Strong density-dependent survival and recruitment regulate the abundance of a coral reef fish

Debate on the control of population dynamics in reef fishes has centred on whether patterns in abundance are determined by the supply of planktonic recruits, or by post-recruitment processes. Recruitment limitation implies little or no regulation of the reef-associated population, and is supported by several experimental studies that failed to detect density dependence. Previous manipulations of population density have, however, focused on juveniles, and there have been no tests for density-dependent interactions among adult reef fishes. I tested for population regulation in Coryphopterus glaucofraenum, a small, short-lived goby that is common in the Caribbean. Adult density was manipulated on artificial reefs and adults were also monitored on reefs where they varied in density naturally. Survival of adult gobies showed a strong inverse relationship with their initial density across a realistic range of densities. Individually marked gobies, however, grew at similar rates across all densities, suggesting that density-dependent survival was not associated with depressed growth, and so may result from predation or parasitism rather than from food shortage. Like adult survival, the accumulation of new recruits on reefs was also much lower at high adult densities than at low densities. Suppression of recruitment by adults may occur because adults cause either reduced larval settlement or reduced early post-settlement survival. In summary, this study has documented a previously unrecorded regulatory mechanism for reef fish populations (density-dependent adult mortality) and provided a particularly strong example of a well-established mechanism (density-dependent recruitment). In combination, these two compensatory mechanisms have the potential to strongly regulate the abundance of this species, and rule out the control of abundance by the supply of recruits.     

Effects of microhabitat characteristics on the settlement and recruitment of a coral reef fish at two spatial scales

Populations of fishes on coral reefs are replenished by the settlement of pelagic larvae to demersal populations. Recruitment varies spatially and temporally and can exert strong effects on the dynamics of reef fish populations. This study examined the effect of microhabitat characteristics on small-scale and large-scale recruitment variation in the three-spot damselfish, Stegastes planifrons (Cuvier). Comparison of 0.25-m² quadrats occupied by three-spots with randomly sampled null quadrats showed that three-spots quadrats contained a higher percent cover of the coral Montastrea annularis than would be expected at random. Manipulative experiments on three types of 1.0-m² patch reefs (living M. annularis, dead Porites Porites and dead Acropora palmata) patch reefs on showed that this non-random distribution was established by microhabitat choice during settlement and not by differential post-settlement survival. The presence of conspecific juveniles did not affect settlement. Recruitment was monitored at nine sites on three islands over 3 years. Recruitment showed no consistent pattern in the relative levels of recruitment among sites. Similarly, no consistent relationship emerged between recruitment levels and microhabitat characteristics at the nine sites. For example, at this large scale, the percent cover of M. annularis explained variation in recruitment in only 1 out of 3 years. These results suggest that small-scale recruitment patterns are influenced by microhabitat choice during settlement, but that these habitat effects do not scale up to influence large-scale variation in recruitment.     

Recruitment pattern of Diadema antillarum in La Parguera, Puerto Rico

Recruitment success is one of the factors that may be influencing the slow population recovery of Diadema antillarum at many locations in the Caribbean. D. antillarum recruitment was measured with recruitment plates at monthly intervals from September 2005 to September 2006 at three reefs along an inshore-offshore gradient in La Parguera, Puerto Rico. A total of 275 recruits of D. antillarum were collected at the shelf-edge reef during this 13-month study. Two recruits were collected at the mid-shelf reef and no recruits were collected at the inner shelf reef. Recruitment varied among months at the shelf-edge, with the highest value (1,067 ind/m²) occurring in July 2006. Previous benthic surveys in the La Parguera region have demonstrated higher densities of adult D. antillarum at inshore reefs. The higher recruitment at the shelf-edge suggests that recruitment is not a major determinant of the spatial distribution of the adult population. Recruitment occurring in this study is indicative that sources of larvae were available upstream and larval survival was occurring.     

