Knut Helge Jensen

Empirical support for optimal virulence in a castrating parasite.

The trade-off hypothesis for the evolution of virulence predicts that parasite transmission stage production and host exploitation are balanced such that lifetime transmission success (LTS) is maximised. However, the experimental evidence for this prediction is weak, mainly because LTS, which indicates parasite fitness, has been difficult to measure. For castrating parasites, this simple model has been modified to take into account that parasites convert host reproductive resources into transmission stages. Parasites that kill the host too early will hardly benefit from these resources, while postponing the killing of the host results in diminished returns. As predicted from optimality models, a parasite inducing castration should therefore castrate early, but show intermediate levels of virulence, where virulence is measured as time to host killing. We studied virulence in an experimental system where a bacterial parasite castrates its host and produces spores that are not released until after host death. This permits estimating the LTS of the parasite, which can then be related to its virulence. We exposed replicate individual Daphnia magna (Crustacea) of one host clone to the same amount of bacterial spores and followed individuals until their death. We found that the parasite shows strong variation in the time to kill its host and that transmission stage production peaks at an intermediate level of virulence. A further experiment tested for the genetic basis of variation in virulence by comparing survival curves of daphniids infected with parasite spores obtained from early killing versus late killing infections. Hosts infected with early killer spores had a significantly higher death rate as compared to those infected with late killers, indicating that variation in time to death was at least in part caused by genetic differences among parasites. We speculate that the clear peak in lifetime reproductive success at intermediate killing times may be caused by the exceptionally strong physiological trade-off between host and parasite reproduction. This is the first experimental study to demonstrate that the production of propagules is highest at intermediate levels of virulence and that parasite genetic variability is available to drive the evolution of virulence in this system.

Variation in phenoloxidase activity and its relation to parasite resistance within and between populations of Daphnia magna

Estimates of phenoloxidase (PO) activity have been suggested as a useful indicator of immunocompetence in arthropods, with the idea that high PO activity would indicate high immunocompetence against parasites and pathogens. Here, we test for variation in PO activity among clones of the planktonic crustacean Daphnia magna and its covariation with susceptibility to infections from four different microparasite species (one bacterium and three microsporidia). Strong clonal variation in PO activity was found within and among populations of D. magna, with 45.6% of the total variation being explained by the clone effect. Quantitative measures of parasite success in infection correlated negatively with PO activity when tested across four host populations. However, these correlations disappeared when the data were corrected for population effects. We conclude that PO activity is not a useful measure of resistance to parasites or of immunocompetence within populations of D. magna. We further tested whether D. magna females that are wounded to induce PO activity are more resistant to infections with the bacterium Pasteuria ramosa than non–wounded controls. We found neither a difference in susceptibility nor a difference in disease progression between the induced group and the control group. These results do not question the function of the PO system in arthropod immune response, but rather suggest that immunocompetence cannot be assessed by considering PO activity alone. Immune response is likely to be a multifactorial trait with various host and parasite characteristics playing important roles in its expression.

Environmental enrichment promotes neural plasticity and cognitive ability in fish

Different kinds of experience during early life can play a significant role in the development of an animals behavioural phenotype. In natural contexts, this influences behaviours from anti-predator responses to navigation abilities. By contrast, for animals reared in captive environments, the homogeneous nature of their experience tends to reduce behavioural flexibility. Studies with cage-reared rodents indicate that captivity often compromises neural development and neural plasticity. Such neural and behavioural deficits can be problematic if captive-bred animals are being reared with the intention of releasing them as part of a conservation strategy. Over the last decade, there has been growing interest in the use of environmental enrichment to promote behavioural flexibility in animals that are bred for release. Here, we describe the positive effects of environmental enrichment on neural plasticity and cognition in juvenile Atlantic salmon (Salmo salar). Exposing fish to enriched conditions upregulated the forebrain expression of NeuroD1 mRNA and improved learning ability assessed in a spatial task. The addition of enrichment to the captive environment thus promotes neural and behavioural changes that are likely to promote behavioural flexibility and improve post-release survival.

