Monogenean body size,but not reproduction,increases with infracommunity density

Body size reveals a plethora of life-history, ecological, and evolutionary information about a species. It plays a critical role in success or failure during competitive, reproductive, or predator–prey interactions. Typically, there is a negative relationship between body size and population density in natural populations and communities. I analysed this relationship within and among multiple populations of two prominent monogenean parasites (>90% prevalence) on Lepomis macrochirus in three lakes in New Jersey (USA), using multiple regression models. To elucidate the causes and benefits of this relationship, I also measured host body condition via a regression index, and reproductive output of the parasite community by measuring parasite eggs shed from the host. The relationship between body size and density of infrapopulations (parasites of a single species on a single host) was positive, and the strength of this relationship for both species depended on which lake they occupied, indicating the potential for Allee effects. This relationship persists at the infracommunity level, where there was a similar positive relationship between a community weighted mean body size and density. However, this relationship did not result in greater reproductive success as measured by infracommunity egg production per individual per 24 h or egg size. The cause of this relationship also remains elusive; it was not explained by host condition or age. The results suggest that there is either no reproductive advantage to this increase in body size or the advantage conferred was not related to these measured fitness components. These findings indicate that researchers should be cautious using body size as a proxy for fitness or reproduction, while also raising further questions about the nature of the relationship between parasites on a host and that between those parasites and the host.     

A new parasitological index for the estimation of peculiarities of the relationships between parasite and its host, and biotope of the host

A new parasitological index (hostal-topical index) for the estimation of the degree of ectoparasite's relationship with its host and biotope of the host is proposed: [formula: see text], where [formula: see text]--hostal-topical index; n--amount of ectoparasites of the given species on the given host species in the biotope; N--amount of ectoparasites of all species from the given taxonomic group on the given host species in the biotope; n1--amount of hosts of the given species in the biotope; N1--amount of hosts of all species from the given taxonomic group in the biotope; n2--amount of ectoparasites of the given species in the biotope; N2--amount of ectoparasites of all species from the given taxonomic group in the biotope. Values [formula: see text] < 0.1 indicate that there is a distinct relationship with the biotope in spite of the host; values fallen into the range 0.1 < [formula: see text] < 0.5 indicate a moderate relationship with the biotope through the host; values [formula: see text] > 0.5 indicate a significant relationship with the host. By means of this index we have analyzed peculiarity of several parasitic species of fleas and gamasid mites to their hosts, biotopes, and biotope through the host. As it was found on the materials from different native zones and subzones of the Omsk Region (Western Siberia, Russia), values of the hostal-topical index for polyhostal parasitic species are lesser than those for oligohostal species. Values of this index can be different for the same species in the different native zones and subzones as well as in the different biotopes of the same native zone (subzone).     

Body size and host range co-determine the altitudinal distribution of Neotropical tephritid flies

Aim We addressed the following questions: (1) Does tephritid body size tend to increase in species found at higher elevations, as predicted by Bergmann's rule? (2) Do tephritids conform to Rapoport's rule, so that species found at higher elevations tend to have broader altitudinal ranges? (3) More generally, how do body size and host range jointly affect the patterns of altitudinal distribution among Neotropical tephritid flies? Location The Mantiqueira mountain range, south‐eastern Brazil, at sites ranging from c. 700 to 2500 m a.s.l. Methods At each site we collected flower heads of all Asteraceae species to rear out endophagous immatures (from January to June in 1998 and 1999). We used structural equation models (SEM) to evaluate jointly the relationships between body size, host range and altitudinal distribution (range and mid‐point). Results Neotropical tephritid body size showed a negative relationship with altitudinal distribution. SE modelling showed no significant direct effect of body size on altitudinal range; however, it had significant indirect negative effects through host range and altitudinal mid‐point. The SE model was a good predictor of observed correlations and accounted for 84% of the variation in tephritid altitudinal range. Main conclusions The altitudinal range of flower‐head‐feeding tephritids is related to host range and is indirectly affected by body size via host range and altitudinal mid‐point. As predicted by Rapoport's rule, tephritids that occur at higher elevations also present wider altitudinal ranges. Bergmann's rule does not apply to Neotropical tephritids along a tropical elevational gradient, but rather its converse was found. Body size may determine host range by imposing a restriction upon large individuals using small flower heads. Host species turnover along the altitudinal gradient may be the main factor explaining the strong relationship between host range and insect elevational distribution.     