Aggression in damselfish: habituation by adults of Pomacentous parritus to juvenile intruders

Synopsis Successful recruitment of juvenile coral reef fishes may depend, in part, upon the aggressive behavior of adults already on the reef. In addition to initial levels of aggression, changes in aggressive behavior of adults, e.g., due to habituation, may have an even greater influence on recruiting juveniles. Adult males of the bicolor damselfish, Pomacentrus partitus, were used as subjects to study habituation of aggression toward conspecific and congeneric (P. variabilis) juveniles. Adults, held in a 1000 liter aquarium, habituated after 4 hours of constant exposure to juveniles restrained in a 1 liter model bottle. Stimulus strength of the juveniles depended on their species identity, size and proximity to the resident adult's shelter. There was a recovery of aggression with a change in stimulus location, but at the same location, a stimulus of greater strength was required to bring about recovery. The implications of these findings for coral reef fish community structure are discussed.     

Territoriality and Habitat Use by Juvenile Blue Tangs, Acanthurus coeruleus

We studied territoriality and habitat use by yellow phase juvenile blue tangs, Acanthurus coeruleus, on a small fringing reef in Barbados, West Indies. Juvenile blue tangs occurred on the reef crest, spurs, and a transition zone between the reef crest and reef flat at a density of about 8 individuals per 100m², but were much rarer on the reef flat. They were solitary and occupied stable home ranges (median=0.85m²) that increased with body size. Observational and experimental data documented aggressive defense of home ranges against conspecific and to a lesser extent congeneric, A. bahianus, juveniles (about 7.5 approaches and attacks per hour directed at intruders). Home range locations were structurally more complex and closer to a vertical face than expected by chance. Although juvenile blue tang territories overlapped considerably with those of larger and more aggressive Stegastes damselfish, which are believed to exclude solitary adult Acanthurus spp. from reef crest and spurs, the tangs avoided Stegastes and were rarely chased (<0.3 fleeing events per hour). Space use and social organization of yellow juvenile blue tangs contrast strikingly with that of both conspecific adults and congeneric juveniles.     

The influence of predation and conspecific adults on the abundance of juvenile Evechinus chloroticus (Echinoidea:Echinometridae)

Summary Reefs dominated by red algae, associated with high echinoid densities, are consistent features of the shallow subtidal around northeastern New Zealand. Factors determining the abundance of juvenile Evechinus chloroticus were investigated in such a habitat. Using a factorial design, a field experiment was used to assess the influence of predators and adult E. chloroticus on juvenile abundance. The use of 2 m² exclusion cages enhanced juvenile E. chloroticus abundance over a 16 month period, an effect independent of conspecific adults. We attributed this effect to the exclusion of benthic-feeding, predatory fish. Several species forage over the study area at high densities and are known from gut content analysis to prey on juvenile E. chloroticus in the field. Invertebrate predators are at very low densities in the area. The possibility of caging and site artefacts confounding this interpretation is discussed.Adult E. chloroticus did not directly affect conspecific juvenile densities during the experimental period. However their removal produced a significant change in community structure toward one dominated by macroscopic brown algae. Echinoid removal led to rapid recruitment of laminarian and fucoid algae, predominantly Ecklonia radiata and Sargassum sinclairii. In addition, densities of herbivorous gastropods, particularly the limpet Cellana stellifera decreased in the echinoid exclusion area, as did the feeding rates of predatory fish. The consequences of E. chloroticus removal may be dependent upon the size of the area from which they are excluded.Despite the high densities of predatory fish, a low though consistent number of juvenile E. chloroticus escape predation. We suggest that these represent sufficient input into the adult grazing population to maintain the habitat. This interpretation argues against a key role for predators in structuring shallow water reef communities in northeastern New Zealand.     

Recruitment variability of resident brown trout in peripheral populations from southern Europe

