Changes in muscle structure associated with somatic growth in Idiosepius pygmaeus, a small tropical cephalopod

Mantle muscle tissue of Idiosepius pygmaeus was examined to describe changes in structure and organization associated with growth. Growth in I. pygmaeus|DD was a function of both an increase in muscle fibre number and fibre size within muscle blocks. Continuous fibre production over the observed life span of I. pygmaeus was indicated by the presence of very small muscle fibres (< 1.0 μm in diameter) in substantial proportions in all sizes of individuals. Muscle blocks became larger as animals increased in size, although new muscle blocks were generated in all sizes of individuals. Mantle muscle fibres had a maximum size of 11 μm. Therefore, for an individual to continue increasing muscle block sizes, new fibres must be produced. This is further evidence of continuous fibre production throughout the size range of I. pygmaeus examined. The relative rates of muscle fibre generation and fibre growth depended on the size of the animal and position along the mantle (anterior, mid or posterior mantle). The predominance of small fibres and blocks at the anterior end of the mantle suggested that this was the primary growth region. Mitochondriapoor and mitochondria-rich muscle fibres from small individuals had much larger mitochondrial cores than muscle fibres from larger animals. Changes in the muscle structure are discussed with respect to the metabolic and energetic requirements of I. pygmaeus , and how these may change with growth.     

A nested parasite species subset pattern in tropical fish: host as major determinant of parasite infracommunity structure

The number of monogenean gill parasite species associated with fish hosts of different sizes is evaluated for 35 host individuals of the West African cyprinid Labeo coubie. The length of host individuals explains 86% of the total variation in monogenean species richness among individuals. Larger hosts harbour more species than smaller ones. The existence of a hierarchical association of parasite species in individuals of L. coubie is demonstrated. Monogenean infracommunities on larger fish hosts consist of all species found on smaller hosts plus those restricted to the larger size categories, suggesting some degree of compositional persistence among host individuals. The findings provide strong support for an interpretation of the relationship between monogenean parasite species richness and host body size in terms of a nested species subset pattern, thus providing a new record of repetitive structure and predictability for parasite infracommunities of hosts.After 15/01/1995, ORSTOM, BP 165 97323 Cayenne cedex Guyane France     

Infracommunity structure of parasites of Hemigymnus melapterus (Pisces: Labridae) from Lizard Island, Australia: the importance of habitat and parasite body size

This study describes the community of all metazoan parasites from 14 individuals of thicklip wrasse, Hemigymnus melapterus, from Lizard Island, Australia. All fish were parasitized, and 4,649 parasite individuals were found. Twenty-six parasite species were identified although only 6 species were abundant and prevalent: gnathiid isopods, the copepod Hatschekia hemigymni, the digenean Callohelmis pichelinae, and 3 morphotypes of tetraphyllidean cestode larvae. We analyzed whether the body size and microhabitat of the parasites and size of the host affected understanding of the structure of the parasite community. We related the abundance, biovolume, and density of parasites with the host body size and analyzed the abundances and volumetric densities of some parasite species within microhabitats. Although the 2 most abundant species comprised 75% of all parasite individuals, 4 species, each in similar proportion, comprised 85% of the total biovolume. Although larger host individuals had higher richness, abundance, and biovolume of parasites than smaller individuals, overall parasite volumetric density actually decreased with the host body size. Moreover, parasites exhibited abundances and densities significantly different among microhabitats; some parasite species depended on the area available, whereas others selected a specific microhabitat. Parasite and habitat size exhibited interesting relationships that should be considered more frequently. Considerations of these parameters improve understanding of parasite community structure and how the parasites use their habitats.     

