and vice versa, was noted with season in the case of some taxonomic groups and species (e.g. Tubulipora sp., Plagioecia sp., Electra pilosa (L.)). The present study emphasizes the need for extended field studies of larval responses to environmental cues, when the focus of interest is in drawing general inferences about naturally occurring behavioural patterns at settlement.     

Field-based video observations of wild barnacle cyprid behaviour in response to textural and chemical settlement cues

Many marine invertebrate larvae respond behaviourally to environmental settlement cues, yet behaviours are often only inferred from settlement patterns or are limited to laboratory studies. The behaviour of wild cypris larvae of Semibalanus balanoides L. was filmed on settlement tiles in the field. Tiles were of five different textures with a nested treatment of crude conspecific adult extract (AE). The effects of texture and AE on eleven defined behaviours were analysed. Texture affected the gross and net exploratory distances, velocity, acceleration and time spent exploring. AE attracted more cyprids during the first minute of immersion and increased the time spent on surfaces. Relatively few arrivals that either travel far and fast, or exit the surface rapidly, may indicate a lower chance of settlement. An increase in time spent on a surface may increase the probability of being in contact with the surface when the sign stimulus to settle occurs.     

Temporal scales of variation in settlement and recruitment of the mussel Perna perna (Linnaeus, 1758)

Population dynamics of many intertidal organisms are strongly affected by the abundance and distribution of larvae arriving on the shore. In particular, not only absolute numbers of settlers but also the degree of synchronisation of settlement can have a strong influence on whether density-dependent or density-independent processes shape adult shape populations. Temporal variation in rates of settlement and recruitment of the mussel Perna perna on the south coast of South Africa was investigated using a nested spatial design at different temporal scales. Variability in settlement at spring tides was examined at two temporal scales: lunar (to investigate the effect of state of the moon on settlement) and tidal (to investigate the influence of state of the tide on mussel settlement). Recruitment over neap tides was examined at one temporal scale, fortnight (to investigate the effect of date on mussel recruitment).Strong temporal variation was evident for both settlement and recruitment, but not at all time scales. Distinct peaks of settler/recruit abundance were observed during the lunar and neap tide studies. Recruitment intensity differed over the course of the year, and pulsing of recruitment was generally synchronised among locations. However, the strength of pulsing differed dramatically among locations, giving a significant interaction between fortnight and location. The finest temporal scale, investigated in the tidal study, did not reveal a significant effect of the state of the tide on settlement. The state of the moon (new or full) was not significant as a main factor (p = 0.052), although generally more settlers arrived on the shore during new moon. Phase of the moon appeared to have an effect on settler abundances, but only when and where densities were high.     

Settlement in different-sized patches by the gregarious intertidal barnacle Chamaesipho tasmanica Foster and Anderson in New South Wales

Most models on settlement of open marine invertebrate populations are based on space-limitation. These models, however, do not recognise that free space may not drive the demography of populations when larval numbers are small or when larval supply varies along a gradient in the habitat. They also do not incorporate the effects of larval choice when settling. It has been hypothesised that, in gregarious barnacles, the effects of adult conspecifics, rather than available free space, may play a primary role in settlement. That is, cues from adults along perimeters of patches, rather than space available, may enhance colonisation. This study therefore aimed to distinguish between these separate influences on populations of Chamaesipho tasmanica, a gregarious barnacle characterised by relatively few larvae arriving to settle each year. Patches of 6, 3 and 1.5 cm diameter were cleared within aggregations of barnacles at three heights (Low, Mid, Upper) of Chamaesipho's distribution at two sites and during 2 years of settlement. Total numbers of settlers in each year were manipulated to determine the separate influences on settlement due to availability of substratum or the effects of conspecific adults. To test for the effects of available free space, numbers of settlers per unit area were analysed. To test for gregarious effects due to the presence of adults, numbers of settlers per unit perimeter were analysed. While available substratum was found not to affect settlement of this barnacle, gregarious settlement in response to adults at perimeters of patches was thought to be confounded by differential larval supply and differential conspecific cues among heights on the shore. Results from this study therefore have important implications for survival of gregarious populations following disturbances, especially in species where larval supply is poor.     

