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.     

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.     

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.     

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.     

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.     

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.     

Cophylogenetic relationships between Dactylogyrus (Monogenea) ectoparasites and endemic cyprinoids of the north-eastern European peri-Mediterranean region

The study of host–parasite coevolution is one of the cornerstones of evolutionary biology. The majority of fish ectoparasites belonging to the genus Dactylogyrus (Monogenea) exhibit a high degree of host specificity. Therefore, it is expected that their evolutionary history is primarily linked with the evolutionary history of their cyprinoid fish hosts and the historical formation of the landmasses. In the present study, we used a cophylogenetic approach to investigate coevolutionary relationships between endemic Cyprinoidea (Cyprinidae and Leuciscidae) from selected regions in southern Europe and their respective Dactylogyrus species. A total of 49 Dactylogyrus species including endemic and non-endemic species were collected from 62 endemic cyprinoid species in the Balkan and Apennine Peninsulas. However, 21 morphologically identified Dactylogyrus species exhibited different genetic variants (ranging from 2 to 28 variants per species) and some of them were recognized as cryptic species on the basis of phylogenetic reconstruction. Phylogenetic analyses revealed several lineages of endemic and non-endemic Dactylogyrus species reflecting some morphological similarities or host affinities. Using distance-based and event-based cophylogenetic methods, we found a significant coevolutionary signal between the phylogenies of parasites and their hosts. In particular, statistically significant links were revealed between Dactylogyrus species of Barbini (Cyprinidae) and their hosts belonging to the genera Aulopyge, Barbus and Luciobarbus. Additionally, a strong coevolutionary link was found between the generalist parasites D. alatus, D. sphyrna, D. vistulae, and their hosts, and between Dactylogyrus species of Pachychilon (Leuciscidae) and their hosts. Cophylogenetic analyses suggest that host switching played an important role in the evolutionary history of Dactylogyrus parasitizing endemic cyprinoids in southern Europe. We propose that the high diversification of phylogenetically related cyprinoid species in the Mediterranean area is a process facilitating the host switching of specific parasites among highly diverse congeneric cyprinoids.     

Evolution of obligate pollination mutualism in the tribe Phyllantheae (Phyllanthaceae)

The landmark discovery of obligate pollination mutualism between Glochidion plants and Epicephala moths has sparked increased interest in the pollination systems of Phyllantheae plants. In this paper I review current information on the natural history and evolutionary history of obligate pollination mutualism in Phyllantheae. Currently, an estimated >500 species are mutualistic with Epicephala moths that actively pollinate flowers and whose progeny feed on the resulting seeds. The Phyllantheae also includes species that are not mutualistic with Epicephala moths and are instead pollinated by bees and/or flies or ants. Phylogenetic analyses indicate that the mutualism evolved independently five times within Phyllantheae, whereas active pollination behavior, a key innovation in this mutualism, evolved once in Epicephala . Reversal of mutualism has occurred at least once in both partner lineages, involving a Breynia species that evolved an alternative pollination system and a derived clade of Epicephala that colonized ant-pollinated Phyllantheae hosts and thereby lost the pollinating habit. The plant–moth association is highly species specific, although a strict one-to-one assumption is not perfectly met. A comparison of plant and moth phylogenies suggests signs of parallel speciation, but partner switches have occurred repeatedly at a range of taxonomic levels. Overall, the remarkable species diversity and multiple originations of the mutualism provide excellent opportunities to address many important questions on mutualism and the coevolutionary process. Although research on the biology of the mutualism is still in its infancy, the Phyllantheae– Epicephala association holds promise as a new model system in ecology and evolutionary biology.     

Spacing pattern and body size composition of the protandrous anemonefish Amphiprion frenatus inhabiting colonial host anemones

The protandrous anemonefish Amphiprion frenatus often forms a group consisting of a large female, a small male, and a smaller nonbreeder at an isolated single host anemone, where home ranges of subordinates were covered with the female's home range. Within the group, the dominant individuals suppress the growth of subordinates, resulting in large size differences. The spacing pattern and body size composition of A. frenatus on colonial hosts were investigated in Ishigaki Island, Okinawa, Japan. Six breeding pairs and 14 nonbreeders inhabited a colony of 157 anemones. Each pair maintained a territory in which pair members used different hosts. Nonbreeders had unstable home ranges on the outskirts of or in the pairs' territories. Body size differences between males and females in pairs and between males and nonbreeders were small. The small size differences in the colony of hosts are caused by reduced suppression of growth of subordinates by the dominant individuals. The total area of host anemones largely affects spacing pattern and social suppression of the anemonefish.     