Factors affecting parasitism by Microctonus aethiopoides (Hymenoptera: Braconidae) and parasitoid development in natural and novel host species

A laboratory study of aspects of parasitoid host acceptance, suitability and physiological regulation in natural and novel host species was carried out to investigate the degree of variability encountered with different hosts and to determine the value of such observations in host range determination. The parasitoid Microctonus aethiopoides Loan was exposed to a natural host, Sitona discoideus Gyllenhal (Coleoptera: Curculionidae) and three novel hosts, the New Zealand native Nicaeana cervina Broun, the introduced weed biological control agent Rhinocyllus conicus (Froehlich), and a congeneric pest species, Sitona lepidus Gyllenhal (all Coleoptera: Curculionidae). Per cent parasitism of these species was 54%, 43%, 39% and 0%, respectively. The results indicated that for both S. discoideus and R. conicus more males than females were parasitized (69% cf. 45%, and 49% cf. 32% respectively) but host size was not a significant factor. Overall, superparasitism was recorded in about 29% of parasitized weevils and there was evidence that host discrimination to avoid superparasitism occurred in the natural host. Conversely, superparasitism occurred more frequently than would be expected in N. cervina (42%) coupled with higher survival of larvae in superparasitized hosts in this species. The frequency distribution of attack of R. conicus by M. aethiopoides was not different from random. Parasitoid development was more rapid in the natural host, S. discoideus, and parasitoid size was positively correlated with host size. There was a strong positive relationship between parasitoid larval survival and the presence of teratocytes in all hosts. Host fecundity and fertility were reduced by parasitism for most species, and in some cases, by exposure to parasitoids in the absence of detectable parasitism. It was concluded that laboratory observations can provide useful information on the compatibility between host and parasitoid which can complement traditional host range tests to predict field host range.     

Gonadosomatic index‐based size at first sexual maturity of male and female Amblygaster clupeoides (Bleeker, 1849) (Clupeidae) on the east coast of the Malaysian peninsular

Size at first sexual maturity and condition of Amblygaster clupeoides were determined in a population at the Malaysian east coast. A total of 887 specimens were collected from commercial fishing vessels between March and November 2012. A random sample was taken each time to avoid bias in the cohort sampling. Size at first sexual maturity was determined by the relationship between the total length and gonadosomatic index (GSI). Similarly, the relationship between the condition factor and GSI was applied to determine the condition at first sexual maturity. The sex ratio was close to 1:1. The total length at first sexual maturity of both males and females were at around 18 cm. Males with a GSI of ≥3.2% and females with a GSI of ≥6.5% were mature, having condition factors of ≥0.90 and ≥0.93, respectively. Based on these results the minimum permissible capture size was identified. This study establishes a strong need for regular and continued monitoring of the changes in fish size at first sexual maturity.     

Feather mites (Acari: Astigmata) and body condition of their avian hosts: a large correlative study

Feather mites are arthropods that live on or in the feathers of birds, and are among the commonest avian ectosymbionts. However, the nature of the ecological interaction between feather mites and birds remains unclear, some studies reporting negative effects of feather mites on their hosts and others reporting positive or no effects. Here we use a large dataset comprising 20 189 measurements taken from 83 species of birds collected during 22 yr in 151 localities from seven countries in Europe and North Africa to explore the correlation between feather mite abundance and body condition of their hosts. We predicted that, if wing‐dwelling feather mites are parasites, a negative correlation with host body condition should be found, while a mutualistic interaction should yield positive correlation. Although negative relationships between feather mite abundance and host body condition were found in a few species of birds, the sign of the correlation was positive in most bird species (69%). The overall effect size was only slightly positive (r =0.066). The effect of feather mite abundance explained <10% of variance in body condition in most species (87%). Results suggest that feather mites are not parasites of birds, but rather that they hold a commensalistic relationship where feather mites may benefit from feeding on uropygial gland secretions of their hosts and birds do not seem to obtain a great benefit from the presence of feather mites.     