Can artificial substrates enriched with crustose coralline algae enhance larval settlement and recruitment in the fluted giant clam (<Emphasis Type="Italic">Tridacna squamosa</Emphasis>)?

Habitat recognition and selection can greatly increase the early-life survival of sessile reef organisms. This study describes the settlement and recruitment responses of the fluted giant clam, Tridacna squamosa, to concrete tablets and tiles containing different concentrations of crustose coralline algae covered coral rubble (CCACR). Crustose coralline algae is known to induce settlement in a variety benthic animals, but it has not been used previously as an aggregate in concrete—potentially a way of encouraging colonization of man-made structures erected on or near coral reefs. After being given the choice of small tablets made with 0%, 30% or 60% CCACR for 4 days, 11 days old larvae preferred the substrate containing the most CCACR. Recruitment responses of juvenile clams to larger tiles made with the same three CCACR concentrations were also tested. These tiles were further divided into rough and smooth surface textures. After 6 weeks, more juvenile clams had recruited to the rough surfaced tiles than the smooth ones, but no significant differences among the CCACR treatments were found. Thus, even though concrete made with CCACR is initially attractive to larvae, it has no effect on recruitment of juvenile T. squamosa. Handling editor: I. Nagalkerken     

Successful determination of larval dispersal distances and subsequent settlement for long-lived pelagic larvae

Despite its importance, we still have a poor understanding of the level of connectivity between marine populations in most geographical locations. Taking advantage of the natural features of the southeast coast of New Zealand's North Island, we deployed a series of settlement stations and conducted plankton tows to capture recent settlers and planktonic larvae of the common intertidal gastropod Austrolittorina cincta (6-8 week larval period). Satellite image analysis and ground truthing surveys revealed the absence of suitable intertidal rocky shore habitat for A. cincta over a 100 km stretch of coastline between Kapiti Island to the south and Wanganui to the north. Fifteen settlement stations (3 replicates × 5 sites), which were used to mimic intertidal habitat suitable for A. cincta, were deployed for two months around and north of Kapiti Island (at 0.5, 1, 5, 15, 50 km). In addition, we also conducted plankton tows at each settlement station when the stations were first deployed to collect A. cincta larvae in the water column. On collection, all newly settled gastropods and larvae in the plankton samples were individually isolated, and a species-specific microsatellite marker was used to positively identify A. cincta individuals. Most of the positively identified A. cincta settlers and larvae were collected at the first three sampling stations (<5 km). However, low numbers of A. cincta settlers and larvae were also recorded at the two more distant locations (15 and 50 km). Dispersal curves modeled from our data suggested that <1% of gastropod larvae would travel more than 100 km. While our data show that most larvae are retained close to their natal populations (<5 km), a small proportion of larvae are able to travel much larger geographic distances. Our estimates of larval dispersal and subsequent settlement are one of only a few for marine species with a long-lived larva.     

Adding coral rubble to substrata enhances settlement of Pocillopora?damicornis larvae

Settlement preferences of Pocillopora damicornis larvae were examined on artificial substrata. Planulation of P. damicornis followed a lunar cycle and the release of larvae occurred after new moon. P. damicornis larvae had the highest rates of settlement within 3 days of being presented settlement substrata. Cumulative settlement gradually increased from 3 to 8 days, and post-settlement mortality was most frequent after 8 days. Settlement experiments showed greatest settlement preference to cement tiles containing 10% coral rubble. This study suggests that physical cues are important in the settlement process, which may be useful for coral reef rehabilitation projects.     

