Acquisition of Breeding Space by Nonbreeders in the Anemonefish Amphiprion clarkii in Temperate Waters of Southern Japan

Behavior of Amphiprion clarkii, especially of nonbreeders, was investigated at the shore of boulders and rocks in Uwa Sea, southern Japan. All individuals except small ones under one year were tagged. Movements of A. clarkii between host sea anemones were usual in this study area where host anemones were abundant, unlike in the general habitat (coral reefs) of Amphiprion, where movements between hosts are restricted. Monogamous pairs established almost contiguous territories containing all hosts except small ones. Nonbreeders had home ranges on the fringes of the pairs' territories and sheltered in relatively small hosts. Breeding spaces were saturated with breeders and were available for nonbreeders only after disappearance of one or both members of an established breeding pair. Home ranges of nonbreeders of similar body size were spaced out, due perhaps to competition for breeding spaces. It is suggested that large nonbreeders refrained from becoming females to keep their gonads ambosexual, so that they could replace either sex in a territory as breeding space became available, or, in pairing with other nonbreeders, the larger fish of the pair could take the female role.     

Mating behavior and sex change of the anemonefish,Amphiprion clarkii,in the temperate waters of southern Japan

Synopsis In the main habitat of the anemonefishes Amphiprion, their movements between host sea anemones are generally restricted because of the low population density of hosts and high predation pressures. On the contrary, movements of A. clarkii between hosts were usual in the present study area (temperate waters of southern Japan), where host anemones are abundant. The general social unit of the anemonefishes is an isolated group consisting of a monogamous breeding pair and a varying number of nonbreeders. In the present study area, however, monogamous pairs established territories almost contiguous to others and nonbreeders had home ranges on the outskirts of the pairs' territories. The high host population density allows A. clarkii to move between hosts for searching for mates and acquiring additional mates. Most pair bonds lasted for more than 6 months, but 13% of the pairs separated because of migration of a mate to another territory. Bigamy occasionally originated from a penetration into a territory of a breeder by a mated neighbor of the opposite sex after the former's mate loss. Among 18 males who had lost their mates, only 3 changed sex and others re-paired with immigrant females, migrated to unmated females' territories or invaded pairs' territories. In the present study area, sex change to female is not the best way for an unmated male to increase his future reproductive success because of a loss of time spent on sex change and an opportunity to re-pair with new mates larger than himself, but is adaptively maintained as the best of a bad situation for the unmated male.     

Patterns of pair formation in protandrous anemonefishes,Amphiprion clarkii,A. frenatus andA. perideraion,on coral reefs of Okinawa,Japan

Synopsis At Sesoko Island, stocks of anemonefishes were observed for 2 years in a 350 m × 150 m area where host sea anemones were sparse. About 40% of the pairs separated, mostly due to typhoon attacks or displacement. Widowed mates remained and acquired new mates on the same sea anemone, except for one case. New mates were immigrant adults inAmphiprion clarkii and A.frenatus, but resident juveniles inA. perideraion. A. clarkii andA. frenatus moved between sea anemones and sometimes displaced smaller consexuals, butA. perideraion rarely moved. The difference in mobility among the 3 species was related to the mean difference in standard length between the largest juveniles in breeding groups and the minimum size of breeding males in each species, and that between breeding males and the minimum size of breeding females. InA. perideraion both of these differences were small, so pairs could be formed quickly by residents after mate loss. On the contrary, inA. clarkii andA. frenatus, either or both differences were large and it would take a longer time to form a pair after mate loss. Differences in mobility affected the patterns of pair formation, and then the size composition of members in breeding groups.     

High mobility of the protandrous anemonefish Amphiprion frenatus: nonrandom pair formation in limited shelter space

Protandry in anemonefishes has been attributed to random pair formation, which results from their limited mobility after random recruitment to isolated host anemones. The recruitment and movement of the anemonefish Amphiprion frenatus were investigated in relation to its group structure and the spatial distribution of its host anemone on a coral reef, where it inhabits isolated single hosts and interhost movement is rare. A juvenile tended to be recruited to a host from which a former resident(s) had disappeared, indicating that larval recruitment is not random. After mass bleaching of corals during which many hosts died, a quarter of adults moved between hosts on average 42m in 3 weeks, indicating that their mobility is potentially high. The few migrations under normal conditions were probably due to the low benefit of movements. Even after the movements, a female was much larger than her mate in a host, and a large female tended to pair with a large male. Although body size of females was positively correlated with their host size, that of males was not. It is suggested that the size of a single host does not allow the coexistence of two or more large fish, and the size composition of each pair is affected by the host size. Protandry in the monogamous fish may be attributed to the nonrandom pair formation in the limited space of a host.     