Parasite prevalence and the size of host populations: an experimental test

Although important in epidemiological theory, the relationship between the size of host populations and the prevalence of parasites has not been investigated empirically. Commonly used models suggest no relationship, but this prediction is sensitive to assumptions about parasite transmission. In laboratory populations, I manipulated the size of Tribolium castaneum flour beetle populations and measured the prevalence and distribution of a parasitic mite, Acarophenax tribolii. I found that parasite prevalence did not vary for a wide range of host population sizes. However, prevalence was lower in populations with less than 40 hosts. This effect cannot be attributed to changes in host population density because host density was held constant among treatments. The reduction in prevalence of small populations below a threshold that I observed is predicted by the extinction debt model, but it is not expected from models of host-parasite interactions that assume density-dependent transmission. The distribution of parasites, measured using Lloyd's patchiness index, was not affected by host population size. The mean crowding of parasites, however, was negatively related with host density. Finally, the prevalence of parasites in large populations did not differ from that found in sets of smaller patches as long as the smaller populations in aggregate were equivalent in size to the large population.     

Secondary production and condition index in Arca zebra (Mollusca: Vivaldia) in the Gulf of Cariaco, Venezuela

Production and condition index of a turkeywing (Arca zebra) population were studied from August 1984 to August 1985 in Pariche, Cariaco Gulf, Venezuela. Production was studied through bimonthly collections using a method designed for populations with continuous reproduction and indistinguishable age classes. The population distribution was stable with a mean density of 37 ind.m-2. A production of 131.61 g dry weight m-2 year-1 was calculated from data on density, biomass and weight increase along the year. Highest production was between February 1985 and August 1985 (65%) and concentrated in individuals 60.00-79.95 mm long (45.73%). There was large seasonal variation in the mean monthly condition index (IC = (Ps/Ph) x 100) in the two size classes examined: it reached a maximum in July 1985 (26.84%) and a minimum in January 1985 (16.31%).     

Evolution and determinants of host specificity in the genus Lamellodiscus (Monogenea)

The evolution and determinants of host specificity in Lamellodiscus species (Monogenea, Diplectanidae) were investigated. The 20 known Mediterranean species were studied, all parasites of fishes from the family Sparidae (Teleostei). An index of specificity, which takes into account the phylogenetic relationships of their fish host species, was defined. The link between specificity and its potential determinants was investigated in a phylogenetic context using the method of independent contrasts. Host specificity in Lamellodiscus species appeared to be highly constrained by phylogeny, but also linked to host size. Mapping specificity onto the parasite phylogenetic tree suggests that specialist species do not represent an evolutionary dead end, and that specialization is not a derived condition. It is hypothesized that the ability to be generalist or specialist in Lamellodiscus is controlled by intrinsic, phylogenetically-related characteristics, and that specialist species tend to use large hosts, which may be more predictable.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77, 431−443.     

Optimal clutch size of the chestnut gall-wasp,Dryocosmus kuriphilus yasumatsu (Hymenoptera: Cynipidae)

Optimal clutch size of the chestnut gall-wasp, Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae), was examined in galls on wild and resistant chestnut trees in 1988 and 1989. The rate of escape success of newly-emerged adults from galls was an average of 60%, irrespective of cell numbers per gall. Dry mass per cell of a gall (as an index of nutritive condition) decreased with increasing cell number per gall, but was proportional to the mean number of mature eggs of new adults per gall. The number of cells per gall that occurred most frequently did not agree with that attained by the maximum survival rate from young larva to adult emergence of the gall-wasp. This discrepancy was examined from the viewpoint of three factors: 1) quality of offspring, 2) defensive response of the host plant causing mortality of the gall-wasp before cell formation, and 3) fitness per gall vs. fitness per egg. It is concluded that the third factor is most likely to be the one best in explaining the discrepancy.     

Body fat and condition in sperm whales,Physeter macrocephalus,from southern Australian waters

Blubber thickness (n=102) and lipid content (n=37) were measured in sperm whales from three mass stranding events on the west and north-west coasts of Tasmania, Australia in February 1998. Blubber thickness was highly variable, ranging from 43.0 to 168.0 mm (mean 98.4+/-18.4 mm) while lipid fat content, also highly variable, ranged from 16.19 to 89.34% (mean 49.2+/-17.9%). Blubber thickness was significantly and positively related to total length, but a blubber thickness index based on the residuals of this relationship was not related to age, sex or reproductive condition. No relationship was found between blubber thickness index and blubber lipid content, indicating that blubber thickness may not provide a comprehensive indication of body fat condition in sperm whales when only measured at a single site. Blubber lipid content was not related to total length, age or sex. Blubber lipid content was stratified vertically throughout the blubber layer, suggesting that the inner blubber layer may be a more active site for lipid deposition and mobilisation, while the outer blubber layer may serve in a structural or thermoregulatory role. The social structure and foraging ecology of this species may serve to minimise the need to rely on stored energy reserves to meet reproductive energy requirements. In addition, the broader role of blubber for structural, buoyancy and insulative functions coupled with high individual variability may cause a lack of obvious relationships between these variables and body size, age, sex and reproductive state in this species.     