Fish kairomone regulation of internal swarm structure in Daphnia pulex (Cladocera: Crustacea)

In this laboratory experiment, swarms of D. pulex were artificially created by placing thirty individuals in small chambers containing 130 ml of water. The swimming behaviour of the animals was measured using a video camera and a programme to digitize observations. The D. pulex swimming in water with fish kairomones had a more uniform swimming speed compared with animals in control water. If aggregated prey individuals benefit from a reduced predation risk, and this risk is further reduced by uniformity of swarm members, the uniformity of swimming speed can be interpreted as an behavioural adjustment to minimize the vulnerability to predation.

A characterization of the ZFL cell line and primary hepatocytes as in vitro liver cell models for the zebrafish (Danio rerio)

The zebrafish (Danio rerio) is a widely used model species in biomedical research. The ZFL cell line, established from zebrafish liver, and freshly isolated primary hepatocytes from zebrafish have been used in several toxicological studies. However, no previous report has compared and characterized these two systems at the level of gene expression. The aim of this study was to evaluate the ZFL cell line in comparison to primary hepatocytes as in vitro models for studying effects of environmental contaminants in zebrafish liver. Using quantitative real-time PCR, the basal level and transcriptional induction potential of key genes involved in toxic responses in the ZFL cell line, primary hepatocytes and whole liver from zebrafish were compared. The study showed that the ZFL cells have lower levels of mRNA of most selected genes compared to zebrafish liver. The induced gene transcription following exposure to ligand was much lower in ZFL cells compared to zebrafish primary hepatocytes at the doses tested. Importantly, oestrogen receptor and vitellogenin genes showed low basal transcription and no induction response in the ZFL cell line. In conclusion, it appears that primary hepatocytes are well suited for studying environmental contaminants including xenoestrogens, but may show large sex-dependent differences in gene transcription. The ZFL cell line shows potential in toxicological studies involving the aryl hydrocarbon receptor pathway. However, low potential for transcriptional induction of genes in general should be expected, especially notable when studying estrogenic responses.

Effects of habitat enrichment and food availability on the foraging behaviour of juvenile Atlantic Cod (Gadus morhua L)

The environment can play an important role in shaping how an animal behaves, and how well the animal performs in a particular environment can be influenced by early experiences. The tradition of releasing captive-reared juveniles into the wild in an effort to strengthen wild fish populations has often had little success owing to high post-release mortality. Fish reared under standard hatchery conditions are provided with fewer stimuli and they receive excess quantities of pellet food that are easy to handle and consume. Captive reared fish therefore appear to be under-stimulated and overfed. Several studies have demonstrated that simple structural enrichment in the rearing facilities promotes flexible behaviour compared to fish reared in plain, standard hatchery tanks. Less attention has been given to the effects of the diet. Here we use a cross-factored design to test the relative role of food ration and spatial enrichment on foraging behaviour. Our results show that fish from enriched environments, regardless of previous food-ration size, were more reluctant to start feeding on the first day in a novel arena. On day two and three, however, fish with prior experience of a low food ration showed greater foraging activity and efficiency than fish fed on full rations. On the second and third day, prior experience with enrichment was less important. We discuss how early feeding experience in combination with structural enrichment may contribute in producing fish that are better suited for release into the wild.

Who benefits from reduced reproduction in parasitized hosts? An experimental test using the Pasteuria ramosa-Daphnia magna system.

We investigated whether parasites or hosts benefit from reduced reproduction in infected hosts. When parasites castrate their hosts, the regain of host reproduction is necessary for castration to be a host adaptation. When infecting Daphnia magna with Pasteuria ramosa, in a lake water based medium, 49 2% of the castrated females regained reproduction. We investigated the relationship between castration level, and parasite and host fitness proxies to determine the adaptive value of host castration. Hosts which regained reproduction contained less spores and had a higher lifetime reproduction than permanently castrated hosts. We also found a negative correlation between parasite and host lifetime reproduction. For hosts which regained reproduction we found no optimal level of castration associated with lifetime reproduction. These results support the view that host castration only is adaptive to the parasite in this system. In addition, we suggest that permanent castration might not be the norm under natural conditions in this system. Finally, we argue that a reduction in host reproduction is more likely to evolve as a property favouring parasites rather than hosts. To our knowledge this is the only experimental study to investigate the adaptive value of reduced host reproduction when castrated hosts can regain reproduction.