Effects of anemonefish on giant sea anemones: expansion behavior,growth, and survival

The symbiosis between giant sea anemones and anemonefish on coral reefs is well known, but little information exists on impacts of this interaction on the sea anemone host. On a coral reef at Eilat, northern Red Sea, individuals of the sea anemone Entacmaea quadricolor that possessed endemic anemonefish Amphiprion bicinctus expanded their tentacles significantly more frequently than did those lacking anemonefish. When anemonefish were experimentally removed, sea anemone hosts contracted partially. Within 1–4 h in most cases, individuals of the butterflyfish Chaetodon fasciatus arrived and attacked the sea anemones, causing them to contract completely into reef holes. Upon the experimental return of anemonefish, the anemone hosts re-expanded. The long-term growth rate and survival of the sea anemones depended on the size and number of their anemonefish. Over several years, sea anemones possessing small or no fish exhibited negative growth (shrinkage) and eventually disappeared, while those with at least one large fish survived and grew. We conclude that host sea anemones sense the presence of symbiotic anemonefish via chemical and/or mechanical cues, and react by altering their expansion behavior. Host sea anemones that lack anemonefish large enough to defend them against predation may remain contracted in reef holes, unable to feed or expose their tentacles for photosynthesis, resulting in their shrinkage and eventual death.     

Anemonefish depletion reduces survival,growth, reproduction and fishery productivity of mutualistic anemone–anemonefish colonies

Intimate knowledge of both partners in a mutualism is necessary to understand the ecology and evolution of each partner, and to manage human impacts that asymmetrically affect one of the partners. Although anemonefishes and their host anemones are iconic mutualists and widely sought by ornamental fisheries, the degree to which anemones depend on anemonefishes, and thus the colony-level effects of collecting anemonefishes, is not well understood. We tracked the size and abundance of anemone Entacmaea quadricolor and anemonefish Amphiprion melanopus colonies for 3 yr after none, some, or all of the resident anemonefish were experimentally removed. Total and partial removal of anemonefish had rapid and sustained negative effects on growth, reproduction and survival of anemones, as well as cascading effects on recruitment and productivity of anemonefish in the remaining colony. As predicted, total removal of anemonefish caused acute declines in size and abundance of anemones, although most anemone colonies (76 %) slowly resumed growth and reproduction after the arrival of anemonefish recruits, which subsequently grew and defended the hosts. Partial removal of anemonefish had similar but typically less severe effects on anemones. Remarkably, the colony-level effects on anemones and anemonefish were proportional to the size and number of anemonefish that were experimentally removed. In particular, anemone survival and anemonefish productivity were highest when one or more adult anemonefish remained in the colony, suggesting that adult fish not only enhanced the protection of anemones, but also increased the recruitment and/or survival of conspecifics. We conclude that the relationship between E. quadricolor and A. melanopus is not only obligate, but also demographically rigid and easily perturbed by anemonefish fisheries. Clearly, these two species must be managed together as a unit and with utmost precaution. To this end, we propose several tangible management actions that will help to minimize fishing effects.     

Socially controlled growth and size-dependent sex change in the anemonefishAmphiprion frenatus in Okinawa,Japan

Social structure, growth and reproductive experience of a protandrous anemonefish,Amphiprion frenatus, were investigated on a coral reef in Okinawa, Japan. In a 67 m × 334 m study area, 24 breeding groups, 10 nonbreeding groups and 2 groups of unknown breeding experience were found around isolated sea anemones. One group usually consisted of 2 or 3 fish. The female in a breeding group was larger than not only her mate but also all males in other breeding groups. The body size and gonadal state of the largest individual in a nonbreeding group were intermediate between the female and male in a breeding group. In both breeding and nonbreeding groups, the largest fish retarded growth of the second largest. After the disappearance or removal of females, their mates took more than 1.5 years to attain the minimum functional female size (about 75 mm in standard length). This delayed sex change can be attributed to strong growth suppression by the female.     

