Drivers of damselfishes distribution patterns in the southwestern Atlantic: tropical and subtropical reefs compared

Abstract Damselfishes are known keystone species of reef environments, however large-scale distribution patterns are poorly studied in the southwestern Atlantic. We evaluated main drivers of distribution of three conspicuous damselfishes, along tropical and subtropical coastal systems, in Brazil. Abundances were assessed against wave exposure, depth (within 1–7 m in tropical and 1–11 m in subtropical reefs) and benthic cover. Despite differences between systems, exposure and depth consistently explained damselfishes distribution. Stegastes fuscus, the larger damselfish species of the genus in the southwestern Atlantic, was dominant in both systems, inhabiting preferably shallow and sheltered reefs. Conversely, Stegastes variabilis occupied shallow habitats with higher exposure. Stegastes pictus was absent from tropical reefs sampled, inhabiting depths >7 m, in subtropical reefs. Species were weakly associated with benthic features, which poorly predicted changes in abundances. Regardless, S. fuscus showed association with articulated calcareous algae, and S. variabilis juveniles associated with erect macroalgae. Despite occurring in very distinctive reef systems, Brazilian damsels habitat requirements are consistent in both tropical and subtropical reefs. While highly persistent species, long term monitoring will inform us how they respond to pervasive global changes and human impacts along Brazilian reefs.

A review of conspecific attraction for habitat selection across taxa

Many species across taxa select habitat based on conspecific presence, known as conspecific attraction. Studies that document conspecific attraction typically provide social information (i.e., cues that indicate the presence of a given species) and then determine if a given species is more likely to settle at locations where the social information is provided compared to those locations that do not. Although the number of studies examining conspecific attraction has grown in recent years, a comprehensive review has not yet been undertaken. Here, we conducted a review of the literature and found 151 studies investigating conspecific attraction across eight taxa. We found that conspecific attraction is widespread with between 80% and 100% of studies, depending on taxa, documenting positive associations between habitat selection and the presence of conspecific cues. Conspecific attraction has been documented more frequently in bird and fish species with less attention given to invertebrate and mammal species. We use the patterns we found to (a) provide an overview of the current state of research on conspecific attraction and (b) discuss how important factors, such as cue characteristics and life history traits, may play a role in shaping conspecific attraction patterns within and across taxa.

Size-specific recolonization success by coral-dwelling damselfishes moderates resilience to habitat loss

Increasing degradation of coral reef ecosystems and specifically, loss of corals is causing significant and widespread declines in the abundance of coral reef fishes, but the proximate cause(s) of these declines are largely unknown. Here, we examine specific responses to host coral mortality for three species of coral-dwelling damselfishes (Dascyllus aruanus, D. reticulatus, and Pomacentrus moluccensis), explicitly testing whether these fishes can successfully move and recolonize nearby coral hosts. Responses of fishes to localized coral loss was studied during population irruptions of coral feeding crown-of-thorns starfish, where starfish consumed 29 (34%) out of 85 coral colonies, of which 25 (86%) were occupied by coral-dwelling damselfishes. Damselfishes were not tagged or individually recognizable, but changes in the colonization of different coral hosts was assessed by carefully assessing the number and size of fishes on every available coral colony. Most damselfishes (> 90%) vacated dead coral hosts within 5 days, and either disappeared entirely (presumed dead) or relocated to nearby coral hosts. Displaced fishes only ever colonized corals already occupied by other coral-dwelling damselfishes (mostly conspecifics) and colonization success was strongly size-dependent. Despite movement of damselfishes to surviving corals, the local abundance of coral-dependent damselfishes declined in approximate accordance with the proportional loss of coral habitat. These results suggest that even if alternative coral hosts are locally abundant, there are significant biological constraints on movement of coral-dwelling damselfishes and recolonization of alternative coral habitats, such that localized persistence of habitat patches during moderate or patchy disturbances do not necessarily provide resilience against overall habitat loss.

