Contents volume 8 1989

Contents volume 8 1989     

The effect of hypoxia on the vulnerability of guppies (Poecilia reticulata,Poeciliidae) to an aquatic predator (Astronotus ocellatus,Cichlidae)

Synopsis When dissolved oxygen concentration was near saturation in a laboratory experiment, guppies that spent (a) more time at the surface and (b) more time moving had a higher probability of being captured by a predatory cichlid fish. With decreasing oxygen concentration surface time and percent time moving increased, but prey risk decreased. In addition, the qualitative correlates of risk changed; under hypoxic conditions, predation risk was lower for prey which spent more time at the surface and in motion. Thus, dissolved oxygen concentration influences both quantitative and qualitative aspects of risk from water-breathing predators.Author to whom reprint requests should be addressed.     

Variation among genotypes in responses to increasing temperature in a marine parasite: evolutionary potential in the face of global warming?

Climates are changing worldwide, and populations are under selection to adapt to these changes. Changing temperature, in particular, can directly impact ectotherms and their parasites, with potential consequences for whole ecosystems. The potential of parasite populations to adapt to climate change largely depends on the amount of genetic variation they possess in their responses to environmental fluctuations. This study is, to our knowledge, the first to look at differences among parasite genotypes in response to temperature, with the goal of quantifying the extent of variation among conspecifics in their responses to increasing temperature. Snails infected with single genotypes of the trematode Maritrema novaezealandensis were sequentially acclimatised to two different temperatures, ‘current’ (15 °C) and ‘elevated’ (20 °C), over long periods. These temperatures are based on current average field conditions in the natural habitat and those predicted to occur during the next few decades. The output and activity of cercariae (free-swimming infective stages emerging from snails) were assessed for each genotype at each temperature. The results indicate that, on average, both cercarial output and activity are higher at the elevated acclimation temperature. More importantly, the output and activity of cercariae are strongly influenced by a genotype-by-temperature interaction, such that different genotypes show different responses to increasing temperature. Both the magnitude and direction (increase or decrease) of responses to temperature varied widely among genotypes. Therefore, there is much potential for natural selection to act on this variation, and predicting how the trematode M. novaezealandensis will respond to the climate changes predicted for the next century will prove challenging.     

Major determinants of the occurrence of a globally invasive parasite in riverine fish over large-scale environmental gradients

The increased rate of outbreaks of infectious diseases in ecosystems is a dramatic consequence of global change, particularly when outbreaks affect important resources such as freshwater fish. However, the links between disease-inducing epizootics and widespread human impacts, including nutrient pollution and high water conductivity, in freshwater organisms are largely unexplored. We used data from extensive surveys in northeastern Spain (99,700 km², 15 river catchments, n = 530 sites) to explore the environmental factors that singly, or in combination, are likely to influence the occurrence of the invasive parasite, Lernaea cyprinacea, after accounting for host fish characteristics. Smaller fish, lower altitudes, higher water conductivity and nutrient pollution were associated with higher probabilities of infection in 19 endemic and widely distributed fish species. We found no evidence that interactive effects among riverine stressors related to water and physical habitat quality better explained the probability of occurrence of L. cyprinacea in fish than did additive-stressor combinations. Nutrient pollution and high water conductivity were two of the major factors contributing to the increased occurrence of L. cyprinacea. Therefore, the improvement of wastewater treatment processes and agricultural practices probably would help to reduce the occurrence of this parasite among native fish.     

Trajectories of participation in organized activities and outcomes in young adulthood

To examine whether trajectories of participation in organized activities during the high school years and beyond (from ages 14 to 20) predicted outcomes at age 21 (externalizing problems, internalizing problems, civic engagement, number of years of education, and perception of physical health), 354 youths (60% girls) were surveyed annually over nine years. Four trajectories were found: (a) “Low and decreasing” (71%), (b) “Moderate and stable” (12%), (c) “High and decreasing after high school” (12%), and (d) “High and increasing after high school” (5%). Results revealed that the predicted outcomes varied according to the different trajectories. Pursuing high levels of activity participation beyond high school was especially beneficial with respect to externalizing problems and educational attainment.     

Body size vs abundance among parasite species: positive relationships?

Across species, abundance usually correlates negatively with body size. This intuitive pattern may result from size-dependent resource requirements in habitats where only finite amounts of resources are available. Among parasite species, it is possible that some resource limitations arc less severe than for free-living animals although this may depend on the type of parasites. The interspecific relationship between body size and abundance (measured as prevalence and intensity of infection) among parasites was tested in two groups of parasites. Among helminth endoparasites of fish, parasite body size correlated positively with prevalence and negatively with intensity of infection. Among copepod ectoparasites of fish, body size correlated positively with both prevalence and intensity. These trends were observed after controlling for the confounding influences of phylogeny and sampling effort. These contrasting patterns may result from the more intense link between body size and intensity-dependent regulation in endoparasites than in ectoparasites. The results of this comparative analysis suggest that parasite body size could be an important factor determining aspects of parasite abundance and distribution, including aggregation.