Population impacts of collecting sea anemones and anemonefish for the marine aquarium trade in the Philippines

Tropical marine ornamentals comprise an increasingly important fishery worldwide. Although the potential for overexploitation of marine ornamentals is great, few studies have addressed the population-level impacts of ornamental exploitation and few ornamental fisheries are managed. Analysis of catch records obtained from collectors over a four-month period in the vicinity of Cebu, Philippines, showed that anemonefish and anemones comprised close to 60% of the total catch. Underwater visual census surveys revealed that both anemone and anemonefish densities were significantly lower in exploited areas than in protected areas. The low density of anemones on exploited reefs accounted for over 80% of the reduced density of anemonefish at those sites. There were similar numbers of anemonefish per unit area of anemone in protected and exploited sites; however, biomass of anemonefish per unit area of anemone was lower in exploited areas. Reduction of anemone removals is recommended to support the sustainable harvest of anemonefish from this region.     

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.     

Mutualism can mediate competition and promote coexistence

Mutualistic interactions are not believed to promote coexistence of competitors because mutualisms produce positive feedbacks on abundances whereas coexistence requires negative feedbacks. Here we show that a mutualism between an anemonefish (Amphiprion) and its sea anemone host mediates the effect of asymmetrical competition for space between the anemonefish and another damselfish (Dascyllus) in a manner that fosters their coexistence. Amphiprion stimulates increases in host area, the shared resource, but social interactions cap the number of anemonefish to two adults per host. Space generated by the mutualism becomes differentially available to Dascyllus because the effectiveness of an anemonefish in excluding its competitor declines with increases in the area it defends. This alters Amphiprion's ratio of per capita intra‐ to interspecific effects and thus facilitates coexistence of the fishes. This mechanism may be prevalent in nature, adding another major pathway by which mutualism can enhance diversity.     

Temperate and tropical algal-sea anemone symbioses

Abstract. In this review, we seek to develop new insights about the nature of algal‐sea anemone symbioses by comparing such associations in temperate and tropical seas. Temperate seas undergo pronounced seasonal cycles in irradiance, temperature, and nutrients, while high irradiance, high temperature, and low nutrients are seasonally far less variable in tropical seas. We compare the nature of symbiosis between sea anemones (= actinians) and zooxanthellae (Symbiodinium spp.) in both regions to test tropical paradigms against temperate examples and to identify directions for future research. Although fewer anemone species are symbiotic in temperate regions, they are locally dominant and ecologically important members of the benthic community compared to the tropics. Zooxanthella densities tend to be lower in temperate anemones, but data are limited to a few species in both temperate and tropical seas. Zooxanthella densities are far more stable over time in temperate anemones than in tropical anemones, suggesting that temperate symbioses are more resistant to fluctuations in environmental parameters such as irradiance and temperature. Light‐saturated photosynthetic rates of temperate and tropical zooxanthellae are similar, but temperate anemone hosts receive severely reduced carbon supplies from zooxanthellae during winter months when light is reduced. Symbiont transmission modes and specificity do not show any trends among anemones in tropical vs. temperate seas. Our review indicates the need for the following: (1) Investigations of other temperate and tropical symbiotic anemone species to assess the generality of trends seen in a few “model’ anemones. (2) Attention to the field ecology of temperate and tropical algal‐anemone symbioses, for example, how symbioses function under seasonally variable environmental factors and how zooxanthellae persist at high densities in darkness and winter. The greater stability of zooxanthella populations in temperate hosts may be useful to understanding tropical symbioses in which bleaching (loss of zooxanthellae) is of major concern. (3) Study of the evolutionary history of symbiosis in both temperate and tropical seas. Continued exploration of the phylogenetic relationships between host anemones and zooxanthella strains may show how and why zooxanthellae differ in anemone hosts in both environments.     