1. Population regulation was studied for seven consecutive years (1992–98) in five rivers at the periphery of the distribution of Salmo trutta, where the fish were living under environmental constraints quite different from those of the main distribution area. 2. Recruitment is naturally highly variable and the populations had been earlier classified as overexploited. Thus we expected that densities of young trout in most populations would be too low for density‐dependent mortality to operate. We tested this by fitting the abundance of recruits to egg densities over seven consecutive years (stock–recruitment relationship), and used the results to judge whether exploitation should be restricted in the interests of conserving the populations. 3. The density of 0+ trout in early September, as well as the initial density of eggs and parents, varied greatly among localities and years. The data for all populations fitted the Ricker stock–recruitment model. The proportion of variance explained by the population curves varied between 32% and 51%. However, in most cases the observations were in the density‐independent part of the stock–recruitment curve, where densities of the recruits increased proportionally with egg densities. 4. Our findings suggest that recruitment densities in most rivers and years were below the carrying capacity of the habitats. Although density‐dependent mechanisms seemed to regulate fish abundance in some cases, environmental factors and harvesting appeared generally to preclude populations from reaching densities high enough for negative feedbacks to operate. The findings thus lend support to Haldane’s (1956) second hypothesis that changes in population density are primarily due to density‐independent factors in unfavourable areas and areas with low density due to exploitation. Exploitation should be reduced to allow natural selection to operate more effectively.     

Population dynamics of an infaunal polychaete: The effect of predators and an adult-recruit interaction

The effect of predation by fish and wading shore birds on the population dynamics of a nereid polychaete (Ceratonereis pseudoerythraeensis Hutchings & Turvey) was studied. The species occurs in high densities on the site studied probably owing to high nutrient input from a nearby sewage treatment farm. Predators were excluded from experimental areas with wire mesh cages. The effects of cage structures on predator behaviour, algal deposition and sediment depth and composition were studied, and the possibility that C. pseudoerythraeensis might migrate in or out of caged areas was experimentally tested. These effects appeared to be of minimal importance. Analyses of size classes of C. pseudoerythraeensis showed that the density of adult worms in control areas decreased relative to caged areas. This was attributed to predation, mainly by flounder and curlew sandpipers. Recruitment of juvenile C. pseudoerythraeensis was greater in control than caged treatments. The hypothesis that increased adult density decreases juvenile recruitment was confirmed by manipulating the densities of adults. The adult-recruit interaction effect apparently counter-balanced losses due to predators, so that if only the total densities of the polychaetes had been studied no effect of predation would have been apparent.     

Consequences of extreme life history traits on population persistence: do short-lived gobies face demographic bottlenecks?

The majority of coral reef goby species are short-lived, with some highly abundant species living less than 100 d. To understand the role and consequences of this extreme life history in shaping coral reef fish populations, we quantitatively documented the structure of small reef fish populations over a 26-month period (>14 short-lived fish generations) at an inshore reef on the Great Barrier Reef, Australia. Most species with life spans >1 yr, such as pomacentrids, exhibited a peak in recruitment during the austral summer, driving seasonal changes in the small fish community composition. In contrast, there were no clear changes in goby community composition, despite the abundance of short-lived, high turnover species. Species of Eviota, the most abundant gobiid genus observed, showed remarkably similar demographic profiles year-round, with consistent densities of adults as well as recently recruited juveniles. Our results demonstrate ongoing recruitment of these small cryptic fishes, which appears to compensate for an exceptionally short life span on the reef. Our results suggest that gobiid populations are able to overcome demographic limitations, and by maintaining reproduction, larval survival and recruitment throughout the year, they may avoid population bottlenecks. These findings also underline the potential trophodynamic importance of these small species; because of this constant turnover, Eviota species and other short-lived fishes may be particularly valuable contributors to the flow of energy on coral reefs, underpinning the year-round trophic structure.     

Density‐dependence in common tree species in a tropical dry forest in Mudumalai,southern India

Abstract. Density‐dependence in tree population dynamics has seldom been examined in dry tropical forests. Using long‐term data from a large permanent plot, this study examined 16 common species in a dry tropical forest in southern India for density‐dependence. Employing quadrat‐based analyses, correlations of mortality, recruitment and population change with tree densities were examined. Mortality in 1–10 cm diameter trees was largely negatively correlated with conspecific density, whereas mortality in > 10 cm diameter trees was positively correlated. Mortality was, however, largely unaffected by the basal area and abundance of heterospecific trees. Recruitment was poor in most species, but in Lagerstroemia microcarpa (Lythraceae), Tectona grandis (Verbenaceae) and Cassia fistula (Fabaceae), species that recruited well, strong negative correlations of recruitment with conspecific basal area and abundance were found. In a few other species that could be tested, recruitment was again negatively correlated with conspecific density. In Lagerstroemia, recruitment was positively correlated with the basal area and abundance of heterospecific trees, but these correlations were non‐significant in other species. Similarly, although the rates of population change were negatively correlated with conspecific density they were positive when dry‐season ground fires occurred in the plot. Thus, the observed positive density‐dependence in large‐tree mortality and the negative density‐dependence in recruitment in many species were such that could potentially regulate tree populations. However, repeated fires influenced density‐dependence in the rates of population change in a way that could promote a few common species in the tree community.     