The effects of host-plant distribution and local abundance on the species richness of agromyzid flies attacking British umbellifers

Abstract. 1. The number of agromyzid species (Diptera: Agromyzidae) attacking British Umbelliferae generally increases with the size of the geographic range of the host, measured as occupied 10 km squares in the Atlas of the British Flora (Lawton & Price, 1979). 2. In the present study we tried to explain the large, residual variation in this species—area relationship using two new variables, namely the local abundance of the host plant, and the number of habitats within which it grows. 3. Local abundance was estimated from eight floras that map plant distributions within English countries by tetrads (2 times 2 km squares). Local abundance was defined as: Total number of occupied tetrads Total number of available tetrads within occupied 10 km squares 4. The number of habitats occupied by each host plant was taken from the only county flora to record plant habitats objectively, that for Warwickshire. 5. We expected to find a correlation between local abundance and the residuals from the national species—area relationship, with locally scarce plants having fewer agromyzids than expected from the sizes of their national ranges, and vice versa. 6. What we found was that size of geographic range and local abundance were highly correlated; hence their relative contributions to agromyzid species richness were difficult to disentangle. Residuals from the national species—area relationship were positively correlated with local abundance, but the relationship marginally failed to achieve statistical significance (P= 0.06). 7. In contrast, the number of habitats occupied by each species of umbellifer in Warwickshire had a marked effect upon agrornyzid species richness, with plants that grow in more habitats supporting more species of insects. Not surprisingly, local abundance and number of habitats occupied were highly correlated. 8. Lawton & Price's observation that aquatic umbellifers are faunally impoverished now emerges as part of the general effect of number of habitats occupied by the host plants on agromyzid species richness. 9. Once the number of habitats occupied by each host plant in Warwickshire was entered into a multiple regression, the effect of size of host geographic range on agromyzid species richness was no longer statistically significant. 10. A combination of the number of habitats occupied, and leaf-form of the host (the latter taken from Lawton & Price, 1979), explains 61% of the variation in agromyzid species richness on British Umbelliferae.     

Condition-dependent tolerance of monoterpenes in an insect herbivore

The ability of a herbivore to tolerate plant defensive chemicals may vary with the herbivore’s energetic state. We investigated the effect of body condition on the survivorship of individual mountain pine beetles, Dendroctonus ponderosae, exposed to host monoterpenes at concentrations comparable to constitutive and induced levels of defence using fumigant exposure. Body condition index was calculated as the residual mass after fitting the relationship between fresh weight and body size. Differences in survivorship among the four monoterpenes tested (α-pinene, myrcene, terpinolene and limonene) were small. Beetles with a higher body condition index survived high monoterpene concentrations better than those in poorer condition. There was no direct effect of sex, but positive effects of body size and fat content on survivorship favoured females, the sex that pioneers attacks on live trees. Higher body condition index corresponded to both higher fat content and fat-free body mass; the same conclusions about monoterpene identity and size-dependent or energy-dependent tolerance of high monoterpene concentrations held if fat or fat-free body mass were used in place of body condition index. This study highlights the need to consider insect body condition in understanding insect–plant interactions.     

Host size,superparasitism and sex ratio in mass-reared <Emphasis Type="Italic">Diachasmimorpha longicaudata</Emphasis>, a fruit fly parasitoid

We analyzed the relationship among host size, superparasitism and sex-ratio in mass reared Diachasmimorpha longicaudata (Hymenoptera: Braconidae). Individual host pupae of Anastrepha ludens (Diptera: Tephritidae) were measured (length and width), and the number of oviposition scars per pupa was used as a reliable indicator of superparasitism. The probability of an emerging parasitoid being a female was positively associated with the number of oviposition scars on the host cuticle, but not with the host size. The number of scars per host pupae from which females emerged was slightly but significantly higher than in those pupae giving raise to males. In D. longicaudata, the influence of host size on sex allocation decisions of individual females seems to be overridden by the level of superparasitism, which itself was positively correlated with pupa length. This suggests that larger pupae could experience a higher number of ovipositions than their smaller counterparts, and that a high level of superparasitism may conduct to a female biased sex ratio. We discuss the relevance of these findings which could provide new elements (e.g., the manipulation of superparasitism) for optimizing the mass rearing of this parasitoid.     