Optimising settlement tiles: The effects of surface texture and energy,orientation and deployment duration upon the fouling community

The aim of this study was to produce a set of tiles for field studies on settlement and biofouling with carefully controlled surface characteristics and practical design, and test them under field conditions. Impressions of precisely defined surface textures were made in silicone. Double sided tiles in epoxy, polyester and silanised epoxy resins were cast from the impressions. Tile characteristics tested were texture (R_a = 0, 0.19, 0.62, 1.1, 2.2 mm), surface free energy (60, 52, 24 mN m^{m 2}), and surface orientation (up, down, into, away). Tiles were deployed in the Red Sea for 4 and 7 months. Measures of community cover, dominance and richness were all significantly affected by each of the factors. The tiles proved durable and robust during the 7 month deployment with no observable changes in surface characteristics and none were lost or broken. These settlement tiles have a wide applicability for both biofouling and ecological studies. The field test demonstrated the complexities of the interactions between just four surface characteristics. This study has also underlined the need for multidimensional analysis of fouling communities for applied and basic research.     

Effects of Adult Dreissena polymorpha on Settling Juveniles and Associated Macroinvertebrates

The impact of Dreissena polymorpha settlement on recruitment of juvenile mussels and density of other macroinvertebrates was studied in field experiments using blank concrete blocks and tiles (control), blocks and tiles with attached empty zebra mussel shells, and blocks and tiles with attached living mussels. On blocks, dominant invertebrate taxa showed colonization patterns coinciding with increased habitat complexity owing to zebra mussel settlement or the biodeposition of faeces and pseudofaeces. Adult and especially juvenile zebra mussels preferred blocks with empty shells to blank blocks and blocks with living mussels; this might possibly be caused by a chemical cue that induces gregarious settlement. Lower recruitment on blocks with attached living mussels compared to blocks with only shells could be the consequence of ingestion of larvae by adult mussels and of competition for food. On tiles, the sediments deposited and the organic content of the sediment were investigated. Sedimentation was significantly higher on shell‐only and live‐mussel tiles compared to blank tiles. Organic matter differed significantly between blank and live‐mussel tiles.     

The effects of larval abundance, settlement and juvenile mortality on the depth distribution of a colonial ascidian

The colonial ascidian Didemnum candidum (Savigny) is more abundant at shallow depths on floating docks than at greater depths along pilings in Pearl Harbor, Oahu, Hawaii. To compare the effects of selective settlement and postsettlement mortality on adult distribution, I determined if settlement was nonrandom relative to depth, if differences in adult abundance were responsible for the depth distribution of settlement, and if juvenile mortality varied with depth. A plankton pump was used to measure variation in larval abundance with depth. Acrylic settling plates were suspended at different depths and sampled nondestructively to measure settlement intensity and juvenile mortality. Settlement and mortality of the didemnid ascidians Diplosoma listerianum Milne-Edwards and Diplosoma sp. were also measured in the field and compared to that of D. candidum. Settlement of D. candidum was indeed nonrandom over depth. Both planktonic larvae and settled juvenils were more numerous at 0.5 than 3 or m. Settlement intensity on vertical plates was greatest within 1–2 cm of the water surface in the laboratory and within 2–3 cm in the field. Directly adjacent to the floating dock, where adults were equally abundant, settlement on horizontal plates was greater at shallow depths, suggesting directed movement of larvae upward. However, the ratio of settled juveniles to planktonic larvae (number of settlers: number of larvae) did not significantly differ with depth, suggesting that larvae were not more likely to settle at a particular depth. Settlement of the Diplosoma species was also heaviest near the surface. Juvenile mortality was greater at 0.5 than at 3 or 6 m for both D. candidum and the Diplosoma species. Experimental settlement showed that mortality, per se, of D. candidum was independent of depth. Rather, mortality was density-dependent, and the higher mortality near the surface was due to the greater number of larvae settling there. At this location, nonrandom settlement appears to determine the adult distribution of D. candidum, despite greater juvenile mortality at shallow depths. The pattern of settlement over depth is largely determined by adult proximity, rather than active larval behavior.     