The Distribution of Planktivorous Damselfishes (Pomacentridae) on the Great Barrier Reef and the Relative Influences of Habitat and Predation

Planktivorous damselfishes (Pomacentridae) are diverse and abundant on the Great Barrier Reef (GBR), are important prey for commercially harvested coral trout (Plectropomus spp.) and their feeding mode plays a central role in transferring energy from the plankton to the reef. However, little is known about their distribution patterns throughout the GBR and how those patterns are influenced by predators and habitat despite increasing pressures on both. Here we quantify the distribution and abundance of GBR planktivorous damselfishes, then examine the role of coral trout and habitat in shaping their assemblages. The assemblage structure of planktivorous damselfishes varied across the continental shelf, yet their total abundances varied sub-regionally, dependent on differences in coral habitat. Latitudinal patterns were relatively weak. Damselfish assemblages generally retained characteristics of their sub-regional setting over 20 years and assemblage degradation was only associated with major coral losses. Damselfish numbers were not negatively influenced by top-down control from coral trout. Instead, numbers of coral trout and damselfishes were both positively associated with coral habitat and each other. Our findings suggest that a complexity of factors and interactions shape reef fish assemblages and reinforce the fundamental importance of coral as the foundation of healthy reef communities.

Spatial patterns of the frog Oophaga pumilio in a plantation system are consistent with conspecific attraction

The conspecific attraction hypothesis predicts that individuals are attracted to conspecifics because conspecifics may be cues to quality habitat and/or colonists may benefit from living in aggregations. Poison frogs (Dendrobatidae) are aposematic, territorial, and visually oriented-three characteristics which make dendrobatids an appropriate model to test for conspecific attraction. In this study, we tested this hypothesis using an extensive mark-recapture dataset of the strawberry poison frog (Oophaga pumilio) from La Selva Biological Station, Costa Rica. Data were collected from replicate populations in a relatively homogenous Theobroma cacao plantation, which provided a unique opportunity to test how conspecifics influence the spatial ecology of migrants in a controlled habitat with homogenous structure. We predicted that (1) individuals entering a population would aggregate with resident adults, (2) migrants would share sites with residents at a greater frequency than expected by chance, and (3) migrant home ranges would have shorter nearest-neighbor distances (NND) to residents than expected by chance. The results were consistent with these three predictions: Relative to random simulations, we observed significant aggregation, home-range overlap, and NND distribution functions in four, five, and six, respectively, of the six migrant-resident groups analyzed. Conspecific attraction may benefit migrant O. pumilio by providing cues to suitable home sites and/or increasing the potential for social interactions with conspecifics; if true, these benefits should outweigh the negative effects of other factors associated with aggregation. The observed aggregation between migrant and resident O. pumilio is consistent with conspecific attraction in dendrobatid frogs, and our study provides rare support from a field setting that conspecific attraction may be a relevant mechanism for models of anuran spatial ecology.

The nose knows: linking sensory cue use, settlement decisions, and post-settlement survival in a temperate reef fish

Habitat selection by animals that migrate or disperse ultimately determines the biotic and abiotic environment they will experience in subsequent life stages. Intuitively, for habitat selection to be adaptive, animals should respond positively to cues produced by habitat characteristics that will enhance their fitness in the new environment. However, there are many examples of dispersing animals where individuals are attracted to cues produced by factors that reduce their fitness after arrival. In this study, we use a temperate reef fish to examine the relative importance of habitat-associated cues in habitat selection decisions, and assess whether use of these cues is adaptive across early life stages. We used a series of laboratory- and field-based manipulative experiments to test: (1) what habitat-associated cues are likely used to locate suitable habitat; (2) whether in situ settlement patterns reflect the cue response tested in the laboratory; and (3) whether the aspects of the habitat that stimulate settlement are the same as those that maximize survival. We observed a positive response to multiple habitat-associated cues, with conspecific cues eliciting the strongest behavioral response in laboratory choice experiments, and a strong inverse density-dependent relationship at settlement. Macroalgal cues also elicited a positive response at settlement, but were associated with higher mortality after settlement, suggesting that habitat selection decisions are not always adaptive. We argue that this non-intuitive behavior may still be adaptive if it improves fitness at an earlier life stage, as habitat selection behavior is the result of tradeoffs in fitness costs across multiple stages.

