Timing of Infections in the Threespine Stickleback (Gasterosteus aculeatus) by Schistocephalus solidus in Alaska.

This study provides direct evidence for the timing of infections by Schistocephalus solidus in the threespine stickleback (Gasterosteus aculeatus) of south-central Alaska. Young-of-the-year fish in Cheney Lake were infected during their first summer within a few months after hatching in May-June. Infections appear to continue under ice cover on the lake during the subsequent fall and winter. Few, if any, 1-yr-old fish seemed to be infected for the first time, although 1-yr-old hosts with established parasites apparently acquired additional infections.

Life-history plasticity in female threespine stickleback.

The postglacial adaptive radiation of the threespine stickleback fish (Gasterosteus aculeatus) has been widely used to investigate the roles of both adaptive evolution and plasticity in behavioral and morphological divergence from the ancestral condition represented by present-day oceanic stickleback. These phenotypes tend to exhibit high levels of ecotypic differentiation. Population divergence in life history has also been well studied, but in contrast to behavior and morphology, the extent and importance of plasticity has been much less well studied. In this review, we summarize what is known about life-history plasticity in female threespine stickleback, considering four traits intimately associated with reproductive output: age/size at maturation, level of reproductive effort, egg size and clutch size. We envision life-history plasticity in an iterative, ontogenetic framework, in which females may express plasticity repeatedly across each of several time frames. We contrast the results of laboratory and field studies because, for most traits, these approaches give somewhat different answers. We provide ideas on what the cues might be for observed plasticity in each trait and, when possible, we inquire about the relative costs and benefits to expressed plasticity. We end with an example of how we think plasticity may play out in stickleback life history given what we know of plasticity in the ancestor.

Rapid shifts in multiple life history traits in a population of threespine stickleback.

Measurement of the rate of phenotypic or genetic change provides data bearing on many questions of fundamental interest to biologists, including how fast changes can proceed, whether shifts occur gradually or in bursts and how long high rates of change can be sustained. Because traits exist in functionally and genetically correlated suites, studies tracking many traits are likely to be the most informative. We quantify very rapid phenotypic changes in egg size (now smaller), clutch size (larger) and the age/size of both breeding females and males (younger, smaller) in an Alaskan population, with these traits shifting at rates from 0.13 to 0.30 haldanes over a 10-year period. In contrast, female reproductive effort and the allometric relationship of clutch size to body size changed little. These shifts appear to be caused by an altered selective landscape, with the presumed selective agent being increasing lake productivity. Some of the traits undoubtedly have at heritable component and thus represent genetic evolution as well as phenotypic.

Host mortality and variability in epizootics of Schistocephalus solidus infecting the threespine stickleback, Gasterosteus aculeatus.

SUMMARY: An analysis of the metrics of Schistocephalus solidus infection of the threespine stickleback, Gasterosteus aculeatus, in Walby Lake, Alaska, showed that an epizootic ended between 1996 and 1998 and another occurred between 1998 and 2003. The end of the first epizootic was associated with a crash in population size of the stickleback, which serves as the second intermediate host. The likely cause of the end of that epizootic is mass mortality of host fish over winter in 1996-1997. The deleterious impact of the parasite on host reproduction and increased host predation associated with parasitic manipulation of host behaviour and morphology to facilitate transmission might also have played a role, along with unknown environmental factors acting on heavily infected fish or fish in poor condition. The second epizootic was linked to relatively high levels of prevalence and mean intensity of infection, but parasite:host mass ratios were quite low at the peak and there were no apparent mass deaths of the host. A number of abiotic and biotic factors are likely to interact to contribute to the occurrence of epizootics in S. solidus, which appear to be unstable and variable. Epizootics appear to depend on particular and, at times, rare sets of circumstances.

Influence of the pseudophyllidean cestode Schistocephalus solidus on oocyte development in the threespine stickleback Gasterosteus aculeatus.

The objective of this study was to investigate the means by which Schistocephalus solidus might reduce annual fecundity in female threespine stickleback fish (Gasterosteus aculeatus) through processes of oocyte development. Histological examinations of specimens from one lake in Alaska in 2000 and 2001 were used to analyse the effects of S. solidus on recruitment of primary growth oocytes into vitellogenesis, atresia of vitellogenic oocytes, and the interspawning interval. The ratio of primary growth to late secondary growth (late vitellogenic) oocytes was significantly greater (P<0.01) among infected fish than uninfected ones in early-season samples from 2000 and 2001, revealing a decrease in recruitment of oocytes from primary growth into vitellogenic oocytes among infected females. The difference was marginally non-significant (P=0.087) in a mid-season sample from 2001 due to reductions in the entire pool of vitellogenic (early and late secondary growth) oocytes recruited prior to the spawning season in this determinate-fecundity species. Atresia among all vitellogenic oocytes was low and did not differ between infected and uninfected females. Histological estimations of the interspawning interval using post-ovulatory follicles showed no significant differences between infected and uninfected fish, suggesting that the number of spawnings in stickleback females each spawning season is unaffected by S. solidus infection. Thus, annual fecundity appears to be reduced only through recruitment of oocytes into vitellogenesis.

Interspawning interval of wild female three-spined stickleback Gasterosteus aculeatus in Alaska.

The interspawning interval, or spawning frequency, of wild three-spined stickleback, Gasterosteus aculeatus, was estimated using histological examination of postovulatory follicles (POF). Females in Alaskan lakes appeared to have as much as a 48 h delay between ovulation and ovoposition, yet the POF method could still be used to estimate the interspawning interval. In two Alaskan lakes the interspawning interval was estimated to range from 2.2 to 7.8 days among individual female G. aculeatus. These estimates were consistent with the range (2.5 to 5 days) of previous estimates among individual females from laboratory observations of spawning G. aculeatus, as well as anecdotal accounts of spawning intervals reported from wild populations in Canada (5-10 days). The interspawning interval of females increased during the course of the spawning season in Alaska, showing that the majority of female spawning activity occurred during the earliest portion of the approximate 6-week reproductive season. The increased interspawning interval appears to be related to a previously reported decrease in body condition in reproductive females during the breeding season. Thus, female G. aculeatus may be unable to sustain the initial rate of reproduction as energy stores that support the rapid growth of vitellogenic oocytes are depleted.