Regional abundance of a planthopper pest: the effect of host patch area and configuration

The effect of host patch area and configuration on the abundance of dispersing individuals of Delphacodes kuscheli Fennah (Homoptera: Delphacidae), the vector of Río Cuarto disease in maize, was investigated in the main maize production area of Argentina. Actively dispersing D. kuscheli individuals were collected from 15 sampling sites during the spring seasons of 1999 and 2000, using sticky traps placed at 6 m above ground level. Host patches were detected and quantified using Landsat 5 TM images for the periods studied. The spatial pattern analysis program FRAGSTATS was used to estimate the total class area, largest patch index, mean proximity index, and patch cohesion index for patches of winter pastures (the main insect host during winter) as observed from the satellite images. Landsat 5 TM estimations showed local variability in the proportion of winter pastures, with patches bigger during 1999 than during 2000, but these patches represented only a very small part of the total landscape. Proximity between host patches was also variable between sites and higher values of cohesion occurred during the first sampling season. The relationship between host area and D. kuscheli mean abundance was adjusted to an exponential (R²= 77.5%) model. Host patch dominance, host patch isolation, and host patch connectivity all showed a positive relationship with D. kuscheli mean abundance, adjusting significantly to linear models (R²= 92%, R²= 90%, and R²= 22%, respectively). Outbreaks of Río Cuarto disease in the main maize production area of Argentina are related to high vector populations. The results indicate that the abundance of D. kuscheli depends on factors related to the abundance and configuration of its host patches.     

Brood parasitism,female condition and clutch reduction in the common eider Somateria mollisima

Conspecific brood parasitism (CBP) is an alternative reproductive tactic found in many animals with parental care. Parasitizing females lay eggs in the nests of other females (hosts) of the same species, which incubate and raise both their own and the foreign offspring. The causes and consequences of CBP are debated. Using albumen fingerprinting of eggs for accurately detecting parasitism, we here analyse its relation to female condition and clutch size in High Arctic common eiders Somateria mollissima borealis. Among 166 clutches in a Svalbard colony, 31 (19%) contained eggs from more than one female, and 40 of 670 eggs (6%) were parasitic. In 6 cases an active nest with egg(s) was taken over by another female. Many suitable nest sites were unoccupied, indicating that CBP and nest takeover are reproductive tactics, not only consequences of nest site shortage. Similarity in body mass between female categories suggests that condition does not determine whether a nesting female becomes parasitised. There was no evidence of low condition in parasites: egg size was similar in hosts and parasites, and parasitism was equally frequent early and late in the laying season. Meta‐analysis of this and 3 other eider studies shows that there is a cost of being parasitised in this precocial species: host females laid on average 7% fewer eggs than other females.     

Correlated evolution of host and parasite body size: tests of Harrison's rule using birds and lice

Large-bodied species of hosts often harbor large-bodied parasites, a pattern known as Harrison's rule. Harrison's rule has been documented for a variety of animal parasites and herbivorous insects, yet the adaptive basis of the body-size correlation is poorly understood. We used phylogenetically independent methods to test for Harrison's rule across a large assemblage of bird lice (Insecta: Phthiraptera). The analysis revealed a significant relationship between louse and host size, despite considerable variation among taxa. We explored factors underlying this variation by testing Harrison's rule within two groups of feather-specialist lice that share hosts (pigeons and doves). The two groups, wing lice (Columbicola spp.) and body lice (Physconelloidinae spp.), have similar life histories, despite spending much of their time on different feather tracts. Wing lice showed strong support for Harrison's rule, whereas body lice showed no significant correlation with host size. Wing louse size was correlated with wing feather size, which was in turn correlated with overall host size. In contrast, body louse size showed no correlation with body feather size, which also was not correlated with overall host size. The reason why body lice did not fit Harrison's rule may be related to the fact that different species of body lice use different microhabitats within body feathers. More detailed measurements of body feathers may be needed to explore the precise relationship of body louse size to relevant components of feather size. Whatever the reason, Harrison's rule does not hold in body lice, possibly because selection on body size is mediated by community-level interactions between body lice.     