The effect of bacterial and diatom biofilms on the settlement of the bryozoan Bugula neritina

Biofilms of marine bacteria and diatoms and their combinations were examined in laboratory choice assays to determine their effects on the attachment and successful metamorphosis of the larvae of the bryozoan Bugula neritina (Linnéus). The larval settlement in response to unfilmed surfaces, a natural biofilm (NBF) and adsorbed cells of three strains of bacteria, five strains of pennate diatoms and combinations of the two at different densities. Bacterial and diatom strains showed different effects on the larval settlement of B. neritina. Bacterial monospecific strains of an unidentified α-Proteobacterium and Vibrio sp. mediated the same percentage of settlement as a filtered seawater control. Biofilms of Pseudoalteromonas sp. caused significantly lower larval settlement. Larval settlement of B. neritina was negatively correlated with increasing densities of Pseudoalteromonas sp. The highest percentages of settlement were mediated by the biofilms of the diatom species Achnanthes sp., Amphora cofeaeformis, Amphora tenerrima, Nitzschia constricta and a 5-day-old natural biofilm, while the lowest settlement was found on a N. frustulum film. A three-way analysis of variance demonstrated that the density of bacteria and the presence of particular species of diatoms and bacteria in combined biofilms, significantly affected the settlement of B. neritina larvae. High settlement of larvae (50-90%) at all treatments indicated that B. neritina larvae are much more indiscriminate settlers than previously expected. Hence, using this species as a monitoring organism to trace ecologically relevant subtle changes of settlement cues in the natural environment should be carefully re-examined.     

The effects of potential larval supply, settlement and post-settlement processes on the distribution of two species of filter-feeding caddisflies

1. Hydropsychid caddisfly larvae are often abundant in fast flow habitats and on highly textured substrata. Such patterns are usually inferred to result from habitat preference, but the roles of larval supply to the benthos and post‐settlement events are rarely examined for stream populations. We describe a manipulative field experiment that examined larval supply, habitat preference and post‐settlement events simultaneously for two co‐occurring species of hydropsychids. 2. Ten artificial channels were constructed in a natural riffle to create fast and slow flow habitats and within each channel we placed six artificial substrata whose surfaces had been modified to create three different texture treatments. We measured discharge through channels and monitored hydropsychid colonisation of substrata every 3–4 days. Half of the substrata had all hydropsychids and nets removed every 3–4 days (short‐term colonisation) whereas the remaining half were counted but nets and hydropsychids left attached (long‐term colonisation). Short‐term counts of nets and individuals were summed to allow comparison with long‐term colonisation substrata. 3. Smicrophylax sp. AV2 larvae were more abundant in fast flow channels than slow flow channels, but these differences were proportional to discharge through each of the channels, suggesting that supply of settlers can explain this pattern. Smicrophylax larvae were least abundant on smooth substrata, which suggests that this species selects habitats based on surface texture. Alternatively, Asmicridea sp. AV1 larvae were only found in fast flow channels and this most likely reflects an active habitat choice by this species, but there was no significant difference between different texture treatments. 4. There were no differences between summed, short‐ and long‐term counts of recruits for either species, but there were more nets than larvae, especially in slow flow channels, by the end of the experiment, suggesting that larval mortality or re‐dispersal after settlement could be considerable. 5. Our results indicate that supply, habitat selection at settlement and post‐settlement processes all contributed variously to the distribution of hydropsychid caddisfly larvae, but that each species was affected differentially by these factors. Larval supply and post‐settlement processes are rarely examined by stream researchers and our results demonstrate these factors deserve much more consideration. Calculating accurate larval supply rates to sites is challenging, but we suggest that such detailed information is necessary if we are to sort out what sets limits to distributions and the underlying population structure of stream invertebrate populations.     

