Dascyllus Aruanus Research Articles

Basin isolation and oceanographic features influencing lineage divergence in the humbug damselfish <I>(Dascyllus aruanus)</I> in the Coral Triangle

The Coral Triangle is a hotspot for marine species diversity as well as for intraspecific genetic diversity. Here, we used nuclear RAG2 and mitochondrial D-Loop genes to identify deep genetic divergence among Dascyllus aruanus (Linnaeus, 1758) populations across relatively short scales within the Coral Triangle. Mitochondrial clades different by >20 mutational steps were completely isolated from one another across the distance between Java and the Lesser Sunda Islands, and also showed frequency differences in the eastern and western Philippines. Evidence for population structure in the Sulu Sea and at the Lesser Sunda Islands was also identified. Our results suggest that the Sulu Sea Throughflow, Bohol Sea Throughflow, Indonesian Throughflow, signatures of extinction events from Pleistocene land barriers, and other past and present forces may be potential factors leading to lineage divergence of D. aruanus, and that these hypotheses should be tested in further studies.

Wind-induced variability in larval retention in a coral reef system: A biophysical modelling study in the South-West Lagoon of New Caledonia

In the present work, a biophysical dispersal model is used to understand the role of the physical environment in determining reef fish larval dispersal patterns in the South-West Lagoon of New Caledonia. We focus on a reef fish species, the humbug damselfish Dascyllus aruanus, to investigate seasonal variability of simulated larval retention at the scale of a reef patch and at the scale of the lagoon, and to explore links between larval retention and wind variability. The model shows that retention exhibits considerable temporal variability and periodically reaches values much larger than anticipated. Non-zero larval settlement occurs over a large part of the lagoon. Nevertheless, settlement values decrease quickly away from the natal reef and mean dispersal distances are of order 25–35km. Cross-correlation analyses indicate that weather conditions characterized by strong south east trade winds lead to low retention rates at both local (reef) and regional (lagoon) scales. By contrast, subtropical weather conditions characterized by weak winds result in high retention rates. These results suggest that large-scale weather regimes can be used as proxies for larval retention of the humbug damselfish in the South-West Lagoon of New Caledonia. Nevertheless, relatively small mean dispersal distances suggest that metapopulation dynamics occur on relatively small spatial scales.

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.

Social group entry rules may limit population resilience to patchy habitat disturbance

Habitat degradation often results in resident displacement. One potentially important determinant of the lethality of such displacement is the nature of interactions between displaced individuals and the occupants of nearby, intact habitat. Here, we show that displaced individuals of the coral-dwelling fish Dascyllus aruanus have very low potential to join a new group due to targeted and cooperative aggression by conspecific resident group members. Residents were found to be relatively non-aggressive towards familiar displaced individuals seeking group re-entry but highly aggressive towards displaced unfamiliar prospective group members. The extent of individual resident aggression towards unfamiliar individuals correlated with group-based dominance rank, resident-to-intruder size similarity, and social group size, such that higher ranking resident group members disproportionately contributed to intruder defence. Subsequent group re-entry by displaced familiar individuals was observed 100% of the time, compared to an 18% group entry success rate for displaced unfamiliar individuals. Results indicate that pre-existing social bonds and associated cooperative aggression can generate greater habitat-disturbance lethality for displaced inhabitants than that predicted by mobility potential and alternative habitat availability alone. This implies that social group entry rules may be an important determinant of population resilience to patchy habitat disturbance.

Initiators, Leaders, and Recruitment Mechanisms in the Collective Movements of Damselfish

Explaining how individual behavior and social interactions give rise to group-level outcomes and affect issues such as leadership is fundamental to the understanding of collective behavior. Here we examined individual and collective behavioral dynamics in groups of humbug damselfish both before and during a collective movement. During the predeparture phase, group activity increased until the collective movement occurred. Although such movements were precipitated by one individual, the success or failure of any attempt to instigate a collective movement was not solely dependent on this initiator's behavior but on the behavior of the group as a whole. Specifically, groups were more active and less cohesive before a successful initiation attempt than before a failed attempt. Individuals who made the most attempts to initiate a collective movement during each trial were ultimately most likely to lead the collective movement. Leadership was not related to dominance but was consistent between trials. The probability of fish recruiting to a group movement initiative was an approximately linear function of the number of fish already recruited. Overall, these results are consistent with nonselective local mimetism, with the decision to leave based on a group's, rather than any particular individual's, readiness to leave.