Muscle markers revisited: activity pattern reconstruction with controls in a central California Amerind population

Anthropologists frequently use musculoskeletal stress markers to reconstruct past activity patterns. Yet, researchers have called into question the reliability of muscle marker measurements in part because body size and age affect muscle marker scores. In this study, the author examined an aggregate upper limb muscle marker to determine if after controlling for the effects of body size and age, one could reconstruct activity patterns of a prehistoric Amerind population. Analyses were made of a sample of 102 (43 males, 59 females) prehistoric central California Amerinds. Muscle markers were measured using two-point observer rating scales; body size was measured by humeral articular surfaces; age and sex were determined previously through standard procedures. Using sex separated rankings and partial correlations, disaggregated muscle markers were examined for correlations with age and size to determine if specific muscle markers may be useful in pinpointing to activity patterns. Aggregate upper limb muscle marker correlated with: age, r = 0.44; humeral size, r = 0.44; and sex, r = 0.43; Ps < 0.001. Older individuals had greater muscle markers, as did larger individuals, and males. Rankings seemed to be confounded by the effect size had on the muscle markers. However, based on partial correlations controlling for size and age, the differences that remained between males and females could be used to reconstruct male activities of throwing in hunting and interpersonal aggression acts.     

Are Sick Individuals Weak Competitors? Competitive Ability of Snails Parasitized by a Gigantism-Inducing Trematode

Parasitized individuals are often expected to be poor competitors because they are weakened by infections. Many trematode species, however, although extensively exploiting their mollusc hosts, also induce gigantism (increased host size) by diverting host resources towards growth instead of reproduction. In such systems, alternatively to reduced competitive ability due to negative effects of parasitism on host performance, larger size could allow more efficient resource acquisition and thus increase the relative competitive ability of host individuals. We addressed this hypothesis by testing the effect of a trematode parasite Diplostomum pseudospathaceum on the competitive ability of its snail host Lymnaea stagnalis. We experimentally examined the growth of snails kept in pairs in relation to their infection status and intensity of resource competition (i.e. food availability). We found that parasitized snails grew faster and their reproduction was reduced compared to unparasitized individuals indicating parasite-induced gigantism. However, growth of the snails was faster when competing with parasitized individuals compared to unparasitized snails indicating reduced competitive ability due to parasitism. The latter effect, however, was relatively weak suggesting that the effects of the parasite on snail physiology may partly override each other in determining competitive ability.     

Epizootiology of blood parasites in an Australian lizard: a mark-recapture study of a natural population

The dynamics of a naturally endemic blood parasite (Hepatozoon hinuliae) were studied in a lizard (Eulamprus quoyii) host population, using 2 years of longitudinal data. We investigated how parasite abundance in the population varied over time, examined whether certain host sub-populations were more prone to infection, and compared parasite loads in relation to host reproductive behaviour. We recorded blood parasite infections of 331 individuals, obtained in 593 captures. Prevalence (the proportion of the host population infected) of blood parasites was high; approximately 66% of the lizard population was infected. Probability of infection increased with host age and size, but did not differ between the sexes. Within individuals, parasite load (the intensity of infection within individuals) did not vary over time, and was independent of host reproductive behaviour. Parasite load was significantly higher in males compared to females.     

The symbiotic shrimp Ascidonia flavomaculata lives solitarily in the tunicate Ascidia mentula: implications for its mating system

Symbiotic crustaceans are expected to live solitarily with their hosts when members of their host species are small (relative to symbiont body size) and structurally simple. We tested the hypothesis of a solitary lifestyle in Ascidonia flavomaculata, a symbiotic shrimp that inhabits the branchial chamber of the relatively small and structurally simple tunicate Ascidia mentula in the subtidal zone of the Islas Baleares, Spain. We found that members of A. flavomaculata dwell as solitary individuals in the branchial chamber of ascidians at a higher frequency than expected by chance alone. Given this host use pattern of A. flavomaculata, we hypothesized that males actively move among host individuals in search of receptive females. We provide several lines of evidence consistent with that hypothesis. First, a positive correlation between shrimp and host body size was detected for females, but not for males, during one of the sampling seasons. If males as well as females spend long periods of time within their host individuals, a positive correlation between shrimp and host body size should have been found for both males and females. Second, the body sizes of individuals in the few male‐female pairs observed during this study were poorly correlated. If males of A. flavomaculata shared their host individuals with females for long periods of time, a positive correlation between the sizes of males and females in a pair should have been found. Lastly, the body sizes of paired males were larger than those of solitary males during summer, when reproduction was more intense. This difference in body size between paired and solitary males additionally suggests competition among putatively roaming males for receptive females. Manipulative experiments and behavioral observations are necessary to reveal the details of the mating system of A. flavomaculata and other symbiotic crustaceans with a solitary lifestyle.     

