Predator-prey interactions in a benthic stream community: a field test of flow-mediated refuges

Ecological theory suggests that the impact of predation can be strongly modified by the existence of regions of the environment in which prey are less accessible to predators, which underscores the need for empirical studies examining the factors influencing the availability and importance of such prey refuges. Our study tested whether benthic microhabitats with high flows provide suspension-feeding larval black flies (Simulium␣vittatum) with a spatial refuge in which the negative impact of predatory flatworms (Dugesia dorotocephala) is reduced. We conducted a short-term field experiment in Chester Creek (southeastern Pennsylvania, United States) to examine how the number of black fly larvae inhabiting tile substrates responded to manipulated variations in flatworm abundance and current speed. The abundance of flatworms declined with increasing current speed, thereby creating the potential for sites with high flows to provide larvae with a refuge from these predators. Multiple regression analysis revealed that the final abundance of larvae exhibited a significant negative relationship to flatworm abundance and a significant positive relationship to current speed. After adjusting for variations in elapsed time and initial larval abundance, flow and predators explained 38% of the variation in the rate of change in larval abundance. The positive correlation between larval abundance and flow had two components: a positive, direct effect of flow on larvae, which arises because these food-limited consumers prefer to reside within sites with faster flows where they can feed at higher rates; and a negative effect of flow on predators, and of predators on larvae, which combine to yield a positive indirect effect of flow on larvae. This indirect effect demonstrates the existence of flow-mediated refuges (i.e., microhabitats in which the impact of predation is reduced due to high flows), although the effect accounts for a small proportion of total variation in larval abundance. A consideration of biomechanical relationships suggests that microhabitats with high flows are likely to create prey refuges in a wide range of freshwater and marine benthic environments. In particular, predators will often experience greater dislodgement forces than prey because of their larger size and because they project farther above the bed where current speeds are faster. Moreover, the ability to resist a given dislodgement force may be greater for many prey, especially those that are sessile or semi- sessile. Received: 31 March 1997 / Accepted: 25 October 1997     

Larval Settlement: The Role of Surface Topography for Sessile Coral Reef Invertebrates

For sessile marine invertebrates with complex life cycles, habitat choice is directed by the larval phase. Defining which habitat-linked cues are implicated in sessile invertebrate larval settlement has largely concentrated on chemical cues which are thought to signal optimal habitat. There has been less effort establishing physical settlement cues, including the role of surface microtopography. This laboratory based study tested whether surface microtopography alone (without chemical cues) plays an important contributing role in the settlement of larvae of coral reef sessile invertebrates. We measured settlement to tiles, engineered with surface microtopography (holes) that closely matched the sizes (width) of larvae of a range of corals and sponges, in addition to surfaces with holes that were markedly larger than larvae. Larvae from two species of scleractinian corals (Acropora millepora and Ctenactis crassa) and three species of coral reef sponges (Luffariella variabilis, Carteriospongia foliascens and Ircinia sp.,) were used in experiments. L. variabilis, A. millepora and C. crassa showed markedly higher settlement to surface microtopography that closely matched their larval width. C. foliascens and Ircinia sp., showed no specificity to surface microtopography, settling just as often to microtopography as to flat surfaces. The findings of this study question the sole reliance on chemical based larval settlement cues, previously established for some coral and sponge species, and demonstrate that specific physical cues (surface complexity) can also play an important role in larval settlement of coral reef sessile invertebrates.     

The effect of natural variation in substrate architecture on the survival of juvenile bicolor damselfish

Coral reefs offer settling fish larvae a spatial mosaic of microhabitats that differ not only in structural complexity but also in the abundance and diversity of predators. This paper provides evidence that interactions between predators and prey are causally linked to variation in relevant architectural characteristics of natural substrates. Juvenile bicolor damselfish, Stegastes partitus, experienced greater mortality on Montastrea annularis boulder coral than on piles of Porites porites coral rubble. This pattern was consistent on both back and fore reef habitats. Architectural differences, variable encounter rates with predators, and access to different food sources all contributed to higher mortality. Spatial and structural differences among refuges provided by the two substrates were most important in affecting survival. Porites rubble contained almost three times the number of crevices (39.9 vs. 14.3 crevices m^–2AM²), had a much smaller mean crevice size (15.9 vs. 48.5 cm²), and had a more complex internal structure than Montastrea coral. Survival of juvenile fish living on Montastrea coral was positively correlated with crevice density (r² = 0.41) whereas survival of fish living on Porites rubble was negatively correlated with crevice size (r² = 0.43). These patterns were evident on fore reef habitats only, whereas on back reef habitats no clear patterns emerged. The effect of these natural differences in architecture on mortality rates of juvenile S. partitus was experimentally tested in the field using a combination of natural and artificial substrates. As the number of large shelters was increased I found that the density of potential predators increased but the survival rates of juvenile bicolor damselfish declined. These results highlight the importance of structural architecture among common reef substrates in affecting predator-prey relationships and in determining the survivorship and small-scale distribution patterns of juvenile reef fish.     