Immigration Rates during Population Density Reduction in a Coral Reef Fish

Although the importance of density-dependent dispersal has been recognized in theory, few empirical studies have examined how immigration changes over a wide range of densities. In a replicated experiment using a novel approach allowing within-site comparison, we examined changes in immigration rate following the gradual removal of territorial damselfish from a limited area within a much larger patch of continuous habitat. In all sites, immigration occurred at intermediate densities but did not occur before the start of removals and only rarely as density approached zero. In the combined data and in 5 of 7 sites, the number of immigrants was a hump-shaped function of density. This is the first experimental evidence for hump-shaped, density-dependent immigration. This pattern may be more widespread than previously recognized because studies over more limited density ranges have identified positive density dependence at low densities and negative density dependence at high densities. Positive density dependence at low density can arise from limits to the number of potential immigrants and from behavioral preferences for settling near conspecifics. Negative density dependence at high density can arise from competition for resources, especially high quality territories. The potential for non-linear effects of local density on immigration needs to be recognized for robust predictions of conservation reserve function, harvest impacts, pest control, and the dynamics of fragmented populations.

Sniffing out the competition? Juvenile coral reef damselfishes use chemical cues to distinguish the presence of conspecific and heterospecific aggregations

Aquatic animals commonly rely on chemical cues to provide information regarding their surroundings. They can respond either by being attracted to (potential mates, preferred habitats) or avoiding (predators, competitors) the source of the stimuli. Coral reef fishes use chemical cues to detect habitats, avoid predators and recognise conspecifics. However, the extent to which chemical cues are used to detect and respond to potential competitors, has received little attention. Here we test olfactory preferences for conspecifics and heterospecifics in newly settled juvenile coral reef fishes. Juveniles of 4 common coral-associated damselfish species: Dascyllus melanurus, Dascyllus reticulatus, Chrysiptera arnazae and Pomacentrus moluccensis, were subjected to olfactory choice tests. Three of the 4 species (excluding P. moluccensis) demonstrated preferences for waterborne conspecifics odours. All species exhibited an avoidance towards heterospecific odours; this aversion was consistently greatest towards P. moluccensis. A neutral response toward heterospecifics was only evident in two instances (1) between the two congeneric Dascyllus species, with D. melanurus toward D. reticulatus, and (2) with C. arnazae toward D. melanurus. While it is already known that the presence of conspecifics plays a vital role in settlement site selection, we show here that the presence of heterospecifics may also be key in determining the spatial distributions of juveniles across areas of coral reef.

The Prevalence and Importance of Competition Among Coral Reef Fishes

Although competition is recognized as a core ecological process, its prevalence and importance in coral reef fish communities have been debated. Here we compile and synthesize the results of 173 experimental tests of competition from 72 publications. We show that evidence for competition is pervasive both within and between species, with 72% of intraspecific tests and 56% of interspecific tests demonstrating a demographically significant consequence of competition (e.g., a decrease in recruitment, survival, growth, or fecundity). We highlight several factors that can interact with the effects of competition and make it more difficult to detect in field experiments. In light of this evidence, we discuss the role of competition in shaping coral reef fish communities and competition's status as one of several processes that contribute to species coexistence. Finally, we consider some of the complex ways that climate change may influence competition, and we provide suggestions for future research.