Forcible eviction and prevention of recruitment in the clown anemonefish

How big an animal group will be depends on how the group's sizeis regulated and on the costs and benefits of living in thegroup. To determine which individuals regulate group size ofthe clown anemonefish, Amphiprion percula, I investigated thestrategies involved in the formation, maintenance, and dissolutionof its groups. Groups composed of a single breeding pair andof zero to four nonbreeding subordinates occupied individualsea anemones (Heteractis magnifica), which provided the fishwith oviposition sites and protection from predators. Groupsize increased linearly with anemone size. I used the residualsof this relationship as a measure of the degree of saturationof each anemone. Residents evicted low-rank subordinates andprevented the recruitment of additional subordinates at anemoneswith a high degree of saturation, but not at anemones with alow degree of saturation. These strategies indicate that residentscontrol group membership of their subordinates, and suggestthat residents might incur costs from the presence of subordinatesin more saturated anemones. In general, whenever residents cancontrol group membership, the prevention of recruitment andthe eviction of subordinates will set an upper limit on groupsize.     

Factors affecting the local abundance of two anemonefishes (Amphiprion frenatus and A. perideraion) around a semi-closed bay in Puerto Galera,the Philippines

Many marine organisms disperse or migrate among habitats, which affects their abundance patterns at individual local habitats. To clarify the factors affecting the distribution patterns of two anemonefishes (Amphiprion frenatus and A. perideraion), we measured the habitat patch size (anemone size), patch isolation (mean distance from other anemones), presence/absence of other anemonefish species, depth, and abundance of the two anemonefishes at each anemone around a semi-closed bay (up to 3.7 km) in Puerto Galera, the Philippines. We assumed that local abundance increases with habitat size and decreases with patch isolation because of greater resource availability and reduced rates of recruitment from other patches. Local abundance of A. frenatus was related to habitat size and the presence of other anemonefish species, whereas that of A. perideraion was affected by the presence of other anemonefish species and water depth. Interspecific competition and/or niche differentiation of habitat can explain the negative relationship between the local abundance of the target species and other anemonefish. Patch isolation was not significant for both species probably because the dispersal rate was not directly proportional to the geographic distance between patches at our study site.     

Determinants of reproductive success in dominant pairs of clownfish: a boosted regression tree analysis

1. Central questions of behavioural and evolutionary ecology are what factors influence the reproductive success of dominant breeders and subordinate nonbreeders within animal societies? A complete understanding of any society requires that these questions be answered for all individuals. 2. The clown anemonefish, Amphiprion percula, forms simple societies that live in close association with sea anemones, Heteractis magnifica. Here, we use data from a well-studied population of A. percula to determine the major predictors of reproductive success of dominant pairs in this species. 3. We analyse the effect of multiple predictors on four components of reproductive success, using a relatively new technique from the field of statistical learning: boosted regression trees (BRTs). BRTs have the potential to model complex relationships in ways that give powerful insight. 4. We show that the reproductive success of dominant pairs is unrelated to the presence, number or phenotype of nonbreeders. This is consistent with the observation that nonbreeders do not help or hinder breeders in any way, confirming and extending the results of a previous study. 5. Primarily, reproductive success is negatively related to male growth and positively related to breeding experience. It is likely that these effects are interrelated because males that grow a lot have little breeding experience. These effects are indicative of a trade-off between male growth and parental investment. 6. Secondarily, reproductive success is positively related to female growth and size. In this population, female size is positively related to group size and anemone size, also. These positive correlations among traits likely are caused by variation in site quality and are suggestive of a silver-spoon effect. 7. Noteworthily, whereas reproductive success is positively related to female size, it is unrelated to male size. This observation provides support for the size advantage hypothesis for sex change: both individuals maximize their reproductive success when the larger individual adopts the female tactic. 8. This study provides the most complete picture to date of the factors that predict the reproductive success of dominant pairs of clown anemonefish and illustrates the utility of BRTs for analysis of complex behavioural and evolutionary ecology data.     