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     

Role of food source and predator avoidance in habitat specialization by an octocoral-associated amphipod

Small marine invertebrates often use sessile organisms as microhabitats, which can provide food sources and/or serve as refuges from predators. Because of the availability of external food items such as epibionts and detritus in the marine environment, these invertebrates may not depend on the sessile organisms as sole food sources. In this study, I hypothesized that habitat specialization by a marine invertebrate is determined by factors other than food. Results of field surveys off the coast of the Izu Peninsula, on the eastern coast of Japan, showed that, with few exceptions, the distribution of the amphipod Incisocalliope symbioticus was restricted to the octocoral Melithaea flabellifera. When presented with several habitat options, I. symbioticus selected M. flabellifera most frequently, although some individuals chose the octocoral Acabaria japonica. The selection was proximately determined by water-borne cues from M. flabellifera that appear to be unrelated to the octocoral as a food source, since the amphipod preferred detritus to the octocoral. As a chemical refuge, M. flabellifera had an allelopathic effect that deterred fish predation on the exposed epifauna. With regard to octocoral habitat in the study area, I. symbioticus may be restricted to M. flabellifera because this was the only abundant octocoral consistently occurring in shallow water (≤10 m), where predation is intensive. The relationship between I. symbioticus and M. flabellifera was commensal and was ultimately driven by the value of M. flabellifera as a chemical refuge from predation, rather than its food value. This study supports the idea that protection from predators, rather than food utilization, can promote ecological specialization.     

Ontogenetic changes in habitat selection during settlement in a coral reef fish: ecological determinants and sensory mechanisms

The behavior of marine larvae during and after settlement can help shape the distribution and abundance of benthic juveniles and therefore the intensity of ecological interactions on reefs. Several laboratory choice-chamber experiments were conducted to explore sensory capabilities and behavioral responses to ecological stimuli to better understand habitat selection by “pre-metamorphic” (larval) and “post-metamorphic” (juvenile) stages of a coral reef fish (Thalassoma hardwicke). T. hardwicke larvae were attracted to benthic macroalgae (Turbinaria ornata and Sargassum mangarevasae), while slightly older post-metamorphosed juveniles chose to occupy live coral colonies (Pocillopora damicornis). Habitat choices of larvae were primarily based upon visual cues and were not influenced by the presence of older conspecifics. In contrast, juveniles selected live coral colonies and preferred those occupied by older conspecifics; choices made by juveniles were based upon both visual and olfactory cues from conspecifics. Overall, the laboratory experiments suggest that early life-history stages of T. hardwicke use a range of sensory modalities that vary through ontogeny, to effectively detect and possibly discriminate among different microhabitats for settlement and later occupation. Habitat selection, based upon cues provided by environmental features and/or by conspecifics, might have important consequences for subsequent competitive interactions.     

Settlement inhibition of fouling invertebrate larvae by metabolites of the marine bacterium halomonas marina within a polyurethane coating

The marine bacterium, Halomonas marina (ATCC 27129), was shown to inhibit settlement and development of the sessile invertebrates Balanus amphitrite and Bugula neritina. Different bacterial treatments were employed to investigate this interaction. Filmed bacteria and liquid suspensions of whole cells, lysed cells and culture filtrate all reduced settlement of B. amphitrite. Polyurethane coatings containing whole cells were partially inhibitory while lysed cells caused complete inhibition of B. amphitrite larval settlement. In contrast, culture filtrate in a polyurethane matrix stimulated settlement of B. amphitrite larvae. Whole cells, culture filtrate, and lysed cells embedded in a polyurethane coating also controlled B. neritina settlement and maturation.     