Habitat characteristics predicting distribution and abundance patterns of scallops in D'Entrecasteaux Channel, Tasmania

Habitat characteristics greatly influence the patterns of distribution and abundance in scallops, providing structure for the settlement of spat and influencing predation risk and rates of survival. Establishing scallop-habitat relationships is relevant to understanding the ecological processes that regulate scallop populations and to managing critical habitats. This information is particularly relevant for the D’Entrecasteaux Channel, south-eastern Tasmania (147.335 W, 43.220 S), a region that has supported significant but highly variable scallop production over many years, including protracted periods of stock collapse. Three species of scallops are present in the region; the commercial scallop Pecten fumatus, the queen scallop Equichlamys bifrons, and the doughboy scallop Mimachlamys asperrima. We used dive surveys and Generalized Additive Modelling to examine the relationship between the distribution and abundance patterns of each species and associated habitat characteristics. The aggregated distribution of each species could be predicted as a function of sediment type and species-specific habitat structural components. While P. fumatus was strongly associated with finer sediments and E. bifrons with coarse grain sediments, M. asperrima had a less selective association, possibly related to its ability to attach on a wide range of substrates. Other habitat characteristics explaining P. fumatus abundance were depth, Asterias amurensis abundance, shell and macroalgae cover. Equichlamys bifrons was strongly associated with macroalgae and seagrass cover, whereas M. asperrima abundance was greatly explained by sponge cover. The models define a set of relationships from which plausible hypotheses can be developed. We propose that these relationships are mediated by predation pressure as well as the specific behavioural characteristics of each species. The findings also highlight the specific habitat characteristics that are relevant for spatial management and habitat restoration plans.

Space partitioning among damselfishes in the Caribbean coast of Panama: the role of habitat preferences

This study aimed to assess abundance and habitat use by juvenile and adult damselfishes, as much as the benthic cover of different reefs on Isla Colon, Bocas del Toro, Panama. Reefs were selected considering different levels of wave exposure and depths. Damselfish and benthic communities were distinct between reefs. The most abundant species in the sheltered deeper reef was Stegastes planifrons followed by S. leucostictus, and they were also recorded in the sheltered shallower reef. Low densities of S. partitus and S. variabilis were also observed in the sheltered deeper reef, as these species are apparently restricted to higher depths. Additionally, these reefs presented patches with high cover of live and dead massive coral. Shallow depths presented high abundances of S. adustus, indicating a preference of this species for shallow habitats and exhibiting a two-fold increase in abundance at higher wave surge. Also, Microspathodon chrysurus reached higher numbers in shallow depths. Furthermore, the exposed reef presented a high cover (%) of algae groups and the fire-coral Millepora alcicornis, acknowledged as a preferred habitat for M. chrysurus and other reef fish. In this study, distinct habitat uses were observed, with patterns regulated by depth and/or wave exposure levels and/or availability of specific benthic cover. For site-attached species as damselfish, habitat specialization, competition and/or non-random recruitment patterns have been found to rule distributional patterns. Similar results for damselfish relative abundances were found compared to studies conducted within Panama and other reefs throughout the Caribbean region over three decades ago, indicating strong habitat affinity for the most abundant species. However, severe reductions of habitat availability following coral mass-mortality events may have disrupted their distributional patterns.

Long-term effects of the cleaner fish Labroides dimidiatus on coral reef fish communities

Cleaning behaviour is deemed a mutualism, however the benefit of cleaning interactions to client individuals is unknown. Furthermore, mechanisms that may shift fish community structure in the presence of cleaning organisms are unclear. Here we show that on patch reefs (61-285 m²) which had all cleaner wrasse Labroides dimidiatus (Labridae) experimentally removed (1-5 adults reef⁻¹) and which were then maintained cleaner-fish free over 8.5 years, individuals of two site-attached (resident) client damselfishes (Pomacentridae) were smaller compared to those on control reefs. Furthermore, resident fishes were 37% less abundant and 23% less species rich per reef, compared to control reefs. Such changes in site-attached fish may reflect lower fish growth rates and/or survivorship. Additionally, juveniles of visitors (fish likely to move between reefs) were 65% less abundant on removal reefs suggesting cleaners may also affect recruitment. This may, in part, explain the 23% lower abundance and 33% lower species richness of visitor fishes, and 66% lower abundance of visitor herbivores (Acanthuridae) on removal reefs that we also observed. This is the first study to demonstrate a benefit of cleaning behaviour to client individuals, in the form of increased size, and to elucidate potential mechanisms leading to community-wide effects on the fish population. Many of the fish groups affected may also indirectly affect other reef organisms, thus further impacting the reef community. The large-scale effect of the presence of the relatively small and uncommon fish, Labroides dimidiadus, on other fishes is unparalleled on coral reefs.