Endophagous insects and structural niches on plants: ecology and evolutionary consequences

1. The endophagous weevil Lixus elongatus was studied on two Carduus species in order to look for the impact of a structural gradient in host plant stem diameter on the life cycle and the genetic structure of this species. 2. Body size of L. elongatus females was correlated positively with their fecundity and the stem diameter at the location of oviposition. 3. Emerging individuals showed a significantly positive relationship of body size with the stem diameter at their pupation and emergence site. Mating behaviour was positively size-assortative. 4. Allozyme analyses indicated a gradient of genetic differentiation along body size classes and a decreasing heterozygosity with decreasing body size. 5. The results led to the conclusion that a structural gradient in plants can promote body size variability in an endophagous herbivore through phenotypic constraints of the host plant and the herbivore. As size-assortative mating translates this variability into differential gene flow between individuals of different body size, this could provide the raw material for speciation processes. Within a single host plant patch, however, further processes are necessary to split the existing genetic gradient into separate gene pools.     

Understanding muscle markers: lower limbs

Musculoskeletal markers are frequently used to reconstruct past lifestyles and activity patterns. Yet the reliability of muscle marker measurements has been called into question because they may be confounded by body size. In this study, an aggregate muscle marker variable was calculated using 20 insertion sites (14 femoral, 6 tibial), and I examined their effects on lower limb size (as a proxy for body size), age, and sex. Analyses were made of a sample of 77 (57 males, 20 females) Native British Columbians (3,500-1,500 years BP) and 18th century Quebec prisoners. Muscle markers were measured using two-point observer rating scales; size was measured by standard methods; and age and sex were determined through pelvic, cranial, and dental morphology. Lower limb muscle markers correlated with: age, r=0.61; lower limb size, r=0.52; and sex, r=0.49; P <0.001. Older individuals had higher muscle marker scores, as did larger individuals and males. Based on partial correlations and regression analyses, age was the best overall predictor of lower limb muscle markers.     

A sex-specific size–number tradeoff in clonal broods

Polyembryonic parasitoids producing single-sex broods of clonal offspring provide an unusually clear window into the classic tradeoff between the number and size of offspring. We conducted a laboratory study of the encyrtid parasitoid Copidosoma bakeri parasitizing the noctuid Agrotis ipsilon to examine the way that size and number of offspring tradeoff in broods of each sex and to determine how the fit between host and parasitoid brood is achieved. We found that brood mass (wasp body mass ×brood size) was proportional to host mass, independent of brood sex, indicating a tight fit between brood and host and ensuring a size–number tradeoff. By correcting brood size and body mass of each brood for host mass, we demonstrated the expected inverse relationship between wasp variables. We postulated that the wasp brood might achieve the fit to the host by (1) adjusting brood size based on information available early in host development before and during division of the embryo, (2) manipulating host size late in host development after completion of embryo division, or (3) simply adjusting individual wasp mass to fill the host. We evaluated host responses to parasitism – and correlations between brood size and host growth early and late in development – for broods of each sex. The data are consistent with adjustment of brood size to the amount of host growth early in host development and with manipulation of host mass late in host development. The tight link between host mass and brood mass also suggests a final adjustment by parasitoid growth to achieve complete filling. Within the tight fit, female broods were smaller but contained larger individuals than male broods. The sex-specific balance point of the tradeoff and sex differences in balancing mechanisms and responses to host size suggest different selection pressures on each sex requiring future investigation.     