Non‐lethal predator effects on the performance of a native and an exotic crayfish species

1. I tested the hypothesis that the potential for non‐lethal effects of predators are more important for overall performance of the fast‐growing exotic signal crayfish (Pacifastacus leniusculus Dana) than for the slower growing native noble crayfish (Astacus astacus L.). I further tested if omnivorous crayfish switched to feed on less risky food sources in the presence of predators, a behaviour that could reduce the feeding costs associated with predator avoidance. 2. In a 2 month long outdoor pool experiment, I measured behaviour, survival, cheliped loss, growth, and food consumption in juvenile noble or signal crayfish in pools with either a caged predatory dragonfly larvae (Aeshna sp.), a planktivorous fish that do not feed on crayfish (sunbleak, Leucaspius delineatus Heckel), or predator‐free controls. Crayfish had access to multiple food sources: live zooplankton, detritus and periphyton. Frozen chironomid larvae were also supplied ad libitum outside crayfish refuges, simulating food in a risky habitat. 3. Crayfish were mainly active during hours of darkness, with signal crayfish spending significantly more time outside refuges than noble crayfish. The proportion of crayfish outside refuges varied between crayfish species, time and predator treatment, with signal crayfish spending more time in refuges at night in the presence of fish. 4. Survival in noble crayfish was higher than in signal crayfish, and signal crayfish had a higher frequency of lost chelipeds, indicating a high level of intraspecific interactions. Crayfish survival was not affected by the presence of predators. 5. Gut‐contents analysis and stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) indicated that the two crayfish species had similar food preferences, and that crayfish received most of their energy from feeding on invertebrates (e.g. chironomid larvae), although detritus was the most frequent food item in their guts. Signal crayfish guts were more full than those of noble crayfish, but signal crayfish in pools with fish contained significantly less food and fewer had consumed chironomids compared with predator‐free controls. Length increase of signal crayfish (35%) was significantly higher than of noble crayfish (20%), but signal crayfish in pools with fish grew less than in control pools. 6. This short‐term study indicates that fish species that do not pose a lethal threat to an organism may indirectly cause reductions in growth by affecting behaviour and feeding. This may occur even though prey are omnivorous and have access to and consume multiple food sources. These non‐lethal effects of predators are expected to be particularly important in exotic crayfish species that show a general response to fish, have high individual growth rates, and when their feeding on the most profitable food source is reduced.     

Fluid-mediated dispersal in streams: models of settlement from the drift

I propose proximal mechanisms that help explain, unify, and expand the predictions of widely accepted empirical models of settlement in streams. I separated the process that leads to settlement of a drifting particle into three stages: (1) initial contact with a substrate, (2) attachment, and (3) settlement sensu stricto. I used physical principles (height above the bed, sinking rate, current speed profile) to predict time until contact (stage 1). I compared these predictions with empirical measurements of settlement of individual black fly larvae (Simulium vittatum) in a laboratory flume. I developed models from empirical data for stages 2 and 3. Each of these models is individual-based and predicts the fate of a single individual. To obtain a population level prediction, models for the three stages were combined and used to simulate the settlement of a group of black fly larvae. The predictions of this simulation were qualitatively similar to population level data from the literature particularly after the incorporation of channel-wide spatial heterogeneity in current speed. The effect of flow heterogeneity on the model agrees with previous work on the lateral transport of stream invertebrates during drift events showing that many organisms settle preferentially in slower areas. By using proximal principles, the approach used in this study brings into focus basic parameters and processes that influence settlement at the scale of the organisms. It also provides a null hypothesis against which to study the effect of local flow heterogeneity on the settlement of stream invertebrates and the capacity of organisms to actively influence settlement. Water currents in streams and rivers commonly transport large numbers of organisms. Consequently, hydrodynamic factors that favor or hamper the settlement of these organisms can potentially influence distributions and abundance. Moreover, if settlement probabilities vary with flow characteristics, this can in turn influence foraging strategies that rely on fluid-mediated dispersal. Received: 9 January 1999 / Accepted: 8 June 1999     

Variation in the effects of larval history on juvenile performance of a temperate reef fish