ROLE OF RECRUITMENT IN STRUCTURING BEDS OF SARGASSUM SPP. (PHAEOPHYTA) AT ROTTNEST ISLAND,WESTERN AUSTRALIA1

The importance of annual recruitment to the structure of adult stands of Sargassum was determined for a mixed species Sargassum bed at Rottnest Island, Western Australia. The morphologically similar species Sargassum spinuligerum Sonder, S. distichum Sonder, and S. podacanthum Sonder grew together in the shallow subtidal (6 m). Positive species determinations were only possible when thalli were reproductive, so recruits, bases, and vegetative annuals for all species were grouped together. Densities of recruits, perennial bases, vegetative annuals, and reproductive annuals were determined at monthly intervals from 20 randomly placed 0.25-m^?2 quadrats. Recruitment and mortality for recruits and adults were further determined at three monthly intervals from 6-×-1-m^?2 permanent quadrats. The density of adults varied little with season (between 32 and 58 m^?2). Growth of annuals was initiated in April, thalli became reproductive by late August–early September, and senescence occurred in December–January. Density of recruits was highly variable (1.6–210 individuals-m^?2) and peaked seasonally during late summer (January–February) and then declined rapidly. Adults showed a complete turnover of thalli in the bed over 25–27 months. Adult mortality was compensated by annual recruitment from propagules (43%) and vegetative regeneration from fragments of holdfasts left on the reef (57%). A seasonal pattern in survivorship was observed for adults that grew from recruits with higher initial numbers and lower mortalities for August and November cohorts. Little seasonally was observed in survivorship of adults that grew vegetatively from remnant crusts. Although initial cohort sizes were smaller for adults grown from recruits than from remnant crusts, mortality was lower, resulting in similar contributions to adult density from both recruits and remnant crusts. Recruitment from propagules and vegetative regeneration played an important role in buffering the adult stand from high rates of mortality and reducing seasonal variation in adult density and contributed to the persistence and seasonal structure of Sargassum beds at Rottnest Island.     

USING ISOLATION BY DISTANCE AND EFFECTIVE DENSITY TO ESTIMATE DISPERSAL SCALES IN ANEMONEFISH

Robust estimates of dispersal are critical for understanding population dynamics and local adaptation, as well as for successful spatial management. Genetic isolation by distance patterns hold clues to dispersal, but understanding these patterns quantitatively has been complicated by uncertainty in effective density. In this study, we genotyped populations of a coral reef fish (Amphiprion clarkii) at 13 microsatellite loci to uncover fine‐scale isolation by distance patterns in two replicate transects. Temporal changes in allele frequencies between generations suggested that effective densities in these populations are 4–21 adults/km. A separate estimate from census densities suggested that effective densities may be as high as 82–178 adults/km. Applying these effective densities with isolation by distance theory suggested that larval dispersal kernels in A. clarkii had a spread near 11 km (4–27 km). These kernels predicted low fractions of self‐recruitment in continuous habitats, but the same kernels were consistent with previously reported, high self‐recruitment fractions (40–60%) when realistic levels of habitat patchiness were considered. Our results suggested that ecologically relevant larval dispersal can be estimated with widely available genetic methods when effective density is measured carefully through cohort sampling and ecological censuses, and that self‐recruitment studies should be interpreted in light of habitat patchiness.     