Size Dependent Male Reproductive Tactic in the Two-Spotted Goby (Gobiusculus flavescens)

Male investment in testes and sperm duct gland in the polygamous nest breeding two-spotted goby Gobiusculus flavescens (Fabricius) was investigated in relation to time in reproductive season and individual physical parameters. This small teleost fish is most likely the most abundant species found along the rocky shores of the North East Atlantic. The two-spotted goby has a single reproductive season, during which nest-caring males can raise several clutches of offspring. According to the literature the males are on average larger than the females. Here we report for the first time a population showing a reversal of this trend, with males on average being smaller than females, a difference likely caused by a large proportion of small males. Early in the breeding season these small males have typical sneaker characters, with relatively large testes and small seminal duct glands compared to the larger dominant territorial males. The presence of these two alternative male reproductive tactics is confirmed by histological studies, which shows the presence of sperm in the sperm duct glands (SDG) of smaller males, but not in the SDG of intermediate and larger males. To our knowledge, males with typical sneaker characters have not been reported in earlier studied populations of two-spotted goby. Interestingly we found that testes investment declined significantly over the course of the breeding season, and that this reduction was significantly more pronounced in small compared to the large males. Further, a significant increase in seminal duct gland (SDG) mass was observed for the smaller males over the breeding season. We propose that this indicates a possible shift in mating tactic by smaller males from a parasitic to a nest-holding tactic over the course of the breeding season. Thus, the observed size dependent plasticity in investment in SDG over time suggests that the reproductive tactic of G. flavescens is conditional, and possibly influenced by mate availability and male—male competition.

How important is light in the aggregation behaviour of Daphnia pulex (Cladocera: Crustacea)?

The influence of light on the aggregation behaviour of Daphnia pulex was tested in a laboratory experiment. The animals were kept in a circular tank containing 16 l of water and the degree of aggregation was estimated by Lloyds Patch Index. By varying the degree of competition for food and light conditions (homogeneous light intensity at the water surface; a gradient in light intensity at the water surface; total darkness), it was possible to test the hypothesis that light has to be present as a proximate factor in the formation of swarms in D. pulex. As the daphnids formed swarms in total darkness, the hypothesis was rejected. However, under a gradient in light intensity at the water surface, the behaviour of daphnids differed from behaviour under the other light regimes by aggregating irrespective of food level, indicating that light is important for aggregation.

Duration of the parasitic phase determines subsequent performance in juvenile freshwater pearl mussels (Margaritifera margaritifera)

Abstract Host–parasite systems have been useful in understanding coevolutionary patterns in sympatric species. Based on the exceptional interaction of the long‐lived and highly host‐specific freshwater pearl mussel (FPM; Margaritifera margaritifera) with its much shorter‐lived host fish (Salmo trutta or Salmo salar), we tested the hypotheses that a longer duration of the parasitic phase increases fitness‐related performance of mussels in their subsequent post parasitic phase, and that temperature is the main factor governing the duration of the parasitic phase. We collected juvenile mussels from naturally and artificially infested fish from eight rivers in Norway. Excysted juvenile mussels were maintained separately for each collection day, under similar temperature and food regimes, for up to 56 days. We recorded size at excystment, post excystment growth, and survival as indicators of juvenile fitness in relation to the duration of the parasitic phase. We also recorded the daily average temperatures for the entire excystment period. We observed strong positive relationships between the length of the parasitic phase and the post parasitic growth rate, size at excystment and post parasitic survival. Temperature was identified as an important factor governing excystment, with higher temperatures decreasing the duration of the parasitic phase. Our results indicate that juvenile mussels with the longest parasitic phase have better resources (larger size and better growth rate) to start their benthic developmental phase and therefore to survive their first winter. Consequently, the parasitic phase is crucial in determining subsequent survival. The temperature dependence of this interaction suggests that climate change may affect the sensitive relationship between endangered FPMs and their fish hosts.