Review Geographic differences in host specialization between the symbiotic water mites Unionicola formosa and Unionicola foili (Acari: Unionicolidae)

The host specificity and population genetic structure of the symbiotic water mites Unionicola foili from the host mussel Utterbackia imbecillis and Unionicola formosa from the mussels Pyganodon cataracta, Pyganodon grandis and Anodonta suborbiculata were examined over a broad geographical scale in order to determine the extent to which specialization by these water mites is structured geographically. The behavioural responses of U. foili and U. formosa were highly host-species specific, with adults of both species exhibiting negative phototaxis in the presence of a chemical factor from the species of mussel with which the mites had been associated in the field. The photobehaviour of these water mites in the presence of water from a non-host mussel varied depending on the species in question. Although U. foili from U. imbecillis exhibited negative phototaxis in water modified by A. suborbiculata, mites from this latter host did not exhibit a directional response in U. imbecillis water. Unionicola foili and U. formosa from A. suborbiculata were positively phototactic when they were exposed to water modified by either species of Pyganodon. The photoresponse of U. formosa from P. cataracta and P. grandis was positive in the presence of water modified by U. imbecillis and A. suborbiculta. However, these mussel-mites showed no directional response in water modified by their alternate species of Pyganodon. Unionicola foili and the host-associated populations of U. formosa were examined for allozyme variation at eight loci to determine the pattern and degree of genetic variation. There was a high degree of genetic differentiation when mite populations from the two species of Pyganodon were compared with U. foili or U. formosa from A. suborbiculata. These populational groupings were fixed for different alleles at two enzyme loci. The results of this study indicate that populations of U. formosa from P. cataracta and P. grandis are reproductively isolated from U. foili from U. imbecillis and from U. formosa from A. suborbiculata and contend that host specificity is an important mechanism in restricting gene flow among these populational groupings. Furthermore, this study indicates that specialization among unionicolid water mites can vary geographically, owing to differences in geographic distribution of available hosts and differences in host use.Exp Appl Acarol 22: 683697 © 1998 Kluwer Academic Publishers     

The symbiotic water mite Unionicola formosa (Acari: Unionicolidae) ingests mucus and tissue of its molluscan host

Unionicola formosa is a symbiotic water mite that passes most of its life cycle in the mantle cavity of freshwater mussels. Although mites of this genus are often referred to as parasitic, little is known about their nutritional biology. A few species reportedly pierce the gill of a host mussel and ingest tissue or hemolymph. The present study was undertaken to identify possible sources of nutrition for U. formosa. To determine if mites ingested particulate matter in the mucous strand produced by a mussel during feeding, mussels with resident mites were exposed to a suspension of fluorescent microspheres. There was no evidence that U. formosa ingested the beads. Histochemical staining did, however, indicate a mucous material present in the midgut of the mites. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic assays revealed a high molecular weight component, consistent with a mucopolysaccharide, present both in the mussel gill and the mites. Results from western blots and an immunoaffinity binding assay with antibodies against mussel gill tissue and hemolymph also indicated that mites ingested host tissue. Whereas U. formosa probably does not ingest particulate material acquired by its host's suspension feeding, it is apparent that this mite utilizes host mucus, gill tissue, or hemolymph for at least part of its nutrition.     

Random amplified polymorphic DNA analysis of kinship within host-associated populations of the symbiotic water mite Unionicola foili (Acari: Unionicolidae)

Kinship relations within populations of unionicolid water mites are not well known, owing to their complex life cycles and the fact that interactions between active and resting stages for some species are transitory. A number of species of unionicolid water mites are, however, obligate symbionts of freshwater mussels and spend most of their life cycle in association with these hosts. Among these species of mites, parents and offspring are more likely to co-occur and thus provide opportunities to address questions related to the structure of the mating system. The present study employs random amplified polymorphic DNA (RAPD) analysis to address kinship within populations of Unionicola foili living in symbiotic association with the host mussel Utterbackia imbecillis. DNA was amplified from adult mites and a representative number of eggs or larvae (n = 20-30) that were removed from mussels collected on three separate occasions (July, November, and March) over a 12-month period. Parsimony analyses of the molecular data for adults and progeny collected from mussels during July, November, and March revealed distinct groupings, that for the most part, corresponded to mites collected from each of the sampling periods. Many of the genetic markers obtained for male and female U. foili were not evident among the larvae or eggs, suggesting that adults obtained from a host mussel at the time of collection were not the parents of a majority of the progeny. However, female mites and eggs collected from mussels during March and November shared more markers than did females and progeny examined during July. Furthermore, many offspring in the July sampling period were found to have one or more parents absent from the sampled population. Overall, RAPD profiling appears to have limited usage in determining kinship within populations of U. foili, due to its recruitment patterns, and the relatively large number of adults and progeny per mussel. It may, however, prove to be a useful method for assessing genetic relatedness among unionicolid mussel-mites that have substantially lower population densities.     

