Exploitation of asymmetric predator–prey interactions by trophically transmitted parasites

The directionality of asymmetric interactions between predators (definitive hosts) and prey (intermediate hosts) should impact trophic transmission in parasites. This study tests the prediction that trophically transmitted parasites are funneled towards asymmetric predator–prey interactions where intermediate hosts have few predators and definitive hosts feed upon many prey (‘downward asymmetry’). The distribution of trophically transmitted parasites was examined in four published food webs in relation to mismatch asymmetry of predator–prey interactions. We found that trophically transmitted parasites exploit downwardly asymmetric interactions in a nonrandom manner, and particular predator–prey pairs contain more trophically transmitted parasites than would be expected by random chance alone. These findings suggest that food web topology has great bearing on the ecology of trophically transmitted parasites, and that consideration of parasite life cycles in the context of food web organization can provide insights into the forces affecting the evolution of trophic transmission.     

Vertebrate evolution: recent perspectives from fish

Recent progress in understanding the evolution of vertebrate genomes has been rapid, and previous notions that all such genomes could be regarded as equivalent in their gene content have been rendered outdated. This notion, often embodied in the representation that vertebrates possess four Hox complexes, still appears in contemporary textbooks of developmental biology. Recent data from the genomes of teleost fish show that this assumption is untrue and suggest that interesting situations might arise from the apparent proliferation of genes among fish.     

Density and distribution of the ringed seal in the Bothnian Bay

A modified strip census of basking ringed seals in the Bothnian Bay was carried out during the last week of April and the first week of May 1988. Of the total ice covered area, 23174 km², 3236 km² (14%) was covered by the transects. The mean density in the area was 0.101 ringed seals km^-2, with substantial regional and local variation. Highest densities were found in the compact drift ice area. The estimated total population of ringed seals on the ice was 2093 in the Bothnian Bay proper and 248 in the North Quark. The 95% confidence limits of the estimates are ±24%.     

Vegetation development on the outer islands of the Bothnian Bay

This paper summarizes succession studies on the outer islands of the Bothnian Bay (Finland). The study area is characterized by consistent and relatively rapid uplift (vertical rise on average 75 cm per century). The spatial and temporal succession has been deduced from the sequence of the littoral and epilittoral vegetation as well as from the historical records of the island development.Four types of successional series are distinguished. The relationships between substrate type (boulder, gravel, sand and clay-silt shores) and 25 typical plant communities are outlined and treated as successional sequences.     

The trophic vacuum and the evolution of complex life cycles in trophically transmitted helminths

Parasitic worms (helminths) frequently have complex life cycles in which they are transmitted trophically between two or more successive hosts. Sexual reproduction often takes place in high trophic-level (TL) vertebrates, where parasites can grow to large sizes with high fecundity. Direct infection of high TL hosts, while advantageous, may be unachievable for parasites constrained to transmit trophically, because helminth propagules are unlikely to be ingested by large predators. Lack of niche overlap between propagule and definitive host (the trophic transmission vacuum) may explain the origin and/or maintenance of intermediate hosts, which overcome this transmission barrier. We show that nematodes infecting high TL definitive hosts tend to have more successive hosts in their life cycles. This relationship was modest, though, driven mainly by the minimum TL of hosts, suggesting that the shortest trophic chains leading to a host define the boundaries of the transmission vacuum. We also show that alternative modes of transmission, like host penetration, allow nematodes to reach high TLs without intermediate hosts. We suggest that widespread omnivory as well as parasite adaptations to increase transmission probably reduce, but do not eliminate, the barriers to the transmission of helminths through the food web.     

Cystidicola farionis as a swimbladder parasite of the whitefish in the Bothnian Bay

Material comprising 3167 specimens of the parasite Cystidicola farionis was assembled from 1741 individuals of the sea-spawning whitefish, Coregonus nasus in the shallow northeastern and deeper central areas of the Bothnian Bay. Occasional parasites were found in the immature fish (≦ 175 mm), but infection increased in the two larger classes as the fish aged, reaching 27% in the northeastern area, and as much as 67% in the southern area. This difference is explained partly by the more frequent feeding on Pontoporeia affinis , the intermediate host for the parasite, the deeper waters extending nearer the coast in the majority of the central and southern Bothnian Bay. The increased intensity of infection with age does not vary significantly between the areas studied. No clear seasonal fluctuations in the prevalence or intensity of infection could be found.     