Habitat associations of juvenile fish at Ningaloo Reef, Western Australia: the importance of coral and algae

Habitat specificity plays a pivotal role in forming community patterns in coral reef fishes, yet considerable uncertainty remains as to the extent of this selectivity, particularly among newly settled recruits. Here we quantified habitat specificity of juvenile coral reef fish at three ecological levels; algal meadows vs. coral reefs, live vs. dead coral and among different coral morphologies. In total, 6979 individuals from 11 families and 56 species were censused along Ningaloo Reef, Western Australia. Juvenile fishes exhibited divergence in habitat use and specialization among species and at all study scales. Despite the close proximity of coral reef and algal meadows (10's of metres) 25 species were unique to coral reef habitats, and seven to algal meadows. Of the seven unique to algal meadows, several species are known to occupy coral reef habitat as adults, suggesting possible ontogenetic shifts in habitat use. Selectivity between live and dead coral was found to be species-specific. In particular, juvenile scarids were found predominantly on the skeletons of dead coral whereas many damsel and butterfly fishes were closely associated with live coral habitat. Among the coral dependent species, coral morphology played a key role in juvenile distribution. Corymbose corals supported a disproportionate number of coral species and individuals relative to their availability, whereas less complex shapes (i.e. massive & encrusting) were rarely used by juvenile fish. Habitat specialisation by juvenile species of ecological and fisheries importance, for a variety of habitat types, argues strongly for the careful conservation and management of multiple habitat types within marine parks, and indicates that the current emphasis on planning conservation using representative habitat areas is warranted. Furthermore, the close association of many juvenile fish with corals susceptible to climate change related disturbances suggests that identifying and protecting reefs resilient to this should be a conservation priority.

An empirical test of recruitment limitation in a coral reef fish

A long-term, large-scale empirical test of the recruitment limitation hypothesis was done by sampling fish populations from the southern Great Barrier Reef after having monitored their recruitment histories for 9 years. After adjustment for demographic differences, recruitment patterns explained over 90 percent of the spatial variation in abundance of a common damselfish among seven coral reefs. The age structures from individual reefs also preserved major temporal variations in the recruitment signal over at least 10 years. Abundance and demography of this small fish at these spatial and temporal scales can be explained almost entirely as variable recruitment interacting with density-independent mortality.

The effect of adult aggression on habitat selection by settlers of two coral-dwelling damselfishes

Coral-reef fishes experience a major challenge when facing settlement in a multi-threat environment, within which, using settlement cues, they need to select a suitable site. Studies in laboratories and artificial setups have shown that the presence of conspecific adults often serves as a positive settlement cue, whose value is explained by the increased survival of juveniles in an already proven fit environment. However, settlement in already inhabited corals may expose the recruits to adult aggression. Daily observations and manipulation experiments were used in the present study, which was conducted in the natural reef. We revealed differential strategies of settlers, which do not necessarily join conspecific adults. Dascyllus aruanus prefer to settle near (not with) their aggressive adults, and to join them only after gaining in size; whereas Dascyllus marginatus settlers in densely populated reefs settle independently of their adult distribution. Our results present different solutions to the challenges faced by fish recruits while selecting their microhabitat, and emphasize the complexity of habitat selection by the naïve settlers. Although laboratory experiments are important to the understanding of fish habitat selection, further studies in natural habitats are essential in order to elucidate the actual patterns of settlement and habitat selection, which are crucial for the survival of coral-reef fish populations.