Random amplified polymorphic DNA analysis of kinship within host-associated populations of the symbiotic water mite Unionicola foili (Acari: Unionicolidae)

Kinship relations within populations of unionicolid water mites are not well known, owing to their complex life cycles and the fact that interactions between active and resting stages for some species are transitory. A number of species of unionicolid water mites are, however, obligate symbionts of freshwater mussels and spend most of their life cycle in association with these hosts. Among these species of mites, parents and offspring are more likely to co-occur and thus provide opportunities to address questions related to the structure of the mating system. The present study employs random amplified polymorphic DNA (RAPD) analysis to address kinship within populations of Unionicola foili living in symbiotic association with the host mussel Utterbackia imbecillis. DNA was amplified from adult mites and a representative number of eggs or larvae (n = 20-30) that were removed from mussels collected on three separate occasions (July, November, and March) over a 12-month period. Parsimony analyses of the molecular data for adults and progeny collected from mussels during July, November, and March revealed distinct groupings, that for the most part, corresponded to mites collected from each of the sampling periods. Many of the genetic markers obtained for male and female U. foili were not evident among the larvae or eggs, suggesting that adults obtained from a host mussel at the time of collection were not the parents of a majority of the progeny. However, female mites and eggs collected from mussels during March and November shared more markers than did females and progeny examined during July. Furthermore, many offspring in the July sampling period were found to have one or more parents absent from the sampled population. Overall, RAPD profiling appears to have limited usage in determining kinship within populations of U. foili, due to its recruitment patterns, and the relatively large number of adults and progeny per mussel. It may, however, prove to be a useful method for assessing genetic relatedness among unionicolid mussel-mites that have substantially lower population densities.     

Behavioral aspects of host recognition by the symbiotic water mite Unionicola formosa (Acarina,Unionicolidae)

Summary The occurrence and specificity of host recognition behavior of adult and nymphal Unionicola formosa and the capability of adult mites to recolonize various mussel species were examined. Adult U. formosa aggregated on excised tissue from their host mussel, Anodonta imbecilis, in preference to that of two other species of mussels. Nymphs also exhibited an aggregation response to host tissue. A radioisotope (⁵¹Cr) technique was used to monitor the recolonization behavior of U. formosa. Adult female mites preferentially re-entered A. imbecilis rather than the sympatric mussel A. cataracta. The specificity of this behavior parallels the distribution of this water mite among potential bivalve hosts in the southeastern U.S. Host recognition by U. formosa may contribute to re-establishing contact with a host after accidental separation and probably helps to maintain mite-mussel symbioses. Whether or not larval U. formosa employ similar host recognition behavior while selecting a potential host has not as yet been determined.     

Host sex preferences and transmission success by the water mite Unionicola foili (Acari: Unionicolidae) parasitic on the midge Chironomus tentans (Diptera: Chironomidae)

This study examined whether ecoparasitic larval Unionicola foili exhibited a sex bias when infecting laboratory populations of the host insect Chironomus tentans and whether an association with male or female midges increased the likelihood of larval mites returning to the aquatic habitat. When laboratory populations of C. tentans were exposed to larval U. foili, there was a higher prevalence of mites among female hosts at emergence (17 of 30 males vs. 25 of 30 females infected by mites). However, there was no significant difference in the distribution or abundance of larvae among infected male (mean = 2.3 larvae per host) and female (mean = 2.6 larvae per host) midges. Larval mites parasitizing both male and female chironomids were more likely to return to water than could be expected by chance. Mite larvae infesting female C. tentans were more likely to return to water when female hosts deposited egg masses in water, suggesting that oviposition plays an important role in cueing larvae parasitizing female midges to detach. The mechanism responsible for increasing the likelihood that mites parasitizing male hosts return to water remains unclear. Future studies will address the possibility of parasite-mediated changes in host behavior.     

