The transmission dynamics of the cercariae of Diplostomum spathaceum were investigated under laboratory conditions using cercariae collected from naturally infected Lymnaea stagnalis. Cercariae were kept in a constant temperature of 20 degrees C and the survival and infectivity to naïve young rainbow trout recorded at 3-h intervals until few cercariae were alive. Mortality initially remained constant but increased rapidly after 20 h. While a model of constant mortality fitted the survival data, an age-dependent model provided a better fit and implied that cercariae tended to carry similar quantities of resources and once these were exhausted the cercariae died. Cercarial infectivity also showed an age-dependent pattern although infectivity tended (P = 0.09) to increase with age over the first 6 h of life and then fall. The per capita transmission rate of cercariae was investigated by experimentally infecting rainbow trout under standardized conditions, first with an increasing cercarial density and second, by keeping density constant but increasing numbers of cercariae. The per capita transmission rate was frequency dependent and averaged 0.341/h (+/- 0.036).

No abstract

A central issue in fish production is the influence of parasitic infections on fish health in fish farms and on survival prospects after fish-stocking into the wild. Is it preferable to produce fish in infection-free conditions or to allow some infection to elicit resistance in fish? We explored the infection of totally naive and previously infected rainbow trout (Oncorhynchus mykiss) by the trematode parasite Diplostomum spathaceum in laboratory conditions. We found that up to 86.1% of the presented cercariae became established in the lenses of naïve fish, which indicated that these fish were highly susceptible to infection and that innate resistance served little or no protection against the parasite. However, acquired resistance after initial infection decreased parasite establishment by 85.0-89.1%. Parasite establishment was also affected by fish host density as fewer parasites were found in fish in higher densities. The implications of these results for the fish-farming industry and fish-stocking protocols are discussed.

A hormone manipulation was performed to examine the effects of testosterone on basal and b-glucan-induced immune functions in wild-caught male and female tench Tinca tinca. Testosterone administration elevated testosterone concentration in plasma, but did not suppress lytic activity of plasma or the chemiluminescence response of blood or head kidney phagocytes in any of the three successive samples or in any of the treatment groups. Both testosterone and b-glucan administrations had a negative effect on the relative mass of the spleen, and testosterone-treated fish lost more mass than control fish. Males had a relatively larger spleen than females, but there were no gender differences in immune function. b-glucan-administration did not affect the plasma testosterone concentration. # 2005 The Fisheries Society of the British Isles

An important factor affecting the life-history of an organism is parental investment in reproduction: reproductive decisions are almost invariably costly. Therefore, reproductive decisions should be beneficial in terms of increased offspring number or fitness. For example, egg laying decisions in many insects can influence resource availability of the offspring through changes in the larval density, and resource availability will have effects on many life-history traits. Here we studied whether female reproductive decisions affect offspring fitness in Callosobruchus maculatus seed beetles. Females laid more eggs on black-eye beans than on mung beans. However, when the difference in the surface area of the beans was accounted for, the number of eggs was not higher in black-eye beans. This together with the poisson distribution of eggs on each of the bean types suggests that females tend to lay their eggs randomly. We found that development time was longer, larval mortality lower and adult survival higher in black-eye beans. We also found interactions between bean type and larval density on size of the offspring such that in mung beans the emergence mass and pronotum width decreased with increasing larval density, but in black-eye beans larval density did not affect the size measures. We conclude that when there is a risk that larval denisty will become high within a bean and there is variable resources available, there exist clear benefits that females might obtain by choosing black-eye beans as a resource for their offspring. However, in contrast to many earlier studies, our results suggest that females may not be making any active oviposition decisions. Therefore, to unequivocally determine whether females do capitalise the potential benefits by active decision making, some further experimentation is required.