Discrete and continuous reproductive tactics in a hermaphroditic society

An important step towards understanding conflicts in animal societies lies in identifying socioecological predictors of individual reproductive tactics. In gonochoristic species, individuals can choose to adopt breeding or nonbreeding tactics, and if they breed, how large a share of reproduction they acquire. In hermaphroditic species, individuals can also adopt male or female breeding tactics. Hermaphrodites’ wider suite of reproductive options makes them interesting models for investigating predictors of reproductive tactics. We used molecular and ecological data to determine socioecological correlates of discrete (breeding versus nonbreeding; males versus female) and continuous (share of reproduction) reproductive tactics in the hermaphroditic coral-dwelling fish, Dascyllus aruanus. The number of potential competitors within groups was positively related to coral size, and the amount of total reproduction over which they competed was associated with the size of the largest individual (i.e. the parental male). Discrete and continuous reproductive tactics were strongly influenced by rank and body size: high-ranking and large individuals were more likely to breed and attain larger reproductive shares and output. High-ranking breeders also obtained a larger reproductive output if they adopted male tactics, whereas low-ranking breeders obtained a larger share if they adopted female tactics, which can explain why these fish show protogynous sex change. Genetic analysis also revealed that subordinates could attain a larger reproductive share than dominants, and that extragroup individuals could contribute to reproduction. Our results shed new light on the causes of variation in reproductive tactics, the payoffs from group membership and the nature conflict in hermaphroditic societies.

Specialization in habitat use by coral reef damselfishes and their susceptibility to habitat loss

While it is generally assumed that specialist species are more vulnerable to disturbance compared with generalist counterparts, this has rarely been tested in coastal marine ecosystems, which are increasingly subject to a wide range of natural and anthropogenic disturbances. Habitat specialists are expected to be more vulnerable to habitat loss because habitat availability exerts a greater limitation on population size, but it is also possible that specialist species may escape effects of disturbance if they use habitats that are generally resilient to disturbance. This study quantified specificity in use of different coral species by six coral-dwelling damselfishes (Chromis viridis, C. atripectoralis, Dascyllus aruanus, D. reticulatus, Pomacentrus moluccensis, and P. amboinensis) and related habitat specialization to proportional declines in their abundance following habitat degradation caused by outbreaks of the coral eating starfish, Acanthaster planci. The coral species preferred by most coral-dwelling damselfishes (e.g., Pocillopora damicornis) were frequently consumed by coral eating crown-of-thorns starfish, such that highly specialized damselfishes were disproportionately affected by coral depletion, despite using a narrower range of different coral species. Vulnerability of damselfishes to this disturbance was strongly correlated with both their reliance on corals and their degree of habitat specialization. Ongoing disturbances to coral reef ecosystems are expected, therefore, to lead to fundamental shifts in the community structure of fish communities where generalists are favored over highly specialist species.

Influence of coral bleaching, coral mortality and conspecific aggression on movement and distribution of coral-dwelling fish

Changes in coral health may have significant impacts on fishes that shelter within live coral habitats. However, disturbances can be spatially patchy and it is unclear whether fishes subject to degradation of their immediate habitat are able to move and exploit alternative nearby healthy habitat patches. This study experimentally investigated the response of a common coral-dwelling damselfish (Dascyllus aruanus) to bleaching and mortality of host coral colonies. Three factors were considered that may influence the initiation and success of movement among habitat patches; i) condition of the host colony, ii) the availability of preferred habitat, and iii) the presence of conspecifics on alternative coral habitat. D. aruanus remained associated with bleached coral colonies and only moved to alternate habitats once the host coral died. On colonies that suffered complete mortality, 67% of associated fish vacated and migrated to healthy coral colonies. The remaining individuals persisted on dead colonies as a result of competition from aggressive resident fish on neighbouring healthy colonies, but relocated once offered an alternative colony without any competitive resident fish. When relocating in degraded reefs, 64% of the fish were successful in migrating to alternative healthy habitats. For these fish the selection of new habitat was based mostly on the presence of conspecifics, with 91% of fish relocating to healthy coral colonies containing conspecifics. The importance of conspecifics presence was further demonstrated by tank experiments where fish selected to associate with habitat containing conspecifics over unoccupied dead or healthy habitat. Movement of coral-dwelling fishes following disturbances will allow individuals to mediate the impact of host coral degradation by relocating to more suitable habitats. However, the potential for relocation will decline as disturbances become more severe and widespread, thereby increasing the distance to suitable habitats and reducing the availability of preferred habitat types. Furthermore, increased densities within habitat patches may increase competitive effects, with potential consequences for growth, survival and reproductive success.