Competition between mobile species using patchy resources: an example from a freshwater,symbiotic assemblage

Summary Three species of freshwater mites that are symbiotic with mussels in St Mark's River, north Florida, have consistently high rates of colonization to and occupy high proportions of mussels. The mites Unionicola poundsi and U. serrata are territorial and have limited numbers/host, whereas the non-territorial U. abnormipes has highly variable numbers/host. U. abnormipes and U. serrata are most common in Villosa villosa and Uniomerus declivis respectively, patterns that can not be explained by host species preferences, whereas U. poundsi is equally abundant in both host species. Field experiments showed that both U. poundsi and U. serrata were limited most by intraspecific competition between adult mites, presumably for access to food and oviposition sites. Additionally, U. serrata did not remain within small hosts, most of which were V. villosa. In contrast, numbers of U. abnormipes were limited by both other mite species although the nature of the interactions differed. U. serrata may prey on U. abnormipes when they co-occur, whereas U. poundsi probably only excludes U. abnormipes from certain areas within hosts. Hence, U. abnormipes occurs mostly in V. villosa because most of these mussels do not contain U. serrata, but even so its numbers are still depressed by U. poundsi. The results were consistent with the general expectation of Holmes and Price (1986) that parasite assemblages where species have high colonization levels should be organized primarily by biotic interactions. However, specific outcomes of competition between mites were consistent with the more general model of Levins (1979) for competition between species using variable resources. Failure of other models to apply to Unionicola pinpointed at least five key biological characters that may form a better basis of comparison than taxonomic or habitat-based contrasts.     

Guild structure in water mites (Unionicola spp.) inhabiting freshwater mussels: choice,competitive exclusion and sex

Summary Unionicolid water mites inhabit freshwater unionid mussels during the nymphal and adult stages of their life-cycle. Regular sampling of mussels from two sites in St. Mark's River, Fl. established that each of four species of water mite (Unionicola abnormipes, U. fossulata, U. serrata and U. formosa) occurred mainly in one or two of the mussel species available at each site.The role of preference for particular mussel species during host location was assessed for the first three mite species by choice experiments, in which mites were offered different mussel species simultaneously. In five out of six experiments, mites entered normally unused mussels as often as they did normally used ones. Additionally, a sexual difference in choice was found for U. fossulata, with males preferring one mussel species and females showing no preference. One mussel species, (Anodonta imbecilis), normally unused but chosen by mite species during the lab. experiments, is inhabited exclusively by the fourth mite species, U. formosa, in the field. An experiment showed that U. formosa excludes other mite species aggressively from Anodonta imbecilis.The results illustrate the sometimes misleading nature of simple sampling data as an indication of host specificity or host preference in parasites. They suggest also that the population dynamics of some parasites might be more fruitfully compared to unrelated, free-living species than to other parasites.     

Host-plant specificity and specialization in eriophyoid mites and their importance for the use of eriophyoid mites as biocontrol agents of weeds

Eriophyoid mites, which are among the smallest plant feeders, are characterized by the intimate relationships they have with their hosts and the restricted range of plants upon which they can reproduce. The knowledge of their true host ranges and mechanisms causing host specificity is fundamental to understanding mite-host interactions, potential mite-host coevolution, and diversity of this group, as well as to apply effective control strategies or to use them as effective biological control agents. The aim of this paper is to review current knowledge on host specificity and specialization in eriophyoid mites, and to point out knowledge gaps and doubts. Using available data on described species and recorded hosts we showed that: (1) 80% of eriophyoids have been reported on only one host species, 95% on one host genus, and 99% on one host family; (2) Diptilomiopidae has the highest proportion of monophagous species and Phytoptidae has the fewest; (3) non-monophagous eriophyoids show the tendency to infest closely related hosts; 4) vagrant eriophyoids have a higher proportion of monophagous species than refuge-seeking and refuge-inducing species; (5) the proportions of monophagous species infesting annual and perennial hosts are similar; however, many species infesting annual hosts have wider host ranges than those infesting perennial hosts; (6) the proportions of species that are monophagous infesting evergreen and deciduous plants are similar; (7) non-monophagous eriophyoid species have wider geographic distribution than monophagous species. Field and laboratory host-specificity tests for several eriophyoid species and their importance for biological control of weeds are described. Testing the actual host range of a given eriophyoid species, searching for ecological data, genetic differentiation analysis, and recognizing factors and mechanisms that contribute to host specificity of eriophyoid mites are suggested as future directions for research.     