Habitat utilization by coral reef fish: implications for specialists vs. generalists in a changing environment

1. The impact of environmental disturbance and habitat loss on associated species is expected to be dependent on a species' level of specialization. We examined habitat use and specialization of coral reef fish from the diverse and ecologically important family Pomacentridae, and determined which species are susceptible to declines in coral cover due to disturbance induced by crown-of-thorns seastar (COTS, Acanthaster planci L.). 2. A high proportion of pomacentrid species live in association with live coral as adults (40%) or juveniles (53%). Adults of many species had strong affiliations with branching corals, while juveniles favoured plating growth forms, reflecting the sizes of refuge provided by coral types. 3. Juveniles of species that associated with coral had narrower niche breadths than adult conspecifics, due to associations with specific coral types. The especially high coral association and narrower niche breadth of juveniles suggest that the presence of live coral is crucial for many species during early life history, and that disturbance-induced coral loss may have serious flow-on effects on adult abundance. 4. Microhabitat availability was a poor predictor of fish species abundance. Significant correlations between coverage of coral types and abundance of five adults and two juvenile species were detected; however, these relationships explained <35% and <10% of the variation in abundance of adult and juvenile species, respectively. 5. Niche breadth explained 74% of the variation in species' mean response to coral decline and it is clear that disturbance has a greater impact on resource specialists, suggesting that increasing frequency and intensity of coral loss will cause reef fish communities to become dominated by habitat generalists at the expense of coral-dwelling specialists.

The morphology and ultrastructure of the peripheral olfactory organ in newly metamorphosed coral-dwelling gobies, Paragobiodon xanthosomus Bleeker (Gobiidae, Teleostei)

We examined the peripheral olfactory organ in newly metamorphosed coral-dwelling gobies, Paragobiodon xanthosomus (SL=5.8mm+/-0.8mm, N=15), by the aid of electron microscopy (scanning and transmission) and light microscopy. Two bilateral olfactory placodes were present in each fish. They were oval-shaped and located medio-ventrally, one in each of the olfactory chambers. Each placode had a continuous cover of cilia. The placode epithelium contained three different types of olfactory receptor neurons: ciliated, microvillous and crypt cells. The latter type was rare. Following a pelagic larval phase, P. xanthosomus settle to the reef and form an obligate association with one species of coral, Seriatopora hystrix. Their well-developed olfactory organs likely enable larvae of P. xanthosomus to detect chemical cues that assist in navigating towards and selecting appropriate coral habitat at settlement. Our findings support past studies showing that the peripheral olfactory organ develops early in coral reef fishes.

Conspecific attraction and shelter selection in gregarious insects

During habitat selection, the presence of conspecifics can frequently drive a nonuniform distribution of animals across habitats of equivalent quality. In group-living species, subgroups of individuals might display mutual attraction while differing in their preferences for environmental resources. The final decision to settle requires individuals to integrate both environmental and social cues. This raises the question of the relative importance of sociality and resources preferences in determining habitat choice. In this study, we examined the interactive influence of conspecific attraction on individual resource preferences on refuge choice in groups of cockroaches. Shelters scaled to the sizes of nymphs and adult males were offered to groups of only nymphs and only males and to mixed groups. The choices of males were consistent across social conditions. Conversely, the preferences of nymphs shifted depending on the social context; the presence of males overrode the affinity nymphs had for scaled-size shelters. We developed a numerical model implementing parameters derived from these experiments to test whether the final spatial distribution of individuals originated from a differential attraction between nymphs and males that was associated with their relative body size. Finally, we propose a general framework for understanding how similar mechanisms can promote the skewed distribution of organisms at different spatial scales.