Modelling transmission of vector-borne pathogens shows complex dynamics when vector feeding sites are limited

The relationship between species richness and the prevalence of vector-borne disease has been widely studied with a range of outcomes. Increasing the number of host species for a pathogen may decrease infection prevalence (dilution effect), increase it (amplification), or have no effect. We derive a general model, and a specific implementation, which show that when the number of vector feeding sites on each host is limiting, the effects on pathogen dynamics of host population size are more complex than previously thought. The model examines vector-borne disease in the presence of different host species that are either competent or incompetent (i.e. that cannot transmit the pathogen to vectors) as reservoirs for the pathogen. With a single host species present, the basic reproduction ratio R(0) is a non-monotonic function of the population size of host individuals (H), i.e. a value [Formula: see text] exists that maximises R(0). Surprisingly, if [Formula: see text] a reduction in host population size may actually increase R(0). Extending this model to a two-host species system, incompetent individuals from the second host species can alter the value of [Formula: see text] which may reverse the effect on pathogen prevalence of host population reduction. We argue that when vector-feeding sites on hosts are limiting, the net effect of increasing host diversity might not be correctly predicted using simple frequency-dependent epidemiological models.     

The influence of host fruit and temperature on the body size of adult Ceratitis capitata (Diptera: Tephritidae) under laboratory and field conditions

The adult body size of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), varies in natural conditions. Body size is an important fitness indicator in the Mediterranean fruit fly; larger individuals are more competitive at mating and have a greater dispersion capacity and fertility. Both temperature during larval development and host fruit quality have been cited as possible causes for this variation. We studied the influence of host fruit and temperature during larval development on adult body size (wing area) in the laboratory, and determined body size variation in field populations of the Mediterannean fruit fly in eastern Spain. Field flies measured had two origins: 1) flies periodically collected throughout the year in field traps from 32 citrus groves, during the period 2003-2007; and 2) flies evolved from different fruit species collected between June and December in 2003 and 2004. In the lab, wing area of male and female adults varied significantly with temperature during larval development, being larger at the lowest temperature. Adult size also was significantly different depending on the host fruit in which larvae developed. The size of the flies captured at the field, either from traps or from fruits, varied seasonally showing a gradual pattern of change along the year. The largest individuals were obtained during winter and early spring and the smallest during late summer. In field conditions, the size of the adult Mediterannean fruit fly seems apparently more related with air temperature than with host fruit. The implications of this adult size pattern on the biology of C. capitata and on the application of the sterile insect technique are discussed.     

Interindividual differences in leg muscle mass and pyruvate kinase activity correlate with interindividual differences in jumping performance of Hyla multilineata

Frog jumping is an excellent model system for examining the structural basis of interindividual variation in burst locomotor performance. Some possible factors that affect jump performance, such as total body size, hindlimb length, muscle mass, and muscle mechanical and biochemical properties, were analysed at the interindividual (intraspecies) level in the tree frog Hyla multilineata. The aim of this study was to determine which of these physiological and anatomical variables both vary between individuals and are correlated with interindividual variation in jump performance. The model produced via stepwise linear regression analysis of absolute data suggested that 62% of the interindividual variation in maximum jump distance could be explained by a combination of interindividual variation in absolute plantaris muscle mass, total hindlimb muscle mass (excluding plantaris muscle), and pyruvate kinase activity. When body length effects were removed, multiple regression indicated that the same independent variables explained 43% of the residual interindividual variation in jump distance. This suggests that individuals with relatively large jumping muscles and high pyruvate kinase activity for their body size achieved comparatively large maximal jump distances for their body size.     

Host patch traits have scale-dependent effects on diversity in a stickleback parasite metacommunity

Many metacommunities are distributed across habitat patches that are themselves aggregated into groups. Perhaps the clearest example of this nested metacommunity structure comes from multi-species parasite assemblages, which occupy individual hosts that are aggregated into host populations. At both spatial scales, we expect parasite community diversity in a given patch (either individual host or population) to depend on patch characteristics that affect colonization rates and species sorting. But, are these patch effects consistent across spatial scales? Or, do different processes govern the distribution of parasite community diversity among individual hosts, versus among host patches? To answer these questions, we document the distribution of parasite richness among host individuals and among populations in a metapopulation of threespine stickleback Gasterosteus aculeatus. We find some host traits (host size, gape width) are associated with increased parasite richness at both spatial scales. Other patch characteristics affect parasite richness only among individuals (sex), or among populations (lake size, lake area, elevation and population mean heterozygosity). These results demonstrate that some rules governing parasite richness in this metacommunity are shared across scales, while others are scale-specific.     