Host specificity in spinturnicid mites: do parasites share a long evolutionary history with their host?

Host specificity in parasites can be explained by spatial isolation from other potential hosts or by specialization and speciation of specific parasite species. The first assertion is based on allopatric speciation, the latter on differential lifetime reproductive success on different available hosts. We investigated the host specificity and cophylogenetic histories of four sympatric European bat species of the genus Myotis and their ectoparasitic wing mites of the genus Spinturnix. We sampled >40 parasite specimens from each bat species and reconstructed their phylogenetic COI trees to assess host specificity. To test for cospeciation, we compared host and parasite trees for congruencies in tree topologies. Corresponding divergence events in host and parasite trees were dated using the molecular clock approach. We found two species of wing mites to be host specific and one species to occur on two unrelated hosts. Host specificity cannot be explained by isolation of host species, because we found individual parasites on other species than their native hosts. Furthermore, we found no evidence for cospeciation, but for one host switch and one sorting event. Host‐specific wing mites were several million years younger than their hosts. Speciation of hosts did not cause speciation in their respective parasites, but we found that diversification of recent host lineages coincided with a lineage split in some parasites.     

Mantle displays of freshwater mussels elicit attacks from fish

1. Gravid females of some North American freshwater mussel species (Bivalvia: Unionidae) display highly modified mantle margins and other reproductive structures which mimic small fish, terrestrial insects, or aquatic macro-invertebrates. We report the responses of fish to these lures, based on the results of laboratory encounters between the following pairs of displaying mussels and fishes: Lampsilis cardium and Micropterus coosae; L . perovalis and M . coosae; and Villosa nebulosa and Percina nigrofasciata . In all three encounters, the lures elicited attacks from fish.
2. Encounters between Lampsilis spp. and M. coosae resulted in gill infestations of the fish by larval mussels, which are obligate parasites on fish. An encounter between V . nebulosa and P . nigrofasciata did not result in infestation.
3. The use of these lures to attract fish may greatly increase the chances of parasite/host encounters and may also reduce the chances of infestation of unsuitable hosts.     

Ecological correlates and phylogenetic signal of host use in North American unionid mussels

Mussels in the order Unionoida comprise ～75% of the world’s freshwater bivalve species and are free-living apart from a brief larval stage that parasitizes fish. We investigated the relationships among species of North American unionid mussels and their known host fishes from a macroevolutionary perspective to test whether and how ecological and evolutionary factors correlate with patterns of host use. A subset of 69 mussel species was chosen based on data availability regarding their fish host repertoires, phylogenetic relationships, and ecology. Despite the brevity of their parasitic life stages, the mussels conformed to the right-skewed distribution of host specificity typical of parasitic taxa, in which most species are specialists and a few are generalists. Phylogenetic least squares regression models identified affinity for low-gradient and riffle habitats, and colonization of post-glacial watersheds as the best predictors for the number of fish host species per mussel. However, the second-best model identified citation number as a predictor of the number of hosts, implying that many mussel–host interactions still remain to be identified. A Multiple Regression Mantel test was performed to identify factors associated with the proportion of hosts shared between pairs of mussel species. Range overlap, citations, genetic distance, and similarity in host infection strategy were significantly correlated with the proportion of hosts shared, yet total variation as explained by the best model was low (R²?=?0.14). There was evidence of a topological association between mussels and their hosts (P?=?0.001) and a significant phylogenetic signal of host specificity (λ?=?0.81, P?=?0.003), indicating closely related mussels that overlap in range are more likely to be competing for hosts. Our results provide an initial macroevolutionary framework for studying the evolution of host infection strategies in these mussels but also highlights gaps still remaining in our fundamental ecological knowledge of this endangered clade.     