Genetic diversity loss associated to high mortality and environmental stress during the recruitment stage of a coral reef fish

We investigated the short-term impact of environmental-induced stress on survival and neutral genetic diversity of recently settled juveniles of a dam- selfish, Dascyllus aruanus, using spatiotemporal caging experiments in various natural environmental conditions in Moorea (French Polynesia). Juveniles' mortality was followed at five study sites and overall four experiments, mortality rates ranged from 0 to 45%. Mortality rate and average daily water temperature were positively correlated (P = 0.018). Juveniles' mortality rate and allelic richness estimated from ten microsatellite loci were negatively correlated (P = 0.046). Together, an overdominance of heterozygotes was observed within hostile environments. These results suggest that an allelic richness loss may be expected as a direct consequence of unfavorable environ- mental conditions. Thus, a worrisome scenario on demo- graphic and genetic consequences may be expected from habitat degradation in the context of global change and human pressure increases.

Trophic Niche Width, Shift, and Specialization of Dascyllus aruanus in Toliara Lagoon, Madagascar

Abstract Intrapopulation diet specializations may result from the use of different dietary items or foraging tactics by individuals within a single population. The damselfish, Dascyllus aruanus, is a highly site-attached coral reef fish living in size hierarchies among branched corals. The trophic niche width and feeding specialization of this species were explored using stable isotopes and stomach content analyses. Intra-group niche variation was mainly related to fish size. Within social groups, D. aruanus gradually shifted its foraging tactics according to size; smaller fish fed on benthic prey such as isopods and copepods, and the larger fish foraged in the water column on planktonic copepods and larger-sized prey. Group density was found to explain some variation in trophic niche characteristics; greater specialization on prey size was observed in the colony having the highest density. All members of the largest colony foraged more frequently in the water column. Knowing that planktonic copepods are ...

Some Observations on the Sexuality of Humbug Damselfish, Dascyllus aruanus (Pisces, Pomacentridae) in the Field

Abstract This study reports preliminary investigations of sexuality in D. aruanus in the field. Topics include sex-ratios amongst groups and solitary individuals, polygyny, protogyny, sexual selection, epigamic characters and sexual strategies. Terminology concerning the sexes in D. aruanus is discussed. The definite sexual phase of individuals was determined by the method of gonadal examination. The sex-ratio was determined amongst solitary individuals and groups of varying sizes. Polygyny tends to be the case but deviations from this are reported and discussed. Results suggest that larger male phase individuals can move to some extent between social groups and that they may select groups according to the number of individuals they contain. Discussion is presented on how aggression controls the extent of such inter-group movements of individuals. Experiments suggest that D. aruanus are protogynous hermaphrodites as concluded by other workers. The behaviour of D. aruanus observed is interpreted and discussed as resulting solely from the effects of selection acting on the individual. Individuals attempt to function in a way that gives them the highest possible reproductive success but are constrained within the limitations of their social system. Zusammenfassung Beim Korallenbarsch Dascyllus aruanus gibt es zwar einzeln lebende männliche und weibliche Individuen, soziale Gruppen aber sind meist polygyne Harems. Es kommen allerdings auch abweichende Gruppierungen und Intersexe vor. Erklärungen dafür werden angeboten. Nimmt man aus einer ansässigen Gruppe das größte Individuum heraus, so wandern männliche Individuen von anderswoher zu. Diese sind immer kleiner als das weggenommene größte Tier war, sind aber größer als die noch vorhandenen Gruppenmitglieder. Individuelle Größenunterschiede sind entscheidend für den Individuenaustausch zwischen Gruppen. Versuche zur Geschlechtsumkehr mit gekäfigten Tieren zeigen, daß dieser Fisch wohl ein protogyner Hermaphrodit ist. Dieses und die Fortpflanzungstaktik mit Harems werden als Ergebnisse der Individualselektion gedeutet.

Aquarium Studies on the Ability of Potential Predators to Learn to Avoid Humbug Damselfish, Dascyllus aruanus (Pisces, Pomacentridae) as Prey

Five species of potential predators of D. aruanus were trained in aquaria using a simple reward versus no-reward training technique in which D. aruanus (no-reward) and a variety of alternative prey species (reward) were presented simultaneously to the former. All the predators tested exhibited an ability to learn to approach the reward species of fish in preference to D. aruanus. Results are discussed in relation to the recently proposed theory that potential predators may learn in the field not to attack D. aruanus but to concentrate on alternative and more easily caught prey.