For organisms with complex life cycles, the transition between life stages can act as a significant demographic and selective bottleneck. Variation in developmental and growth rates among individuals present in one stage (e.g. larvae), due to initial differences in parental input and/or environmental conditions experienced, can propagate to future stages (e.g. juveniles), and such ‘carry‐over effects’ can shape fitness and phenotypic distributions within a population. However, variation in the strength of carry‐over effects between life stages and the intensity of selective mortality acting on intrinsic variation, and how these might be mediated by environmental variability in natural systems, is poorly known. Here, we evaluate variation in the strength to which larval growth histories can mediate juvenile performance (growth and survival), for a reef fish (Forsterygion lapillum) common to rocky reefs of New Zealand. We used otoliths to reconstruct demographic histories of recently settled fish that were sampled across cohorts, sites and microhabitats. We quantified sources of variation in the strength of carry‐over effects and selective mortality that operate on larval growth histories. We found overall that individuals that grew fast as larvae tended to experience proportional growth advantages as juveniles. However, the strength of growth advantages being maintained into the juvenile period varied among cohorts, sites and microhabitats. Specifically, a stronger growth advantage was found on some microhabitats (e.g. mixed stands of macroalgae) relative to others (e.g. monocultures of Carpophyllum maschalocarpum) for some cohorts and sites only. For other cohorts and sites, the degree of coupling between larval and juvenile growth rates was either indistinguishable between microhabitats or else not evident. Similarly, the intensity of growth‐based selective mortality varied among cohorts, sites and microhabitats: for the cohort and site where carry‐over effects differed between microhabitats, we also observed difference in the intensity to which fish with rapid larval growth rates were favoured. Overall, our results highlight how this spatial and temporal patchiness in extrinsic factors can interact with intrinsic variation of recruiting individuals to have a major influence on the resulting distribution of juveniles and their phenotypic traits.     

The influence of pre‐settlement and early post‐settlement processes on the adult distribution and relative dominance of two invasive mussel species

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Rising CO2 concentrations affect settlement behaviour of larval damselfishes

Reef fish larvae actively select preferred benthic habitat, relying on olfactory, visual and acoustic cues to discriminate between microhabitats at settlement. Recent studies show exposure to elevated carbon dioxide (CO₂) impairs olfactory cue recognition in larval reef fishes. However, whether this alters the behaviour of settling fish or disrupts habitat selection is unknown. Here, the effect of elevated CO₂ on larval behaviour and habitat selection at settlement was tested in three species of damselfishes (family Pomacentridae) that differ in their pattern of habitat use: Pomacentrus amboinensis (a habitat generalist), Pomacentrus chrysurus (a rubble specialist) and Pomacentrus moluccensis (a live coral specialist). Settlement-stage larvae were exposed to current-day CO₂ levels or CO₂ concentrations that could occur by 2100 (700 and 850 ppm) based on IPCC emission scenarios. First, pair-wise choice tests were performed using a two-channel flume chamber to test olfactory discrimination between hard coral, soft coral and coral rubble habitats. The habitat selected by settling fish was then compared among treatments using a multi-choice settlement experiment conducted overnight. Finally, settlement timing between treatments was compared across two lunar cycles for one of the species, P. chrysurus. Exposure to elevated CO₂ disrupted the ability of larvae to discriminate between habitat odours in olfactory trials. However, this had no effect on the habitats selected at settlement when all sensory cues were available. The timing of settlement was dramatically altered by CO₂ exposure, with control fish exhibiting peak settlement around the new moon, whereas fish exposed to 850 ppm CO₂ displaying highest settlement rates around the full moon. These results suggest larvae can rely on other sensory information, such as visual cues, to compensate for impaired olfactory ability when selecting settlement habitat at small spatial scales. However, rising CO₂ could cause larvae to settle at unfavourable times, with potential consequences for larval survival and population replenishment.     

Community Structure of Fishes and Habitat Complexity on a Tropical Rocky Shore

The relationship between the variables of reef fish community structure (fish richness, fish diversity and total number of fishes) and those of habitat complexity (total surface area, substratum diversity, topographic complexity, number of holes, percent cover of turf algae, zoanthids, millepores, massive corals, bare rock, encrusting calcareous algae, urchins, other sessile organisms and Sargassum) were examined on three different rocky shores (FA, FB and FT) at Arraial do Cabo, a tropical region located on Brazilian southeastern coast (23° S, 42° W). Fish abundance and vertical distribution were assessed by a visual census technique using strip transects. Percentage cover of benthic organisms and other substratum types were calculated by replicated transects using a chain laid down on the substratum. Topographic complexity was determined by the chain link method and number of holes were estimated by direct counts on replicated transects. More than 91 fishes belonging to 37 families were seen in all study sites during one year of visual census effort. FA and FB sites had similar fish community structure and habitat complexity characteristics, while FT showed different habitat characteristics and higher fish diversity and richness, plus total number of fishes. Vertical distribution of fishes along the rocky shores studied seemed to be predictable and was determined by factors such as feeding habits and behavior, availability of refuges and social interactions. The habitat variables that best explained the higher diversity and number of fishes observed in FT site were total surface area of rocky shores and the abundance of benthic sessile invertebrates; these conditions were typical of rocky shores more exposed to wave surge.     