Territorial Behavior and Ecology of the Anemonefish Amphiprion melanopus on Guam1)

Field observations and experiments on the Indo-Pacific anemonefish Amphiprion melanopus at Guam show that it colonizes the aggregating sea anemone Physobrachia douglasi extensively and nearly exclusively. There are an average of four fish per colony. The total standard length of anemonefish at each aggregation is highly correlated with the total area covered by resident anemones, suggesting both a carrying capacity and some form of social control of growth of anemonefish²). Inter-colony migration of juveniles, as well as larval recruitment, may contribute to maintenance of the optimum colony size. Adults do not migrate. Both juveniles and adults defend territory. Juvenile territories are mutually exclusive areas within the confines of the anemone aggregation, while adult territories are considerably larger than the area covered by the anemone aggregation. Though territories of mated ♀♀ and ♂♂ overlap completely, female emphasis is peripheral relative to the ♂. Adult conspecific intruders are attacked more heavily and at greater distances than are juveniles. Intraspecific territoriality in juveniles probably reflects the limited availability of critical habitat. In adults it may function to protect the pair bond. Interspecific aggression is less intense and appears to protect both the spawn and host anemones from various predators.     

Cophylogenetic analysis of the relationship between anemonefish Amphiprion (Perciformes: Pomacentridae) and their symbiotic host anemones (Anthozoa: Actiniaria)

ABSTRACT

The association between anemonefish and anemone is a classical example of mutualism in coral reefs. Although mutualism is probably the key innovation that triggered the adaptive radiation of anemonefish into a wide range of habitats, the coevolutionary history between the groups has not been thoroughly tested in a phylogenetic framework. We examined the evolutionary history of the association via distance-based (Parafit and PACo) and event-based methods (Core-PA, Jane). Mitochondrial DNA sequences (COI mtDNA, Cytb, 16S rDNA and 12S rDNA) were used to reconstruct the phylogenies of tropical Amphiprion species and their host anemones by using maximum likelihood with best-fit models selected. Neither distance-based analyses nor event-based analyses revealed global significant congruence between the phylogenies of the hosts and the symbionts, and thus no evidence for coevolution between anemone-anemonefish. However, at the individual pair level, the fish showed some dependence on anemone hosts. Even though living in close association and benefiting from each other, the change of genetic composition of one species (anemonefish) does not always evolve in response to changes in the other (anemones). These findings expand our understanding of the pattern and the role of evolutionary events to allow a better prediction of the future of the anemonefish-anemone relationship.     

Benefits to host sea anemones from ammonia contributions of resident anemonefish

Large ectosymbionts (especially fishes and crustaceans) may have major impacts on the physiology of host cnidarians (sea anemones and corals), but these effects have not been well quantified. Here we describe impacts on giant sea anemone hosts (Entacmaea quadricolor) and their endosymbiotic zooxanthellae (Symbiodinium spp.) from the excretion products of anemonefish guests (Amphiprion bicinctus) under laboratory conditions. Starved host anemones were maintained with anemonefish, ammonia supplements (= NH₃ gas and NH₄⁺ ion), or neither for 2 mo. In the presence of external ammonia supplements or resident anemonefish, the zooxanthellae within host anemones increased in abundance (173% and 139% respectively), and provided the hosts with energy that minimized host body size loss. In contrast, anemones cultured with neither ammonia nor anemonefish harbored significantly lower abundances of zooxanthellae (84% of initial abundance) and decreased > 60% in body size. Although they maintained higher zooxanthella abundances, anemones cultured with either ammonia supplements or resident anemonefish exhibited significantly lower ammonia uptake rates (0.065 ± 0.005 µmol g^- 1 h^- 1, and 0.052 ± 0.018 µmol g^- 1 h^- 1 respectively) than did control anemones (0.119 ± 0.009 µmol g^- 1 h^- 1), indicating that their zooxanthellae were more nitrogen sufficient. We conclude that, in this multi-level mutualism, ammonia supplements provide essentially the same level of physiological contribution to host anemones and zooxanthellae as do live resident fish. This nutrient supplement reduces the dependence of the zooxanthellae on host feeding, and allows them to provide abundant photosynthetically-produced energy to the host.     