Macroalgae Inhibits Larval Settlement and Increases Recruit Mortality at Ningaloo Reef,Western Australia

Globally, many coral reefs are degraded and demonstrate reduced coral cover and increased macroalgal abundance. While negative correlations between macroalgae and coral recruitment have commonly been documented, the mechanisms by which macroalgae affects recruitment have received little attention. Here we examined the effect of macroalgae on larval settlement and the growth and survival of coral recruits, in a field experiment over nine months. Exclusion treatments were used to manipulate herbivory and macroalgal biomass, while settlement tiles measured coral settlement and survival. After nine months the volume of macroalgae was up to 40 times greater in the caged treatments than in controls and the settlement of coral larvae on the undersides of tiles within caged plots was 93% lower than in the uncaged treatments. The growth and survival of coral recruits was also severely reduced in the presence of macroalgae: survival was 79% lower in caged treatments and corals were up to 58% smaller with 75% fewer polyps. These data indicate that macroalgae has an additive effect on coral recruitment by reducing larval settlement and increasing recruit mortality. This research demonstrates that macroalgae can not only inhibit coral recruitment, but also potentially maintain dominance through a positive feedback system.     

Settlement behaviour and early post-settlement predation of the sea urchin Strongylocentrotus droebachiensis

We examined behaviour of competent larvae of the green sea urchin Strongylocentrotus droebachiensis provided with pebbles encrusted with coralline red algae, a strong settlement inducer, in laboratory experiments. Larvae settled at greater frequencies on upward-facing coralline surfaces and in small gaps between coralline and glass surfaces than expected by a random distribution of settlement. These patterns may be explained by encounter rate with inductive cues. There was no change in settler distributions within ~ 1 week of settlement, indicating no net movement between adjacent microhabitats. In flow, live and recently killed larvae settled or were passively entrapped at greater frequencies on high- than low-rugosity coralline crusts. Recent settlers (0.5-1 mm test diameter) were consumed by small decapod crustaceans and bulldozed by periwinkles. Juveniles became less vulnerable to predation by hermit crabs with increasing size, and reached a growth refuge at ~ 10 mm test diameter. Our laboratory findings suggest that the cryptic distribution of recent settlers is probably not due to microhabitat selection by settling larvae or early post-settlement migration, at least not in response to physical cues such as light or surface texture. Differential rates predation of young juveniles between exposed and cryptic habitats cannot be ruled out as an important determinant of this pattern.     

Recruitment, early survival and growth of the Mediterranean red coral Corallium rubrum (L 1758), a 4-year study

Recruitment, early survival and growth rates of the precious Mediterranean red coral Corallium rubrum (L 1758) are poorly known. We examined these life history traits by means of artificial long-term settlement plates. Eighteen marble tiles placed off the coast of Leghorn (Tuscany, Italy) at two depths (25 and 35 m) were photographed monthly over the 4-year period from 1998 to 2002. Overall, 864 transparencies were examined to follow the individual life histories of red coral colonies belonging to four successive cohorts. Red coral planulae settled on tiles each year between July and September. Overall, 388 settlers colonized the tiles (244 at 25 m and 144 at 35 m), and their respective densities varied between 12.37±6.1 and 2.75±2.4 dm⁻². Heavy mortality affected these colonies (−24.35±9.12 colonies % y⁻¹), but, after 4 years, the tiles still harboured a persistent population (19±4.97 and 9.75±2.87 colonies dm⁻², respectively, at 25 and 35 m) with positive net recruitment rates. Only in 1999 did the net recruitment rate show a negative trend, although only at the shallower depth. At the same time (late summer 1999), a thermal anomaly affected several epibenthic communities in the Ligurian Sea. After 4 years, the tiles were removed, and the colonies that settled on them were measured. The average annual growth rate of colonies was low (0.62±0.19 mm y⁻¹ in diameter), and a marked reduction in growth with age was observed. Our findings suggest that the populations of this slow-growing long-lived octocoral exhibit a high capacity for colonization and seem to be quite resilient to environmental variability.     