The ploys of sex: relationships among the mode of reproduction,body size and habitats of coral-reef brittlestars

Observations were made of 33 species of brittlestars (3980 specimens) from specific substrata collected in four zones on the Belize Barrier Reef, Caribbean Sea. The body size of most species of brittlestars with planktonic larvae differs significantly among different substrata. Generally, individuals from the calcareous alga Halimeda opuntia are smallest, those found in corals (Porites porites, Madracis mirabilis, and Agaricia tenuifolia) are larger, and those from coral rubble are the largest. This suggests that brittlestars with planktonic larvae move to new microhabitats as they grow. In contrast, most brooding and fissiparous species are relatively small and their size-distributions are similar among all substrata. Halimeda harbours denser concentrations of brittlestars and more small and juvenile individuals than the other substrata. Juveniles of the brooding and fissiparous species are most common in Halimeda on the Back Reef whereas juveniles developing from planktonic larvae are most common in Halimeda patches in deeper water. Fissiparity and brooding may be means for individuals (genomes) of small, apomictic species to reach large size (and correspondingly high fecundities) in patchy microhabitats that select for small body sizes. Small brittlestar species and juveniles are most numerous in the microhabitats called refuge-substrata, such as Halimeda, which may repel predators and reduce environmental stress. Whether young brittlestars are concentrated in refuge-substrata through settlement behavior, migration, or differential survival remains unknown. Experiments revealed that coral polyps kill small brittlestars, perhaps accounting for the rarity of small and juvenile brittlestars in coral substrata.     

Interaction of a biotic factor (predator presence) and an abiotic factor (low oxygen) as an influence on benthic invertebrate communities

We examined the response of benthic invertebrates to hypoxia and predation risk in bioassay and behavioral experiments. In the bioassay, four invertebrate species differed widely in their tolerance of hypoxia. The mayfly, Callibaetis montanus, and the beetle larva, Hydaticus modestus, exhibited a low tolerance of hypoxia, the amphipod, Gammarus lacustris, was intermediate in its response and the caddisfly, Hesperophylax occidentalis, showed high tolerance of hypoxia. In the behavioral experiments, we observed the response of these benthic invertebrates, which differ in locomotor abilities, to vertical oxygen and temperature gradients similar to those in an ice-covered pond. With adequate oxygen, invertebrates typically remained on the bottom substrate. As benthic oxygen declined in the absence of fish, all taxa moved above the benthic refuge to areas with higher oxygen concentrations. In the presence of fish mayflies increased activity whereas all other taxa decreased activity in response to hypoxia. Mayflies and amphipods remained in the benthic refuge longer and endured lower oxygen concentrations whereas the vertical distribution of caddisflies and beetle larvae was not influenced by the presence of fish. As benthic oxygen declined in the presence of fish, all but the beetle larva reduced activity over all oxygen concentrations compared to when fish were absent. As benthic oxygen continued to decline, mayflies and amphipods moved above the benthic refuge and were preyed upon by fish. Thus, highly mobile taxa unable to tolerate hypoxia (mayflies and amphipods) responded behaviorally to declining oxygen concentrations by migrating upward in the water column. Taxa that were less mobile (beetle larvae) or hypoxia-tolerant (caddisflies) showed less of a response. Taxa most vulnerable to fish predation (mayflies and amphipods) showed a stronger behavioral response to predator presence than those less vulnerable (caddisflies and beetle larvae). Because invertebrates differ in their ability to withstand hypoxia, episodes of winter hypoxia could have long-lasting effects on benthic invertebrate communities either by direct mortality or selective predation on less tolerant taxa.     