Assemblage and interaction structure of the anemonefish-anemone mutualism across the Manado region of Sulawesi,Indonesia

The Manado area (Indonesia–North Sulawesi), a marine high diversity hot-spot, hosts 7 species of anemonefish (family Pomacentridae, subfamily Amphiprioniae) living in symbiosis with 9 species of sea anemones (family Stichodactylidae and Actiniidae). This high biological diversity −27% and 80%, respectively, of the total known diversity of anemonefish and sea anemones—allows us to test different hypotheses focused on the obligate mutualism between anemonefish and sea anemones. In the Manado area, species richness of anemones and anemonefish across several sites was not correlated, but all anemones contained at least one fish individual, and there was a strong positive correlation between the numbers of individual anemonefish and anemones. As expected, each fish species had a preferred anemone host; also a partial niche overlap (Pianka’s Index) was often detected. The analysis of unique species composition suggests that competition is not an important factor determining the presence or absence of particular combinations of either anemonefish or host anemones (no evidence of competitive exclusion). The NODF algorithm showed that, at both a regional and local scale, the interaction between anemonefish and host anemones is not significantly nested, as a result of a combination of local conditions with competition, forcing species that regionally are more generalist to become more specialist.     

Vertical and horizontal distribution patterns of the giant sea anemone Heteractis crispa with symbiotic anemonefish on a fringing coral reef

The distribution patterns of the leathery sea anemone, Heteractis crispa, which contains an algal endosymbiont (zooxanthellae) and anemonefish, were investigated in relation to size distribution on a shallow fringing reef (3.2 ha, 0–4 m depth) in Okinawa, Japan. Individual growth and movements were also examined. Large individuals (>1,000 cm²) inhabited reef edges up to a depth of 4 m, while small anemone (<500 cm²) inhabited shallow reefs including inner reef flats. Individuals rarely moved, and their sizes were significantly correlated with their water depths. Growth of small anemones was negatively correlated with their distance from the reef edge, suggesting that reef edges provide more prey and lower levels of physiological stress. This study suggested that deep reef edges are suitable habitats for H. crispa. Large anemones were inhabited by large Amphiprion perideraion or large Amphiprion clarkii, both of which are effective defenders against anemone predators. Anemones that settle in deep reef edges may enjoy a higher survival rate and attain a large size because of their symbiotic relationship with anemonefish. However, early settlers do not harbor anemonefish. Their mortality rate would be higher in the deep edges than in shallow edges, the complicated topography of which provides refuge.     

Growth, reproduction and survival of a tropical sea anemone (Actiniaria): benefits of hosting anemonefish

The ecological performance of the sea anemone Heteractis magnifica was examined during a 36-month experiment with respect to season and the presence and numbers of a mutualist (orange-fin anemonefish Amphiprion chrysopterus). Anemones primarily grew during the autumn, with most asexual reproduction occurring in winter; mortality was not strongly seasonal. Individual growth rates did not differ between anemones harboring one or two anemonefish, but these rates were three times faster than for anemones lacking Amphiprion. Anemones with two anemonefish had the highest fission rate, whereas those without anemonefish had the lowest. By contrast, anemones that were not defended by anemonefish suffered higher-than-expected mortality. As a consequence, anemones with two Amphiprion had the greatest net increase in surface area, and those lacking anemonefish had a negligible gain that was statistically indistinguishable from zero after three years. Anemonefish not only enhanced anemone survivorship as previously believed, they also fostered faster growth and more frequent asexual reproduction.     