Bayesian inference supports the host selection hypothesis in explaining adaptive host specificity by European bitterling

Generalist parasites have the capacity to infect multiple hosts. The temporal pattern of host specificity by generalist parasites is rarely studied, but is critical to understanding what variables underpin infection and thereby the impact of parasites on host species and the way they impose selection on hosts. Here, the temporal dynamics of infection of four species of freshwater mussel by European bitterling fish (Rhodeus amarus) was investigated over three spawning seasons. Bitterling lay their eggs in the gills of freshwater mussels, which suffer reduced growth, oxygen stress, gill damage and elevated mortality as a result of parasitism. The temporal pattern of infection of mussels by European bitterling in multiple populations was examined. Using a Bernoulli Generalized Additive Mixed Model with Bayesian inference it was demonstrated that one mussel species, Unio pictorum, was exploited over the entire bitterling spawning season. As the season progressed, bitterling showed a preference for other mussel species, which were inferior hosts. Temporal changes in host use reflected elevated density-dependent mortality in preferred hosts that were already infected. Plasticity in host specificity by bitterling conformed with the predictions of the host selection hypothesis. The relationship between bitterling and their host mussels differs qualitatively from that of avian brood parasites.     

Host, macrohabitat, and microhabitat specificity in the gill parasite Afrodiplozoon polycotyleus (Monogenea)

Studies on species of Monogenea have shown that these parasites often infect only a specific host species, genus, or family, and that they attach only to specific sites within hosts. Few studies, however, examine habitat specificity across host and habitat scales. In this study, we focused on host, macrohabitat, and microhabitat specificity in the monogenean diplozoon Afrodiplozoon polycotyleus, a gill parasite of African cyprinid fishes, Barbus spp. We first compared the occurrence of A. polycotyleus among 4 species of Barbus from a single location in the Mpanga River of western Uganda; Barbus neumayeri was the only species infected with the parasite. We then quantified parasite prevalence and mean abundance in B. neumayeri from a series of river and swamp sites in the same drainage, looking for environmental predictors of diplozoon prevalence and abundance over a broad habitat scale. The prevalence and mean abundance of A. polycotyleus on gills of B. neumayeri was highest in the hypoxic swamp habitat, followed by the intermittent stream sites, and faster flowing river sites. Parasite prevalence and mean abundance across habitats were negatively related to both water current and dissolved oxygen concentration. Within hosts, A. polycotyleus was strongly specific among hemibranchs in poorly oxygenated water and was found on arch 2, hemibranch 4 most frequently.     

Parasitism in species of Bathymodiolus (Bivalvia: Mytilidae) mussels from deep-sea seep and hydrothermal vents

Bivalve species, especially mussels, are biomass dominants in many deep-sea chemosynthetic ecosystems. As in shallow-water environments, parasites are likely to be important factors in the population dynamics of bivalve communities in chemosynthetic ecosystems, but there has been little study of parasitism in deep-sea seep or vent molluscs. In this study, parasite types, diversity, prevalence, infection density and non-infectious indicators of stress or disease as related to host age, reproductive condition, and endosymbiont density were assessed in mussels (Bathymodiolus heckerae) from 2 seep sites and mussels (B. puteoserpentis) from 2 vent sites. We identified 10 microbial or parasitic agents in histological sections. Parasite types included 3 viral-like gut inclusions, 2 rickettsia-like gill inclusions, a rickettsia-like mantle inclusion, a bacterial gill-rosette, a chlamydia-like gut inclusion, gill-dwelling ciliates, and an unidentified inclusion in gut tissues. Parasite species richness was greater in seep mussels than in vent mussels, with the seep mussels possessing 9 types of parasites compared to 2 in the vent mussels. One of the viral-like inclusions infecting the seep mussel B. heckerae was pathogenic, causing lysis of the digestive tubules. The prevalence and intensity of infection by this pathogen were greater in hosts with shell lengths less than 100 mm. Mussels from all 4 sites also exhibited intense infiltration of tissues and blood spaces by enlarged hemocytes. Hemocytic infiltration (hemocytosis) showed variable degrees of severity that were not associated with other host factors examined.     