Group structure in a restricted entry system is mediated by both resident and joiner preferences

The benefits of grouping behaviour may not be equally distributed across all individuals within a group, leading to conflict over group membership among established group members, and between residents and outsiders attempting to join a group. Although the interaction between the preferences of joining individuals and existing group members may exert considerable pressure on group structure, empirical work on group living to date has focussed on free entry groups, in which all individuals are permitted entry. Using the humbug damselfish, Dascyllus aruanus, we examined a restricted entry grouping system, in which group residents control membership by aggressively rejecting potential new members. We found that the preferences shown by joining members were not always aligned with strategies that incurred the least harm from resident group members, suggesting a conflict between the preferences of residents and preferences of group joiners. Solitary fish preferred to join familiar groups and groups of size-matched residents. Residents were less aggressive towards familiar group joiners. However, resident aggression towards unfamiliar individuals depended on the size of the joining individual, the size of the resident and the composition of the group. These results demonstrate that animal group structure is mediated by both the preferences of joining individuals and the preferences of residents.

Evolution of ontogenetic allometry shaping giant species: a case study from the damselfish genus Dascyllus (Pomacentridae)

The evolution of body size, the paired phenomena of giantism and dwarfism, has long been studied by biologists and paleontologists. However, detailed investigations devoted to the study of the evolution of ontogenetic patterns shaping giant species are scarce. The damselfishes of the genus Dascyllus appear as an excellent model for such a study. Their well understood phylogeny reveals that large-bodied species have evolved in two different clades. Geometric morphometric methods were used to compare the ontogenetic trajectories of the neurocranium and the mandible in both small-bodied (Dascyllus aruanus and Dascyllus carneus; maximum size: 50–65 mm standard length) and giant (Dascyllus trimaculatus and Dascyllus flavicaudus; maximum size: 90–110 mm standard length) Dascyllus species. At their respective maximum body size, the neurocranium of the giant species is significantly shorter and have a higher supraoccipital crest relative to the small-bodied species, whereas mandible shape variation is more limited and is not related to the ‘giant’ trait. The hypothesis of ontogenetic scaling whereby the giant species evolved by extending the allometric trajectory of the small-bodied ones (i.e. hypermorphosis) is rejected. Instead, the allometric trajectories vary among species by lateral transpositions. The rate of shape changes and the type of lateral transposition also differ according to the skeletal unit among Dascyllus species. Differences seen between the two giant species in the present study demonstrate that giant species may appear by varied alterations of the ancestor allometric pattern. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99, 99–117. ADDITIONAL KEYWORDS: coral reef fishes – development – geometric morphometrics – giantism – growth – heterochrony – trajectory – shape – size – skull.

Genetic relatedness in groups of the humbug damselfish Dascyllus aruanus: small, similar‐sized individuals may be close kin

Kin selection plays an important role in the evolution of social behaviour in terrestrial systems. The extent to which kin selection influences the evolution of social behaviour in marine systems is largely unexplored. Generally, it is considered that kin selection is irrelevant in marine systems, because it is assumed that the dispersing larval phase of marine organisms will break up kin associations. Here, we challenge this assumption and investigate the opportunity for kin selection in a coral reef fish: the humbug damselfish Dascyllus aruanus. This fish lives in groups composed of a large male and a number of smaller females and nonbreeders. We use 10 polymorphic microsatellite loci to assess the relatedness of 265 individuals from 35 groups. The mean coefficient of relatedness among group members is 0.01 +/- 0.04, suggesting that individuals are not associated with close relatives. However, the distribution of pairwise relatedness of individuals within groups has an overabundance of positive values, and indicates that there might be 35 pairs of close relatives within groups. Further analyses reveal that close relatives likely are similar in size and small in size, suggesting that they might have recruited together. We conclude that it is possible for kin selection to operate in D. aruanus, but kin recognition will be a prerequisite for such selection. This study reveals that individuals can be associated with close relatives, and there is a hidden potential for kin selection, during certain parts of the life cycle of coral reef fishes.