Oyster Larvae Settle in Response to Habitat-Associated Underwater Sounds

Following a planktonic dispersal period of days to months, the larvae of benthic marine organisms must locate suitable seafloor habitat in which to settle and metamorphose. For animals that are sessile or sedentary as adults, settlement onto substrates that are adequate for survival and reproduction is particularly critical, yet represents a challenge since patchily distributed settlement sites may be difficult to find along a coast or within an estuary. Recent studies have demonstrated that the underwater soundscape, the distinct sounds that emanate from habitats and contain information about their biological and physical characteristics, may serve as broad-scale environmental cue for marine larvae to find satisfactory settlement sites. Here, we contrast the acoustic characteristics of oyster reef and off-reef soft bottoms, and investigate the effect of habitat-associated estuarine sound on the settlement patterns of an economically and ecologically important reef-building bivalve, the Eastern oyster (Crassostrea virginica). Subtidal oyster reefs in coastal North Carolina, USA show distinct acoustic signatures compared to adjacent off-reef soft bottom habitats, characterized by consistently higher levels of sound in the 1.5–20 kHz range. Manipulative laboratory playback experiments found increased settlement in larval oyster cultures exposed to oyster reef sound compared to unstructured soft bottom sound or no sound treatments. In field experiments, ambient reef sound produced higher levels of oyster settlement in larval cultures than did off-reef sound treatments. The results suggest that oyster larvae have the ability to respond to sounds indicative of optimal settlement sites, and this is the first evidence that habitat-related differences in estuarine sounds influence the settlement of a mollusk. Habitat-specific sound characteristics may represent an important settlement and habitat selection cue for estuarine invertebrates and could play a role in driving settlement and recruitment patterns in marine communities.     

Characteristics of Settling Coral Reef Fish Are Related to Recruitment Timing and Success

Many marine populations exhibit high variability in the recruitment of young into the population. While environmental cycles and oceanography explain some patterns of replenishment, the role of other growth-related processes in influencing settlement and recruitment is less clear. Examination of a 65-mo. time series of recruitment of a common coral reef fish, Stegastes partitus, to the reefs of the upper Florida Keys revealed that during peak recruitment months, settlement stage larvae arriving during dark lunar phases grew faster as larvae and were larger at settlement compared to those settling during the light lunar phases. However, the strength and direction of early trait-mediated selective mortality also varied by settlement lunar phase such that the early life history traits of 2–4 week old recruit survivors that settled across the lunar cycle converged to more similar values. Similarly, within peak settlement periods, early life history traits of settling larvae and selective mortality of recruits varied by the magnitude of the settlement event: larvae settling in larger events had longer PLDs and consequently were larger at settlement than those settling in smaller pulses. Traits also varied by recruitment habitat: recruits surviving in live coral habitat (vs rubble) or areas with higher densities of adult conspecifics were those that were larger at settlement. Reef habitats, especially those with high densities of territorial conspecifics, are more challenging habitats for young fish to occupy and small settlers (due to lower larval growth and/or shorter PLDs) to these habitats have a lower chance of survival than they do in rubble habitats. Settling reef fish are not all equal and the time and location of settlement influences the likelihood that individuals will survive to contribute to the population.     

Effects of sedimentation on settlement of Acropora millepora

Sedimentation is assumed to affect communities of reef-building corals by inhibiting larval settlement, however there has been no quantitative evidence to support this supposition. The effects of varying rates of sedimentation (0.5 to 325 mg cm^-2d^-1) on settlement rates of Acropora millepora larvae were examined experimentally, in aquaria. Settlement and juvenile orientation onto conditioned cut coral plates were recorded after two days. Higher sedimentation rates reduced the number of larvae settling on uppersurfaces, but total numbers of settled larvae were not significantly affected by sedimentary regime. While total settlement was unaffected by sediment under the confined experimental conditions, it is likely to be reduced under field conditions, since accumulation of sediment on upward-facing surfaces will greatly reduce the overall amount of suitable substratum available.     

Selective settlement of the barnacle Semibalanus balanoides (L.) facilitates its growth and reproduction on mussel beds in the Wadden Sea

On the unstable sedimentary tidal flats of the Wadden Sea, a suitable attachment substrate for sessile organisms is generally lacking. Epibenthic mussel beds (Mytilus edulis L.) provide the only and strongly limited settlement sites available for the barnacle, Semibalanus balanoides (L.). Field investigations showed that barnacles were non-randomly distributed within a mussel bed. They preferentially occurred near the siphonal apertures of living mussels but rarely grew on dead mussels or shell fragments. Field experiments revealed that this was due to selective settlement of barnacle cyprid larvae. Growth of barnacles was significantly higher upon living mussels than on empty mussel shells. Moreover, a higher reproductive output was obtained by individuals on living mussels which produced twice as many nauplii larvae than barnacles attached to empty shells. This study shows that selective settlement of S. balanoides cyprid larvae on living mussels is adaptive with respect to individual fitness. Received in revised form: 15 January 2001 Electronic Publication