扁颅蝠和褐扁颅蝠体表寄生革螨的宿主选择

该文通过野外调查和室内行为实验,研究了扁颅蝠(Tylonycteris pachypus)和褐扁颅蝠(T.robustula)与其体表寄生革螨(雷氏巨刺螨(Macronyssus radovskyi)及拟雷氏巨刺螨(M.pararadovskyi))之间的关系.在野外自然条件下,雌性扁颅蝠的体表寄生革螨负荷量与宿主健康指数呈正相关(Spearman:rs=0.55,P＜0.01,n=24),而在雄性扁颅蝠以及雌、雄褐扁颅蝠中则无相关性(P＞0.05).室内原宿主感染实验发现,扁颅蝠和褐扁颅蝠体表寄生革螨均明显倾向于选择各自的雄性宿主,扁颅蝠雌、雄性感染率分别为(42±12)％和(58±12)％(t=-3.6,df=31,P＜0.01);褐扁颅蝠雌、雄性感染率分别为(37±11)％和(63±11)％(t=-6.1,df=26,P＜0.001).用扁颅蝠体表寄生革螨(拟雷氏巨刺螨)对扁颅蝠与褐扁颅蝠交叉感染后发现,寄生革螨明显选择其原宿主扁颅蝠,扁颅蝠与褐扁颅蝠感染率分别为(71±13)％和(29±13)％(t=9.1,df=29,P＜0.001).以上结果表明,扁颅蝠和褐扁颅蝠的体表寄生革螨负荷量与宿主身体状态无明显相关性,而对宿主性别表现不同偏好;扁颅蝠的体表寄生革螨对宿主表现明显的专一性.     

Size matters for lice on birds: Coevolutionary allometry of host and parasite body size

Body size is one of the most fundamental characteristics of all organisms. It influences physiology, morphology, behavior, and even interspecific interactions such as those between parasites and their hosts. Host body size influences the magnitude and variability of parasite size according to Harrison's rule (HR: positive relationship between host and parasite body sizes) and Poulin's Increasing Variance Hypothesis (PIVH: positive relationship between host body size and the variability of parasite body size). We analyzed parasite–host body size allometry for 581 species of avian lice (～15% of known diversity) and their hosts. We applied phylogenetic generalized least squares (PGLS) methods to account for phylogenetic nonindependence controlling for host and parasite phylogenies separately and variance heterogeneity. We tested HR and PIVH for the major families of avian lice (Ricinidae, Menoponidae, Philopteridae), and for distinct ecological guilds within Philopteridae. Our data indicate that most families and guilds of avian lice follow both HR and PIVH; however, ricinids did not follow PIVH and the “body lice” guild of philopterid lice did not follow HR or PIVH. We discuss mathematical and ecological factors that may be responsible for these patterns, and we discuss the potential pervasiveness of these relationships among all parasites on Earth.     

Aspects of the ecology and natural history of Paraergasilus rylovi (Copepoda, Ergasilidae) parasitic in unionids of Finland

The distribution of Paraergasilus rylovi in 17 populations of unionids was investigated. In 1 unionid population, the parasite was studied regarding host age, size, sex, and the reproductive period (occurrence of egg sacs). Results from pooled material from the years 1987--1989 and 1996 (southern Finland, 11 populations) indicated that Anodonta piscinalis (n = 1,359) is the main host (total mean prevalence 71% and intensity +/-SE of infection 16.4+/-0.6). Pseudanodonta complanata (n = 106) was infected occasionally (3% and 1.3+/-0.3), whereas Unio pictorum (n = 108) and U. tumidus (n = 17) were not infected. Results from 17 A. piscinalis populations showed that P. rylovi occurs in southern Finland but not in northern Finland. In A. piscinalis, the mean intensity of infection was higher in lake populations than in river populations. Both host age and length had a negative relationship with the intensity of P. rylovi infection. Host sex did not affect the intensity of infection. Egg sacs of P. rylovi were found from June to August. There was a tendency for higher intensities of infection in autumn. Infection by the digenean Rhipidocotyle fennica had no effect on the intensity of P. rylovi infection.