An experimental test of density- and distant-dependent recruitment of mahogany (Swietenia macrophylla) in southeastern Amazonia

According to the Janzen–Connell model, high mortality of seeds and seedlings in proximity to conspecific adults can help maintain species diversity in tropical forests. Using a natural population of big-leaf mahogany (Swietenia macrophylla King), we tested the model’s mechanism by examining seed predation and juvenile recruitment in the forest understory and in treefall gaps in the vicinity of both isolated and clumped adults. We used tethered seeds placed in three types of exclosure plots: (1) complete access to seeds, (2) semi-access (access by small-sized seed predators) and (3) no access (all mammals excluded). Exclosure treatments were applied within the understory (both near and far from adults) and in gaps at eight fruiting adults in the late dry season (2001) and scored ten months later. Significantly more seeds were removed in canopy gaps near clumped adults than at isolated adults; otherwise, none of the treatment factors significantly influenced seed predation. In contrast, understory juvenile recruitment was significantly enhanced by distance from adults and was twice as high at isolated than clumped adults, providing novel support for the Janzen–Connell mechanism. No-access exclosures protected significantly more seeds than semi- and full-access exclosures, implicating small mammals in seed losses. Across the eight trees, juvenile recruitment in the no-access exclosures decreased significantly with conspecific adult densities, implicating non-mammalian density-responsive factor(s) in mortality following germination; likely a known specialist invertebrate herbivore. When all treatments were combined, conspecific adult basal area and total DBH explained 72 and 90% of variation in overall juvenile recruitment, respectively. Collectively, these results indicate that Janzen–Connell effects can operate in S. macrophylla, especially during the seed-to-seedling transition, and will likely reduce recruitment in areas of high conspecific densities. They also suggest that further research into the causes of density-dependence in tropical trees should investigate mortality agents following germination.     

Population regulation by post-settlement mortality in two temperate reef fishes

 Input of individuals dispersing into open populations can be highly variable, yet the consequences of such variation for subsequent population densities are not well understood. I explored the influence of variable input (”supply”) on subsequent densities of juveniles and adults in open local populations of two temperate reef fishes, the bluebanded goby (Lythrypnus dalli) and the blackeye goby (Coryphopterus nicholsii). Variable recruitment was simulated by stocking a natural range of densities of young fishes on replicate patch reefs. Density and mortality of the stocked cohorts were followed over time, until the fishes reached maturity. Over the first day of the experiments, mortality of both species was significantly density-dependent; however, there was still a very strong relationship between density on day 1 and density on day 0 (i.e., simulated recruitment was still an excellent predictor of population density). At this point in the study, the main effects of density-dependent mortality were to reduce mean densities and variation about the mean. Over the period from the start of the experiments until the time when maturity was reached by each species (about 1 and 3 months for Lythrypnus and Coryphopterus, respectively), mortality was strongly density-dependent. Such strong density-dependent mortality virtually eliminated any linear relationship between adult density and ”recruit” density. However, for both species, the relationship between these two variables was well fit by an asymptotic curve, with the asymptotic density of adults equal to c. 3/m² for Coryphopterus, and c. 10/m² for Lythrypnus. Natural recruitment (via settlement of larvae) to the reefs over the period of the study (9 months) was above the asymptotic densities of adults for the two species, even though the study did not encompass the periods of peak annual recruitment of either species. This suggests that adult populations of these two gobies may often be limited, and regulated, by post-settlement processes, rather than by input of settlers. Other studies have shown that mortality of the two species is density-independent, or only weakly density-dependent, on reefs from which predators have been excluded. Hence, it appears that predators cause density-dependent mortality in these fishes. Received: 26 November 1996 / Accepted: 5 April 1997     

Recruitment in the redlip blenny Ophioblennius atlanticus: is space limiting?

This study investigates pre-and post-recruitment characteristics of a population of the redlip blenny (Ophioblennius atlanticus) on a fringing reef in Barbados, W.I. Recruits were observed in 6 of the 8 months of the study, but 90% of all recruitment occurred within a single month. Monthly recruitment rate was not correlated with the monthly mortality of residents, suggesting that the rate at which space becomes available on the reef does not control the timing of blenny recruitment. Most recruitment occurred when adult blenny density was low, i.e. when most total space was available on the reef. Postrecruitment territory size of resident blennies was half that of pre-recruitment territory size, and was better predicted by fish size than it was before recruitment. Aggression by resident blennies increased with blenny density, and was primarily directed at recruiting conspecifics. These results suggest increased competition for territorial space after recruitment. Following the major recruitment pulse, the monthly percentage of recruits dying was correlated with density. This may indicate density-dependent mortality, but could result from the covariance of density with age. We suggest that whether reef fish populations are space-limited or recruitment-limited may vary between species and within species between locations.     