Density-dependent habitat selection and performance by a large mobile reef fish

Many exploited reef fish are vulnerable to overfishing because they concentrate over hard-bottom patchy habitats. How mobile reef fish use patchy habitat, and the potential consequences on demographic parameters, must be known for spatially explicit population dynamics modeling, for discriminating essential fish habitat (EFH), and for effectively planning conservation measures (e.g., marine protected areas, stock enhancement, and artificial reefs). Gag, Mycteroperca microlepis, is an ecologically and economically important warm-temperate grouper in the southeastern United States, with behavioral and life history traits conducive to large-scale field experiments. The Suwannee Regional Reef System (SRRS) was built of standard habitat units (SHUs) in 1991-1993 to manipulate and control habitat patchiness and intrinsic habitat quality, and thereby test predictions from habitat selection theory. Colonization of the SRRS by gag over the first six years showed significant interactions of SHU size, spacing, and reef age; with trajectories modeled using a quadratic function for closely spaced SHUs (25 m) and a linear model for widely spaced SHUs (225 m), with larger SHUs (16 standardized cubes) accumulating significantly more gag faster than smaller 4-cube SHUs (mean = 72.5 gag/16-cube SHU at 225-m spacing by year 6, compared to 24.2 gag/4-cube SHU for same spacing and reef age). Residency times (mean = 9.8 mo), indicative of choice and measured by ultrasonic telemetry (1995-1998), showed significant interaction of SHU size and spacing consistent with colonization trajectories. Average relative weight (W(r)) and incremental growth were greater on smaller than larger SHUs (mean W(r) = 104.2 vs. 97.7; incremental growth differed by 15%), contrary to patterns of abundance and residency. Experimental manipulation of shelter on a subset of SRRS sites (2000-2001) confirmed our hypothesis that shelter limits local densities of gag, which, in turn, regulates their growth and condition. Density-dependent habitat selection for shelter and individual growth dynamics were therefore interdependent ecological processes that help to explain how patchy reef habitat sustains gag production. Moreover, gag selected shelter at the expense of maximizing their growth. Thus, mobile reef fishes could experience density-dependent effects on growth, survival, and/or reproduction (i.e., demographic parameters) despite reduced stock sizes as a consequence of fishing.

Site fidelity and homing in tropical coral reef cardinalfish: are they using olfactory cues?

A number of tropical coral reef fish hold station and display restricted home ranges. If artificially displaced, they will return to their home site. We questioned if marine fish are using the same mechanisms for home site detection as many freshwater fish, that is, by olfactory sensing of chemical signals deposited on the substrate by conspecific fish. Behavioral experiments were conducted on Lizard Island Research Station, Queensland, Australia, in 2001 and 2002. Five-lined cardinalfish (Cheilodipterus quinquelineatus) were tested in groups with split-branded cardinalfish (Apogon compressus) as a reference species and individually against Apogon leptacanthus as well as conspecifics of another reef site. The group tests showed that both species preferred artificial reef sites that had previously been occupied by conspecifics. Individual C. quinquelineatus preferred scent of conspecifics from their own reef site to that from another site. They also preferred the scent released by artificial reefs previously occupied by conspecifics of their reef site to that of similar reefs previously occupied by conspecifics of another reef site. No discrimination between species from the same reef site was obtained in experiments with individual fish. Our data suggest that cardinalfish are keeping station and are homing by use of conspecific olfactory signals.

The role of conspecifics in habitat settlement in a territorial grasshopper

Conspecifics may play a positive, neutral or negative role in the habitat choices of prospective settlers. I investigated the effect of conspecifics on habitat choice in a territorial grasshopper, Ligurotettix coquilletti, in three field experiments using both broadcast calls and live males as stimuli. In a two-bush choice experiment, males chose bushes from which conspecific calls were broadcast over either silent bushes or bushes broadcasting calls of another grasshopper species. When presented with a choice between broad-cast calls and a live conspecific male, however, males preferred bushes that contained a live male. In the third experiment, males were more strongly attracted to bushes occupied by a single territorial male than to unoccupied bushes, but had a lower probability of remaining on occupied bushes over an extended period. Despite the lower overall probability of remaining on a bush, males remained clustered on the occupied bushes at the end of this experiment because of their high initial attraction to occupied bushes. Copyright 1998 The Association for the Study of Animal Behaviour

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.