Mobility of Stichodactyla Gigantea Sea Anemones and Implications for Resident False Clown Anemonefish, Amphiprion Ocellaris

For reef fishes that do not move between habitat patches following settlement, habitat selection is expected during settlement. Although false clown anemonefish, Amphiprion ocellaris, are sedentary following settlement, they are not especially discriminating during settlement, and are commonly found occupying anemones at which no apparent nest site exists. In this study I report on mobility of Stichodactyla gigantea sea anemones, including anemones with resident false clowns. I argue that anemone mobility can help explain why settling false clowns are not more discriminating: although the per annum probability of an anemone moving is low, the probability of that anemone moving over the course of a resident's life is considerably higher. Therefore, an anemone's current microhabitat may not be a good predictor of its microhabitat and suitability as a host in the future.     

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.     

Detrimental effects of host anemone bleaching on anemonefish populations

Coral bleaching and related reef degradation have caused significant declines in the abundance of reef-associated fishes. Most attention on the effects of bleaching has focused on corals, but bleaching is also prevalent in other cnidarians, including sea anemones. The consequences of anemone bleaching are unknown, and the demographic effects of bleaching on associated fish recruitment, survival, and reproduction are poorly understood. We examined the effect of habitat degradation including host anemone bleaching on fish abundance, egg production, and recruitment of the panda anemonefish (Amphiprion polymnus) near Port Moresby, Papua New Guinea. Following a high-temperature anomaly in shallow waters of the region, most shallow anemones to a depth of 6 m (approximately 35% of all the anemones in this area) were severely bleached. Anemone mortality was low but bleached anemones underwent a ~34% reduction in body size. Total numbers of A. polymnus were not affected by bleaching and reduction in shelter area. While egg production of females living in bleached anemones was reduced by ~38% in 2009 compared to 2008, egg production of females on unbleached anemones did not differ significantly between years. Total recruitment in 2009 was much lower than in 2008. However, we found no evidence of recruiting larvae avoiding bleached anemones at settlement suggesting that other factors or different chemical cues were more important in determining recruitment than habitat quality. These results provide the first field evidence of detrimental effects of climate-induced bleaching and habitat degradation on reproduction and recruitment of anemonefish.     

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.     

Flexibility of nutritional strategies within a mutualism: food availability affects algal symbiont productivity in two congeneric sea anemone species

Mutualistic symbioses are common, especially in nutrient-poor environments where an association between hosts and symbionts can allow the symbiotic partners to persist and collectively out-compete non-symbiotic species. Usually these mutualisms are built on an intimate transfer of energy and nutrients (e.g. carbon and nitrogen) between host and symbiont. However, resource availability is not consistent, and the benefit of the symbiotic association can depend on the availability of resources to mutualists. We manipulated the diets of two temperate sea anemone species in the genus Anthopleura in the field and recorded the responses of sea anemones and algal symbionts in the family Symbiodiniaceae to our treatments. Algal symbiont density, symbiont volume and photosynthetic efficiency of symbionts responded to changes in sea anemone diet, but the responses depended on the species of sea anemone. We suggest that temperate sea anemones and their symbionts can respond to changes in anemone diet, modifying the balance between heterotrophy and autotrophy in the symbiosis. Our data support the hypothesis that symbionts are upregulated or downregulated based on food availability, allowing for a flexible nutritional strategy based on external resources.     

Comparing host and parasite phylogenies: gyrodactylus flatworms jumping from goby to goby

The combination of exceptionally high species diversity, high host specificity, and a complex reproduction system raises many questions about the underlying mechanisms triggering speciation in the flatworm genus Gyrodactylus. The coevolutionary history with their goby hosts was investigated using both topology- and distance-based approaches; phylogenies were constructed of the V4 region of the 18S rRNA and the complete ITS rDNA region for the parasites, and 12S and 16S mtDNA fragments for the hosts. The overall fit between both trees was significant according to the topology-based programs (TreeMap 1.0, 2.0 beta and TreeFitter), but not according to the timed analysis in TreeMap 2.0 beta and the distance-based method (ParaFit). An absolute timing of speciation events in host and parasite ruled out the possibility of synchronous speciation for the gill parasites, favouring the distance-based result. Based on this information together with the biological background of host and parasite, the following TreeMap solution was selected. The group of gill parasites evolved from a host switch from G. arcuatus, parasitizing the three-spined stickleback onto the gobies, followed by several host-switching events among the respective goby hosts. The timing of these events is estimated to date back to the Late Pleistocene, suggesting a role for refugia-mediated mixing of parasite species. In contrast, it is suggested that co-speciation in the fin-parasites resulted in several host-associated species complexes. This illustrates that phylogenetically conserved host-switching mimics the phylogenetic signature of co-speciation, confounding topology-based programs.