Host plant adaptation in Drosophila mettleri populations

The process of local adaptation creates diversity among allopatric populations, and may eventually lead to speciation. Plant-feeding insect populations that specialize on different host species provide an excellent opportunity to evaluate the causes of ecological specialization and the subsequent consequences for diversity. In this study, we used geographically separated Drosophila mettleri populations that specialize on different host cacti to examine oviposition preference for and larval performance on an array of natural and non-natural hosts (eight total). We found evidence of local adaptation in performance on saguaro cactus (Carnegiea gigantea) for populations that are typically associated with this host, and to chemically divergent prickly pear species (Opuntia spp.) in a genetically isolated population on Santa Catalina Island. Moreover, each population exhibited reduced performance on the alternative host. This finding is consistent with trade-offs associated with adaptation to these chemically divergent hosts, although we also discuss alternative explanations for this pattern. For oviposition preference, Santa Catalina Island flies were more likely to oviposit on some prickly pear species, but all populations readily laid eggs on saguaro. Experiments with non-natural hosts suggest that factors such as ecological opportunity may play a more important role than host plant chemistry in explaining the lack of natural associations with some hosts.     

The phenomenon of phylogenetic synhospitality in acariform mites (acari: acariformes)--the permanent parasites of vertebrates

The term synhospitality means the association of two or more closely related parasite species with one host species (Eichler, 1966). The cases of two or three synhospitalic species are known from the same host species, and especially ones where parasites were recorded from different parts of the host range, are quite common. The most ordinary reason causing synhospitality in permanent parasites is the host switching. Nevertheless, there are a number of synhospitality cases, where the parasite complex is monophyletic because evolved on a single host species. The special term--"phylogenetic synhospitality" (FS) is proposed for these cases of synhospitality. Most known cases of FS in acariform mites, permanent parasites of vertebrates, are analysed. It is found out that both astigmatan and prostigmatan parasite mites demonstrate a numbers of FS. The majority of these examples represent parasitism of two or three synhospitalic parasite species. Impressive examples of FS involving a number of synhospitalic species is shown by only astigmatan mites inhabiting the fur of mammals or plumage of birds. Most known examples involving four or more mite species are discussed: 51 mite species of the genus Schizocarpus (Chirodiscidae) parasitizing Castor fiber and C. canadensis (Castoridae); 6 species of Listrophorus spp. (Listrophoridae) from Ondatra zibethicus (Cricetidae); 23 species of Listrophoroides s. 1. (Atopomelidae) from Maxomys surifer (Muridae); 21 species of Cytostethum (Atomelidae) from Potorous tridactylus (Potoridae); 4 species of Listrophoroides (Afrolistrophoroides) from Malacomys longipes (Muridae); 7 species of Fainalges (Xolalgidae) from Aratinga holochlora (Psittacidae); 4 species of Zygepigynia (Pteronyssidae) from Chrysocolaptes lucidus (Picidae). The main reason of FS is that, in spite of the Fahrenholz's rule, the speciation of many parasites proceeds much more intensively than in their hosts because of the more rapid replacement of the parasitic generations. The first factor causing FS is the mite speciation it temporary segregated populations of the host (allopatric speciation). In this case, the "multispecies complexes" appeared after the subsequent reintegration of the host populations formerly isolated. The second factor is the speciation due to the specialization of mites to local microhabitats in the fur or plumage of host (sympatric or synxenic speciation). The second way of speciation is most characteristic for mites with highly specialized attaching structures. The phenomenon of FS more resides in ectoparasites of mammals rather than in feather mites in spite of much more structural complicacy of plumage rather than the fur. The high mobility of birds and wide dispersion of their new generations probably embarrass the process of sympatric speciation in their parasites. As a rule, only really significant geographical barriers play role for population isolation in birds. Thus, it could be concluded that two independent factors or their combination lead to FS. (i) The complex and/or disjunctive host range giving a possibility for allopatric speciation in parasites. (ii) The deep mite specialization to local microhabitats on the host body causing sympatric (synxenic) speciation. Fur of mammals and plumage of birds are very complicated in structure and microconditions and provide a considerable number of different microhabitats for mites inhabiting them. The prevalence of one of these two factors depends on the biological peculiarities of both parasites and their hosts. In mites with lesser specialized attaching organs, for example in atopomelids, allopatric speciation dominates. In mites with strongly specialized attaching organs, for example in listrophorids or chirodiscids, both pathways of speciation may take place. In feather mites, sympatric speciation should be more probable due to quite complicate and various structure of feathers in avian hosts. In fur mites, sympatric speciation is more likely in mites parasitizing hosts with peculiar ecology, for example in semiaquatic rodents possessing quite different fur structure in different parts of the body.     