The influence of plant cover on surfperch abundance at an offshore temperate reef

Synopsis At an offshore reef near Santa Barbara, southern California, young-of-year (young) of five surfperch species (Embiotocidae: Embiotoca jacksoni, E. lateralis, Hypsurus caryi, Rhacochilus toxotes, Damalichthys vacca) once thrived in a dense kelp understory of Pterygophora californica and Laminaria farlowii, but disappeared after a severe storm in February, 1980 deforested their habitat. Measurements of fish density and kelp cover made before deforestation indicated that the young surfperch appeared in the spring and disappeared in the fall as cover increased and declined. Adult surfperch and large kelp bass (Paralabrax clathratus), which can eat young but not adult surfperch, remained all year. We tested to see if kelp cover was an essential refuge for the young by pruning back (thinning) kelp blades from one of two transects. The thinning caused a significant redistribution of young which clearly avoided open spaces, but not of adults which are less vulnerable to predation. Measurements of fish density made after the storm-induced deforestation showed that adult surfperch and kelp bass still remained abundant even after the young surfperch were gone. Only after an abrupt reforestation in 1983, more than a year after the present study was terminated, were young once again seen on the reef. Although young surfperch may seek tiny prey living on kelp blades, most lines of evidence indicated that the distribution of the young is more a response to risk of predation. Hence the extent of kelp understory was probably the main determinant of the survival of young surfperch on the reef.     

Effect of habitat characteristics on the distribution and abundance of damselfish within a Red Sea reef

For coral reef fish with an obligate relationship to their habitat, like Pomacentrid damselfish, choosing a suitable home amongst the reef structure is key to survival. A surprisingly small number of studies have examined patterns in adult damselfish distributions compared to other ontogenetic phases. The aim of this study was to determine which reef and coral colony characteristics explained adult damselfish distribution patterns in a Red Sea reef. The characteristics investigated were reef type (continuous or patchy), coral species (seven species of Acropora), and coral morphology (coral size and branching density). The focal damselfish species were Dascyllus aruanus, D. marginatus, Chromis viridis, and C. flavaxilla. Occupancy (presence or absence of resident damselfish), group size and fish species richness were not significantly different between the seven Acropora species. However, within each coral species, damselfish were more likely to occupy larger coral colonies than smaller coral colonies. Occupancy rates were also higher in patchy reef habitats than in continuous sections of the reef, probably because average coral colony size was greater in patchy reef type. Fish group size increased significantly with coral colony volume and with larger branch spacing. Multi-species groups of fish commonly occurred and were increasingly likely with reduced branching density and increased coral size.     

Different Surrounding Landscapes may Result in Different Fish Assemblages in East African Seagrass Beds

Few studies have considered how seagrass fish assemblages are influenced by surrounding habitats. This information is needed for a better understanding of the connectivity between tropical coastal ecosystems. To study the effects of surrounding habitats on the composition, diversity and densities of coral reef fish species on seagrass beds, underwater visual census surveys were carried out in two seagrass habitat types at various locations along the coast of Zanzibar (Tanzania) in the western Indian Ocean. Fish assemblages of seagrass beds in a marine embayment with large areas of mangroves (bay seagrasses) situated 9 km away from coral reefs were compared with those of seagrass beds situated on the continental shelf adjacent to coral reefs (reef seagrasses). No differences in total fish density, total species richness or total juvenile fish density and species richness were observed between the two seagrass habitat types. However, at species level, nine species showed significantly higher densities in bay seagrasses, while eight other species showed significantly higher densities in reef seagrasses. Another four species were exclusively observed in bay seagrasses. Since seagrass complexity could not be related to these differences, it is suggested that the arrangement of seagrass beds in the surrounding landscape (i.e. the arrangement on the continental shelf adjacent to the coral reef, or the arrangement in an embayment with mangroves situated away from reefs) has a possible effect on the occurrence of various reef-associated fish species on seagrass beds. Fish migration from or to the seagrass beds and recruitment and settlement patterns of larvae possibly explain these observations. Juvenile fish densities were similar in the two types of seagrass habitats indicating that seagrass beds adjacent to coral reefs also function as important juvenile habitats, even though they may be subject to higher levels of predation. On the contrary, the density and species richness of adult fish was significantly higher on reef seagrasses than on bay seagrasses, indicating that proximity to the coral reef increases density of adult fish on reef seagrasses, and/or that ontogenetic shifts to the reef may reduce adult density on bay seagrasses.     

