Juvenile three-spined sticklebacks avoid parasitized conspecifics

Synopsis Juvenile three-spined sticklebacks,Gasterosteus aculeatus, were given a series of four choice tests to determine whether they avoided schools of conspecifics in which individuals were parasitized with the ectoparasiteArgulus canadensis. Results from these tests indicate that juvenile sticklebacks can avoid schools of parasitized conspecifics. Furthermore, parasites alone did not elicit an avoidance response, suggesting that it is both the presence of the parasite and its effect on stickleback behavior that causes avoidance of parasitized individuals.     

The contribution of post-copulatory mechanisms to incipient ecological speciation in sticklebacks

Ecology can play a major role in species diversification. As individuals are adapting to contrasting habitats, reproductive barriers may evolve at multiple levels. While pre-mating barriers have been extensively studied, the evolution of post-mating reproductive isolation during early stages of ecological speciation remains poorly understood. In diverging three-spined stickleback ecotypes from two lakes and two rivers, we observed differences in sperm traits between lake and river males. Interestingly, these differences did not translate into ecotype-specific gamete precedence for sympatric males in competitive in vitro fertilization experiments, potentially owing to antagonistic compensatory effects. However, we observed indirect evidence for impeded development of inter-ecotype zygotes, possibly suggesting an early stage of genetic incompatibility between ecotypes. Our results show that pre-zygotic post-copulatory mechanisms play a minor role during this first stage of ecotype divergence, but suggest that genetic incompatibilities may arise at early stages of ecological speciation.     

Colour perception in three-spined sticklebacks: sexes are not so different after all

Sexual selection theory implies a tight coupling between the evolution of male sexual display and the sensory capabilities of the female. In sexually dimorphic species it is proposed that this might lead to sex differences in a species’ perceptive abilities. However, supporting evidence for this is rare, and to date there is only one example; three-spined sticklebacks, Gasterosteus aculeatus. Here, the female’s visual system is reported to become more red-sensitive during the summer breeding season; a time when sexually mature males display a red throat and belly to potential mates. In contrast, a shift in sensitivity is not apparent in males. These results, although commonly quoted, are surprising because previous observations suggest that both sexes may benefit from the detection of the male’s red colour patch; in females the intensity of red coloration can influence the choice of mate, and in males the conspicuous red colouration can aid the detection of rival males. To investigate this paradox we repeated the original optomotor experiment using a fully controlled design. In contrast to the earlier result, we found that both males and females exhibit a shift in their sensitivity to red during the reproductive period. These new data therefore do not support the hypothesis that sex differences in perceptual abilities occur in sexually dimorphic species.     

Shoaling behaviour of sticklebacks infected with the microsporidian parasite, <Emphasis Type="Italic">Glugea anomala</Emphasis>

We compared the shoaling behaviour of three-spined sticklebacks, Gasterosteus aculeatus, infected with the microsporidian, Glugea anomala, to that of non-infected conspecifics. Infected fish lost significantly more weight than non-infected fish during a period of food deprivation, suggesting a metabolic cost to parasitism. In binary shoal choice tests, non-infected test fish showed an association preference for a shoal of non-infected over a shoal of infected conspecifics; infected test fish displayed no preference. Infected fish, however, showed a higher overall tendency to shoal than non-parasitised fish. Furthermore, infected fish occupied front positions within a mixed school. We consider the behavioural differences between infected and uninfected fish in the context of their potential benefits to the fish hosts and the parasites.     

Food selection in juvenile three-spined sticklebacks studied in relation to size,abundance and biomass of prey

The food selection and diet of juvenile three-spined sticklebacks (Gasterosteus aculeatus L.) was studied in the littoral of a brackish-water sea area. The sampling stations were located in a eutrophicated area, where the plankton was characterized by rotifers, in a noneutrophicated zone, where copepods predominated, and in the intermediate zone, characterized by cladocerans. In all areas the fish preferred microcrustaceans to rotifers, but they did not feed strictly according to the size-selection hypothesis. Nor did they take the most abundant prey species. With some exceptions the species chiefly eaten by and also selected by fish, were those which had the largest biomass in the plankton.     