Understanding muscle markers: aggregation and construct validity

Musculoskeletal markers are frequently used to reconstruct past lifestyles and activity patterns. Yet, the reliability of muscle marker measurements has been called into question because they allegedly fail to correlate with cross-sectional properties and exercise patterns, and are confounded by body size. In this study, the principle of aggregation was used to sum muscle markers over 7 insertion sites (4 humeral, 2 radial, and 1 ulnar) and examine the effects on them of body size, age, sex, and cross-sectional properties. Analyses were made of a sample of 91 (66 males, 25 females) Native British Columbians (3500-1500 years BP) and 18th century Quebec prisoners. Muscle markers were measured using three-point observer rating scales; size was measured by standard methods; age and sex were determined through pelvic, cranial, and dental morphology; and cross-sectional properties were calculated from radiographs. Whereas any single muscle marker component failed to correlate with age, size, sex, or cross-sections, aggregate muscle marker correlated with: age, r = 0.49; size, r = 0.38; sex, r = 0.40; and, cross-sections, r = 0.38; P < 0.001. Older individuals had greater muscle markers, as did larger individuals, males, and those with more robust cross-sections. Based on partial correlations and regression analyses, age was the best overall predictor of aggregate muscle marker.     

Life history cost of trematode infection in Helisoma anceps using mark-recapture in Charlie's Pond

Parasitism has the potential to affect key life history traits of an infected host. Perhaps the most studied interactions are in snail-trematode systems, where infection can result in altered growth rates, survival, and/or fecundity of the individual. Positive correlations between host size and parasite prevalence are often attributed to changes in growth rates or mortality, which have been observed in the laboratory. Extending lab-based conclusions to the natural setting is problematic, especially when environmental conditions differ between the laboratory and the field. The present study uses reproduction experiments and mark-recapture methods to directly measure key life history traits of the pulmonate snail Helisoma anceps in Charlie's Pond. Based on previous laboratory and field experiments on H. anceps, we predict a significant reduction in fecundity, but not growth rate or survival, of infected snails. Individual capture histories were analyzed with multistate models to obtain estimates of survival and infection probabilities throughout the year. Recaptured individuals were used to calculate specific growth rates. Trematode infection resulted in complete castration of the host. However, neither survival nor growth rates were found to differ between infected and uninfected individuals. The probability of infection exhibited seasonal variation, but it did not vary with size of the snail. These results suggest that the correlation between host size and trematode prevalence is not due to differential mortality or changes in growth rates. Instead, the infection accumulates in large snails via the growth of smaller, infected individuals.     

How host size may constrain the evolution of parasite body size and clutch size. The parasitic isopod Ichthyoxenus fushanensis and its host fish, Varicorhinus bacbatulus, as an example

The flesh-burrowing parasitic isopod Ichthyoxenus fushanensis was found infecting the body cavity of a freshwater fish, Varicorhinus bacbatulus , in heterosexual pairs. Herein we investigate the question of how the host body size may constrain the parasite size and clutch size by analyzing the interactions among the body size and clutch size of the parasite, and host size. Due to the low transmission rate of I. fushanensis to its host and the positive relationship between clutch size and female size, selection may favor larger females with larger clutch sizes to compensate for massive losses of manca (the free-living juveniles). The path model reveals that clutch size depends not only directly on female size, but also on the sizes of her host and mate. Female size also depends on the sizes of the host and her mate. A negative correlation exists between the body sizes of the paired males and females. This negative correlation may be regarded as a consequence of competition for limited available space or other resources provided by the host. The effects of host size on parasite size, however, act on the total volume of both sexes as a whole, not specifically on either the female or the male. In this case, the available space/resources may not allow both individuals of different sexes to evolve toward a larger size simultaneously. Under the constraint of host size, a strategy of reducing the body size of the paired male may provide a way to increase the body size of the paired female and achieve a larger clutch size.     