Life on the edge: conspecific attraction and recruitment of populations to disturbed habitats

Variation in the recruitment of benthic marine invertebrates is often attributed to the interaction of the supply of new individuals to a habitat and the availability of space for colonisation when they arrive. Also important in determining variation in recruitment is the response of the larvae to the characteristics of the habitat. Larvae of many benthic marine invertebrates have shown great specificity of requirements in setting their limits of distribution at the time of selection of a habitat. The tubeworm Galeolaria caespitosa shows great variation in recruitment from place to place on rocky intertidal seashores and is a gregarious animal with larvae showing directed responses to conspecific adults on the substratum. I hypothesised that, if variation in recruitment of G. caespitosa were independent of conditions on the substratum, the magnitude of recruitment in patches of the same shape but different sizes cleared within continuous mats of conspecific adults would be directly related to the area available for colonisation in the patch. Alternatively, if variation in recruitment were due to the response of larvae to conspecific adults on the substratum, the magnitude of recruitment would be a function of the perimeter of the patch, which, given patches of the same shape, is a measure of the influence of conspecific adults in that patch. To distinguish between these alternatives, small (area = 25 cm²; perimeter = 20 cm) and large (area = 225 cm²; perimeter = 60 cm) square patches were cleared within continuous mats of conspecific adults at four sites and recruitment of G. caespitosa was monitored over two seasons of recruitment. The density of recruits per unit area was, on average, almost three times greater in small than in large patches, indicating that recruitment of G. caespitosa is not directly related to the area of the patch. In contrast, the density of recruits per unit perimeter was not significantly different between small and large patches, indicating that recruitment of G. caespitosa is related to the proximity of conspecific adults in the patch. Therefore, at a given site, the perimeter of patches within mats of G. caespitosa is a better predictor of the relative magnitude of recruitment among patches than that provided by their areas. These results are contrary to many models of invertebrate recruitment that assume close linkage between available space on the substratum and settlement. Moreover, they highlight the importance of behavioural responses of animals at the time of selection of habitat in accounting for variation in recruitment. For populations of organisms that display gregarious behaviour at settlement, or conspecific attraction, this direct relationship between the perimeter of patches and recruitment could be used as a tool in restoring populations to disturbed habitats. The added benefit of such facilitative interactions in restoring populations is that they provide increasing returns to the population for a given supply of potential colonists to a habitat. Received: 1 November 1996 / Accepted: 20 January 1997     

Larval dispersal,recruitment, and adult distribution of the brooding stony octocoral<Emphasis Type="Italic"> Heliopora coerulea</Emphasis> on Ishigaki Island,southwest Japan

Larval dispersal and recruitment are important factors that determine the distribution of adult corals. The relationships between larval dispersal, recruitment, and the adult distribution of the blue octocoral, Heliopora coerulea, were investigated on Shiraho Reef, Ishigaki Island, southwest Japan. Heliopora coerulea is a surface brooder that releases planulae in June or July on Shiraho Reef. We observed planulae between 1998 and 2000 and found that they did not swim actively; instead, they crawled into their settlement positions after becoming grounded on the substratum. Planulae occurred throughout the water column and were dispersed by tidal and wind-driven currents around the parent population on the reef flat. Recruitment was observed only within 350 m of the parent populations, including areas between the branches of the adult colony. The planulae of H. coerulea had a narrow dispersal range as a result of their mostly benthic, shorter larval duration, and the influence of weaker currents. Thus, the dispersal distance of larvae is determined by their position in the water column, the currents that deliver the larvae, and the competency period of the larvae. The narrow dispersal range of H. coerulea was consistent with recruitment of sexually derived larvae onto their natal reef.