Effects of freshwater and marine overwintering environments on life histories of threespine sticklebacks: evidence for adaptive variation between anadromous and resident freshwater populations

Summary Life history theory predicts that migratory fishes should delay reproduction, be larger at first reproduction, and have higher fecundities than nonmigrants. We tested this hypothesis by comparing life histories of anadromous (estuary) and resident freshwater (upstream) threespine sticklebacks (Gasterosteus aculeatus L.) from the Navarro River, California, USA. Using a split-brood, two-environment breeding design, families from cach population were divided and reared in both freshwater and seawater overwintering environments. In both treatments, the more migratory estuary sticklebacks were larger at first reproduction and had large initial clutch sizes; in the freshwater treatment, the estuary sticklebacks matured later than the upstream fish. Population means varied little across treatments, indicating that the average effects of the different overwintering conditions were slight. The responses of individual families to a given overwintering treatment were highly variable in both populations, as reflected in significant family x treatment effects for all traits. Phenotypic correlations among life history traits were significant and positive for most traits, and were similar in magnitude in both populations. Differences in the relative degree of specialization for migration may in part explain variation in life history between these populations.     

Introduced Siberian chipmunks are more heavily infested by ixodid ticks than are native bank voles in a suburban forest in France

By serving as hosts for native vectors, introduced species can surpass native hosts in their role as major reservoirs of local pathogens. During a 4-year longitudinal study, we investigated factors that affected infestation by ixodid ticks on both introduced Siberian chipmunks Tamias sibiricus barberi and native bank voles Myodes glareolus in a suburban forest (Forêt de Sénart, Ile-de-France). Ticks were counted on adult bank voles and on adult and young chipmunks using regular monthly trapping sessions, and questing ticks were quantified by dragging. At the summer peak of questing Ixodes ricinus availability, the average tick load was 27-69 times greater on adult chipmunks than on adult voles, while average biomass per hectare of chipmunks and voles were similar. In adult chipmunks, individual effects significantly explained 31% and 24% of the total variance of tick larvae and nymph burdens, respectively. Male adult chipmunks harboured significantly more larvae and nymphs than adult females, and than juveniles born in spring and in summer. The higher tick loads, and more specifically the ratio of nymphs over larvae, observed in chipmunks may be caused by a higher predisposition - both in terms of susceptibility and exposure - to questing ticks. Tick burdens were also related to habitat and seasonal variation in age- and sex-related space use by both rodents. Introduced chipmunks may thus have an important role in the dynamics of local vector-borne pathogens compared with native reservoir hosts such as bank voles.     

Variation in the reproductive potential of Schistocephalus infected male sticklebacks is associated with 11-ketotestosterone titre

Parasites can impact host reproduction by interfering with host endocrine systems, but the adaptive nature of such effects is disputed. Schistocephalus solidus plerocercoids are parasites of three-spined sticklebacks Gasterosteus aculeatus that are often associated with impaired host reproduction. Here, we relate reproductive behavior and physiology to levels of the androgen 11-ketotestosterone (11KT) in naturally infected and non-infected male sticklebacks from two UK populations. In one population infected males harbored heavy infections and showed uniformly reduced 11KT titres and kidney spiggin (nesting glue protein) content compared to non-infected fish. However in a second population infection levels were more variable and males with smaller infections recorded 11KT and spiggin titres that overlapped those of non-infected fish; among infected males from this population 11KT and kidney spiggin also both correlated negatively with infection severity. Male reproductive behavior correlated closely with 11KT titre in both populations, and infected males with high 11KT levels exhibited normal reproductive behavior. Our results suggest that Schistocephalus infection per se does not block reproductive development in male sticklebacks, and that some male fish may have the ability to breed whilst infected. Our results are not consistent with the hypothesis that Schistocephalus adaptively castrates male hosts via endocrine disruption; rather they support the hypothesis that reproductive disruption is a side effect of the energetic costs of infection.