Two levels of spacing and limits to local population density for settled larvae of the ascidian Clavelina moluccensis: a nearest-neighbour analysis

Among sessile marine invertebrates, the dispersion patterns of settling larvae are likely to strongly affect the nature and strength of subsequent intraspecific interactions. We use a recently developed graphical analysis technique to examine in situ spacing among settled larvae of the colonial ascidian Clavelina moluccensis. Larvae were mapped at settlement each day for 1 month at the time of their release from adults. Experimental plots measuring 90×90 mm were located on naturally occurring bare patches of wood on pier pilings which were extensively encrusted with sponges and ascidians. An inclusive one-tailed nearest-neighbour analysis applied at the time of the maximum number of individuals per plot revealed consistent fine-scale regular spacing between settled larvae. Furthermore, a two-tailed goodness-of-fit analysis indicated two modes of spacing: one at about 4.5 mm and the other at around 9 mm. No contact between larvae was observed at settlement and the spatial regularity is interpreted as a response of settling larvae to water-borne cues released from settled individuals. The two modes of spacing indicate the possibility of kin recognition as has been demonstrated for bryozoans. Water-borne cues may also limit local population density, as those plots with a high density of settlers showed a much greater decrease in settlement rate over time than low-density plots. Our findings are consistent with the hypothesis that intraspecific competitive interactions shape the patterns of dispersion of these invertebrates on hard substrata in the field.     

Coral Settlement on a Highly Disturbed Equatorial Reef System

Processes occurring early in the life stages of corals can greatly influence the demography of coral populations, and successful settlement of coral larvae that leads to recruitment is a critical life history stage for coral reef ecosystems. Although corals in Singapore persist in one the world’s most anthropogenically impacted reef systems, our understanding of the role of coral settlement in the persistence of coral communities in Singapore remains limited. Spatial and temporal patterns of coral settlement were examined at 7 sites in the southern islands of Singapore, using settlement tiles deployed and collected every 3 months from 2011 to 2013. Settlement occurred year round, but varied significantly across time and space. Annual coral settlement was low (~54.72 spat m^-2 yr^-1) relative to other equatorial regions, but there was evidence of temporal variation in settlement rates. Peak settlement occurred between March–May and September–November, coinciding with annual coral spawning periods (March–April and October), while the lowest settlement occurred from December–February during the northeast monsoon. A period of high settlement was also observed between June and August in the first year (2011/12), possibly due to some species spawning outside predicted spawning periods, larvae settling from other locations or extended larval settlement competency periods. Settlement rates varied significantly among sites, but spatial variation was relatively consistent between years, suggesting the strong effects of local coral assemblages or environmental conditions. Pocilloporidae were the most abundant coral spat (83.6%), while Poritidae comprised only 6% of the spat, and Acroporidae <1%. Other, unidentifiable families represented 10% of the coral spat. These results indicate that current settlement patterns are reinforcing the local adult assemblage structure (‘others’; i.e. sediment-tolerant coral taxa) in Singapore, but that the replenishment capacity of Singapore’s reefs appears relatively constrained, which could lead to less resilient reefs.     

Habitat differentiation in the early life stages of simultaneously mass-spawning corals

The settlement process of coral larvae following simultaneous mass-spawning remains poorly understood, particularly in terms of population and community parameters. Here, the larval settlement patterns of Acropora corals, which are the most diverse genera of scleractinian corals at the species (haplotype) level, were investigated within a single subtropical reef. Across a 4-year period (2007–2010), the mitochondrial and nuclear molecular markers of 1,073 larval settlers were analyzed. Of the 11 dominant haplotypes of recruited populations, nine exhibited non-random patterns of settlement distribution. This result suggests that the actual habitat segregation starts during the early swimming larval stages of their life history, rather than by natural selection after random settlement. In addition, the presence of a depth-related settlement pattern supports that species-specific vertical zonation of coral larvae may play a role in the establishment of habitat segregation. Moreover, in some species that showed a preference toward the shoreward area of the bay, the settlement pattern was consistent with that of the adult distribution. This result indicates that the gametes were not mixed between fore and back reefs in the period from fertilization to settlement during the mass-spawning event, even within a single small reef. Another compatible hypothesis of this pattern is that the larvae are able to recognize various types of environmental information, facilitating the selection of optimal micro-habitats. Overall, Acropora coral larvae that are produced from a simultaneous mass-spawning event may have adapted to complex reef topography by means of multi-step habitat selection at settlement, corresponding to different spatial scales.