Controlled Breeding and Larval Rearing Techniques of Marine Ornamental Fishes

International trade of marine ornamental fishes has been expanding rapidly in recent years, and the fact that nearly 98% of the species traded are collected from reef habitats is of vital concern for the conservation of the fragile coral reef ecosystem. Hence, it is widely accepted that the ultimate answer to a long-term sustainable trade of marine ornamental fishes is only through the development of hatchery production technologies. The techniques for broodstock development, breeding and seed production of three species of damsel fishes viz. the three spot damsel, Dascyllus trimaculatus, the humbug damsel, Dascyllus aruanus and the blue damsel, Pomacentrus caeruleus, were developed and standardised, which can be scaled up for commercial level production. Broodstock development was done in one-tonne Fiber Reinforced Plastic (FRP) tanks with biological filter and by feeding with natural feeds. The size range of broodstock fish of D. trimaculatus, D. aruanus and P. caeruleus were 9-10, 7-8 and 7-9 cm, respectively. The number of eggs per spawning ranged from 5000 to 15000. The interval between two successive spawnings ranged from 3 to 14 days. The eggs were attached either on the sides of the broodstock tank or on the substratum provided in the broodstock tank. Parental care by the male was noted. Hatching occurred on the evening of the fourth day of incubation. The larvae were altricial type with no mouth opening at the time of hatching for D. trimaculatus and D. aruanus. The larvae of P. caeruleus were with mouth opening at the time of hatching. The length range of newly hatched larvae was 1.5-2.5 mm and the range of mouth opening was 150-200 µ. Larviculture was done in five-tonne capacity FRP tank by employing greenwater technique. Copepod nauplii were used as the starter feed and after about two weeks when the mouth opening of the larvae had reached around 450 µ, newly hatched Artemia nauplii were supplemented. The metamorphosis period ranged from 20 to 40 days. Several batches of the three species were hatchery produced, and the technique can be scaled up for commercial level production for ornamental fish trade.

Sound production in four damselfish (Dascyllus) species: phyletic relationships?

Most studies of fish sounds show that the sounds are species-specific, with unique spectral and timing characteristics. This raises the question as to whether these sounds can be used to understand phyletic relationships between species and which acoustic parameters are subject to variation between species. In the present study, 597 sounds (and 2540 pulses) related to signal jumps of four Dascyllus species (Dascyllus aruanus, Dascyllus trimaculatus, Dascyllus albisella, and Dascyllus flavicaudus) from different geographic regions (Madagascar, Moorea, Rangiroa, and Hawaii) were analysed. It was possible to discern species-specific sounds, but also variation in sounds between populations. Large variations in sound length were found between Dascyllus species, whereas differences in interpulse duration were found to be variable between populations. In the regions where species live in sympatry, it appears that they restrict the variability in their sounds. This could comprise evidence of adaptation with character displacement of sonic characteristics where different species co-occur. However, sonic characteristics still overlapped substantially between species, suggesting that females would need to sample more than one sound and potentially use other cues to discriminate between species. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 97, 928‐940.

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.

Use of Copepods as Live Feed for Larviculture of Damselfishes

It is well established that small-sized live feed with sufficient DHA, EPA and ARA as starter feed is the key factor for the success in larviculture of marine fishes with altricial larvae having limited yolk, which are in an undeveloped state at hatching. Copepods form a major component of the natural diet of many fish larvae and the wide range of body sizes of copepods both within and between species is extremely useful for employing the early stage nauplii and copepodites as starter feed for very small larvae with small mouth opening. Improved survival, growth and normal pigmentation have been documented in the larviculture of several marine finfishes reared with the early stage nauplii and copepodites. This is generally attributed to the levels of DHA, EPA and or arachidomic acid (ARA) in the diet and particularly to the DHA: EPA ratio in the diet. Two species of copepods viz. Euterpina acutifrons, a harpacticoid copepod and Pseudodiaptomus serricaudatus, a calanoid copepod were selected and cultured. They were employed as starter feed for the larviculture of three species of damsel fishes viz. the three spot damselfish, Dascyllus trimaculatus, the humbug damselfish, Dascyllus aruanus and the blue damselfish, Pomacentrus caeruleus. It was found that co-culturing of copepods in greenwater in the larviculture tank is the most effective method for initiating the exogenous feeding of the species studied.

Isolation and characterization of 16 microsatellite loci in the humbug damselfish, Dascyllus aruanus (family Pomacentridae)

Here we report on 16 microsatellite loci designed for the damselfish Dascyllus aruanus. All loci were tested on 98 individuals and were polymorphic (seven to 35 alleles). Expected heterozygosity ranged from 0.705 to 0.942. Six loci showed Hardy-Weinberg disequilibrium due to the occurrence of null alleles. Cross-species amplifications conducted within the genus Dascyllus (D. carneus, D. strasburgi, D. trimaculatus) lead to polymorphic fragments in 32 out of 48 tests. These 16 loci will enable future research into the behavioural ecology and population ecology of Dascyllus aruanus throughout the Indo-Pacific.