Some consequences for a parasitic herbivore,the milkweed longhorn beetle,Tetraopes tetrophthalmus,of a host-plant shift from Asdepias syriaca to A. verticillata

Summary The impact of differences in host plants on individuals and populations of insect herbivores was investigated using the milkweed longhorn beetle, Tetraopes tetrophthalmus (Forster), larvae of which feed parasitically on host rhizomes. One host, Asclepias syriaca L., was larger in stem and rhizome diameter and grew in cooler soil than the other host, A. verticillata L. The major effects on beetles were retarded phenology at the cooler site and reduced size on the smaller host. Reduced size of beetles was correlated with several important individual attributes: reduced length of life, number of ovarioles, egg size, and a reduced probability of mating with large beetles. The population consequences of these characteristics, largely inferred from these observations, were little or no outbreeding in the small population on A. verticillata.     

Selection of large host plants for oviposition by a monophagous leaf beetle: nutritional quality or enemy‐free space?

Abstract.  1. Oviposition site selection is crucial for the reproductive success of herbivorous insects. According to the preference–performance hypothesis, females should oviposit on host plants that enhance the performance of their offspring. More specifically, the plant vigour hypothesis predicts that females should prefer large and vigorously growing host plants for oviposition and that larvae should perform best on these plants.
2. The present study examined whether females of the monophagous leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae) prefer to oviposit on large host plant individuals of the meadow clary and whether large host plants are of higher nutritional quality than small host plants. Subsequently, it was tested whether the female preference correlates with offspring performance and survival.
3. In the field, females preferred large host plant individuals for oviposition and host plant quality, i.e. leaf nitrogen content, was significantly higher in leaves of large than of small host plants.
4. In the laboratory, larval development time was shorter on leaves of large host plant individuals than on small host plant individuals, but this could not be shown in the field.
5. However, a predator-exclusion experiment in the field resulted in a higher survival of larvae on large host plants than on small host plants when all predators had free access to the plants. On caged host plants there was no difference in survival of larvae between plant size categories.
6. It is concluded that females of C. canaliculata select oviposition sites that enhance both performance and survival of their offspring, which meets the predictions of the plant vigour hypothesis.     

Monogamy does not last long in Pontonia mexicana, a symbiotic shrimp of the amber pen-shell Pinna carnea from the southeastern Caribbean Sea

Theory suggests that symbiotic species should be monogamous and form long-lasting heterosexual pairs in/on hosts when inhabiting scarce and small hosts in environments where predation risk away from hosts is high (e.g., tropical subtidal). This prediction was tested with Pontonia mexicana which inhabits the relatively small and scarce amber pen-shell Pinna carnea in the tropical Caribbean. In agreement with theory, P. mexicana were found dwelling as heterosexual pairs in the mantle cavity of pen-shell individuals with more frequency than expected by chance alone. However, additional observations suggested that male-female pairing of P. mexicana does not last long. First, males paired with females that were close to molt and become sexually receptive more frequently than expected by chance alone. In monogamous species in which pairing appears to be long lasting, males occur with females in the same host, independent of the reproductive condition of the female. Second, the body size of paired shrimps was poorly correlated and the relationship between host and shrimp body size was weak. If males and females of P. mexicana were staying within host individuals for long periods of time, a tight correlation between host and shrimp size and between males and females in a pair would have been found. Lastly, sexual dimorphism in terms of cheliped size was evident; males invested considerably more resources to this body structure compared to females. In monogamous species in which pairing appears to be long-term, sexual dimorphism is low or absent given that sexual selection is weak in this mating system. Overall, our data suggests that monogamy in P. mexicana does not last long and that males switch among host individuals in search of receptive females. Manipulative experiments are necessary to understand the conditions favoring short- and long-term monogamy in symbiotic crustaceans.