Genetic differences among host-associated populations of water mites (Acari: Unionicolidae: Unionicola): allozyme variation supports morphological differentiation

Unionicola poundsi and U. lasallei are recognized as closely related, morphologically distinct species of water mites living in symbiotic association with the mussels Villosa villosa and Uniomerus declivus, respectively. However, results of a transplant experiment suggested that the morphological characters used to separate these species are plastic and are influenced by the host species in which these mites metamorphose. These results indicate that U. poundsi and U. lasallei are variants of the same species. To test the validity of these contrasting notions, the genetic structure of mite populations from Uniomerus declivus and V. villosa was compared. An examination of allozyme variation at 9 enzyme loci revealed a high degree of genetic differentiation between these host-associated populations, with mites from U. declivus and V. villosa being fixed for different alleles at 3 loci and exhibiting significant allele heterogeneity at 71% of their polymorphic loci. Coefficients of genetic similarity and genetic distance for mites from U. declivus and V. villosa were 0.36 and 0.95, respectively. The results of this study suggest that mite populations from U. declivus and V. villosa are genetically distinct and complement morphological data recognizing them as valid species.     

No evidence for host specialization or host-race formation in the European bitterling (Rhodeus amarus), a fish that parasitizes freshwater mussels

Coevolutionary relationships between parasites and hosts can elevate the rate of evolutionary changes owing to reciprocal adaptations between coevolving partners. Such relationships can result in the evolution of host specificity. Recent methodological advances have permitted the recognition of cryptic lineages, with important consequences for our understanding of biological diversity. We used the European bitterling (Rhodeus amarus), a freshwater fish that parasitizes unionid mussels, to investigate host specialization across regions of recent and ancient sympatry between coevolving partners. We combined genetic data (12 microsatellite and 2 mitochondrial markers) from five populations with experimental data for possible mechanisms of host species recognition (imprinting and conditioning). We found no strong evidence for the existence of cryptic lineages in R. amarus, though a small proportion of variation among individuals in an area of recent bitterling–mussel association was statistically significant in explaining host specificity. No other measures supported the existence of host‐specific lineages. Behavioural data revealed a weak effect of conditioning that biased behavioural preferences towards specific host species. Host imprinting had no effect on oviposition behaviour. Overall, we established that populations of R. amarus show limited potential for specialization, manifested as weak effects of host conditioning and genetic within‐population structure. Rhodeus amarus is the only species of mussel‐parasitizing fish in Europe, which contrasts with the species‐rich communities of bitterling in eastern Asia where several host‐specific bitterling occur. We discuss costs and constraints on the evolution of host‐specific lineages in our study system and more generally.     

Competitive Exclusion,Harem Behaviour and Host Specificity of the Water Mite Unionicola ypsilophora (Hydrachnellae,Acari) Inhabiting Anodonta cygnea (Unionidae)

The co-occurring freshwater mussels Anodonta cygnea and A. anatina serve as hosts for the water mites Unionicola ypsilophora and U. intermedia, respectively. Male U. ypsilophora display a territorial behaviour. They fight with other males, and as a result, there is usually only one male per host. As a consequence, this intrasexual aggression results in female-defence polygyny, or a harem mating system. In contrast, U. intermedia shows no antagonistic behaviour between males. A. cygnea can serve as a host for U. intermedia, but this mite species apparently is excluded from the mussel by U. ypsilophora. In this way, U. intermedia is restricted by competitive exclusion to the mussel A. anatina.