Ammonia flux,physiological parameters,and <Emphasis Type="Italic">Symbiodinium</Emphasis> diversity in the anemonefish symbiosis on Red Sea coral reefs

Despite the ecological importance of anemonefish symbioses, little is known about how nutritional contributions from anemonefish interact with sea anemone physiology and Symbiodinium (endosymbiotic dinoflagellate) genetic identity under field conditions. On Red Sea coral reefs, we measured variation in ammonia concentrations near anemones, excretion rates of anemonefish, physiological parameters of anemones and Symbiodinium, and genetic identity of Symbiodinium within anemones. Ammonia concentrations among anemone tentacles were up to 49% above background levels, and anemonefish excreted ammonia significantly more rapidly after diurnal feeding than they did after nocturnal rest, similar to their excretion patterns under laboratory conditions. Levels of 4 physiological parameters (anemone protein content, and Symbiodinium abundance, chlorophyll a concentration, and division rate) were similar to those known for laboratory-cultured anemones, and in the field did not depend on the number of anemonefish per anemone or depth below sea surface. Symbiodinium abundance varied significantly with irradiance in the shaded reef microhabitats occupied by anemones. Most anemones at all depths harbored a novel Symbiodinium 18S rDNA variant within internal transcribed spacer region 2 (ITS2) type C1, while the rest hosted known ITS2 type C1. Association with Symbiodinium Clade C is consistent with the symbiotic pattern of these anemones on other Indo-Pacific reefs, but the C1 variant of Symbiodinium identified here has not been described previously. We conclude that in the field, anemonefish excrete ammonia at rapid rates that correlate with elevated concentrations among host anemone tentacles. Limited natural variation in anemonefish abundance may contribute to consistently high levels of physiological parameters in both anemones and Symbiodinium, in contrast to laboratory manipulations where removal of fish causes anemones to shrink and Symbiodinium to become less abundant.     

Group formation and anemone use in captively reared anemonefish (<Emphasis Type="Italic">Amphiprion frenatus</Emphasis>)

We studied group formation and anemone use in captively reared anemonefish Amphiprion frenatus. Four small fish (23–32 mm total length) were simultaneously released into a central area of the aquaria (ca. 160 l) that was divided in half with an anemone placed on each half, with the opportunity to settle on either side. Four-day trials were conducted for analyzing settlement patterns. Because of the monogamous mating system, we predicted that small fish should settle in pairs. However, all four small fish settled on the same side in 7 of 14 replicates, which was a higher than expected value under random settlement.     

Searching for a Toxic Key to Unlock the Mystery of Anemonefish and Anemone Symbiosis

Twenty-six species of anemonefish of the genera Amphiprion and monospecific Premnas, use only 10 species of anemones as hosts in the wild (Families: Actiniidae, Stichodactylidae and Thalassianthidae). Of these 10 anemone species some are used by multiple species of anemonefish while others have only a single anemonefish symbiont. Past studies have explored the different patterns of usage between anemonefish species and anemone species; however the evolution of this relationship remains unknown and has been little studied over the past decade. Here we reopen the case, comparing the toxicity of crude venoms obtained from anemones that host anemonefish as a way to investigate why some anemone species are used as a host more than others. Specifically, for each anemone species we investigated acute toxicity using Artemia francisca (LC₅₀), haemolytic toxicity using ovine erythrocytes (EC₅₀) and neurotoxicity using shore crabs (Ozius truncatus). We found that haemolytic and neurotoxic activity varied among host anemone species. Generally anemone species that displayed greater haemolytic activity also displayed high neurotoxic activity and tend to be more toxic on average as indicated by acute lethality analysis. An overall venom toxicity ranking for each anemone species was compared with the number of anemonefish species that are known to associate with each anemone species in the wild. Interestingly, anemones with intermediate toxicity had the highest number of anemonefish associates, whereas anemones with either very low or very high toxicity had the fewest anemonefish associates. These data demonstrate that variation in toxicity among host anemone species may be important in the establishment and maintenance of anemonefish anemone symbiosis.     

Small Subordinate Male Advantage in the Zebrafish

Dominance is an important determinant of reproductive success in many species, and size is usually an indicator of dominance status, with larger, dominant individuals physically and physiologically preventing smaller subordinates from mating. However, small size may be advantageous in some mating contexts because enhanced manoeuvrability enables males to get closer to females during mating. Here, we determined the paternity success and testes size of dominant and subordinate male zebrafish (Danio rerio), in pairs that controlled for social status. There was no statistical difference in both body size and testes size between dominant and subordinate males. Dominant males sired significantly more offspring than subordinates, but when subordinates were small, they had a greater share of the paternity than larger subordinates. Small male advantage may be one mechanism by which variation in body size is maintained in this species.