Metechinorhynchus salmonis infection and diet in the river-spawning whitefish of the Bothnian Bay

A total of 206 river-spawning whitefish Coregonus lavaretus (L.) from the Bothnian Bay was investigated for food objects and acanthocephalans in the years 1975–78. The only acanthocephalan species found was Metechinorhynchus salmonis (Müller 1780), the total number of which was 1295.
The whitefish is shown to be highly susceptible to infection by M. salmonis , with as many as 88·9% of specimens infected in a sample in June 1978, although the mean intensity was only 8·9 parasites per host. The river-spawning whitefish does not favour the intermediate host, Pontoporeia affinis , in its summer diet in the Bothnian Bay preferring to feed on eggs and molluscs, so that the incidence of M. salmonis infection decreases to 33·3% by the middle of summer. This is further reduced in the late summer by the fast associated with the migration to their upriver spawning grounds. The fact that the other acanthocephalan inhabiting the river-spawning whitefish in the southern Gulf of Bothnia, Echinorhynchus gadi Müller, 1776 is not to be found in the Bothnian Bay population suggests that the fish captured in spring have remained in the Bothnian Bay throughout the winter.     

Migratory behaviour of adult cisco, Coregonus albula L., in the Bothnian Bay

The population of adult cisco in the Bothnian Bay consists of a number of spawning populations. Each one of these returns to the same inshore area for spawning every autumn. In the summer these populations are mixed and occur mainly offshore. These are the main conclusions of an experiment in which cisco were marked with Carlin-type tags. The cisco were caught mainly in fyke nets both in the summer and in the autumn and released after tagging in the area where they had been caught. Fish tagged in the autumn and recaptured in the autumn one or more years later showed a strong affinity to the point of release, while fish from the same group recaptured in the summer had migrated far from the release site. The distribution of cisco that were tagged in the summer and caught in the autumn showed no correlation with the release site.     

Further observations on flagellates within sea ice in northern Bothnian Bay, the Baltic Sea

To improve our knowledge of flagellates inhabiting the Baltic Sea ice and water column during the winter, material was obtained from northern Bothnian Bay in March/April 1995. Light microscopical observations on live and fixed material and further transmission electron microscopy of whole mounts revealed 47 nanoflagellate taxa. In addition, detached scales of eight taxa were encountered. It is now evident that nearly all nanoflagellate classes are present within Bothnian Bay sea ice. The most common groups were cryptomonads, dinoflagellates, chrysophytes, prasinophytes, choanoflagellates and heterotrophic flagellates of unknown systematic position (Protista incertae sedis). Most flagellates in Baltic Sea ice biota apparently thrive in both the water column and the sea ice, while some, e.g. Paraphysomonas spp. (Chrysophyceae), heterotrophic euglenids, volvocalean chlorophytes, and some taxa of uncertain systematic affinity, are more frequently found within the sea ice. Received: 9 February 1997 / Accepted: 23 September 1997     

A comparative study of helminth parasites from the fish Tilapia zillii and Oreochromis leucostictus in Lake Naivasha and Oloidien Bay,Kenya

The parasitic fauna of two fish species, namely gill-netted samples of 652 Oreochromis leucostictus and 448 Tilapia zillii from Lake Naivasha and Oloidien Bay was investigated during the period from the end of October 1995 to September 1996. Five larval helminth parasites were recovered including the nematode, Contracaecum sp., the acanthocephalan Polyacanthorhynchus kenyensis, the digenetic trematode, Clinostomum sp. and two cestodes, Amirthalingamia sp. and Cyclustera sp. Both prevalence and intensity of the infection of these helminths increased in larger sized fish, whereas male fish were more heavily infected than females. No seasonality in infection level were observed. The health status of both fish species remained unaffected, although O. leucostictus from Oloidien Bay which harboured heavy infections of Contracaecum exhibited stuntedness and the lack of fatty deposits around the digestive caecum.     

Interference and the persistence of vertically transmitted parasites

Given their ubiquity in nature, understanding the factors that allow the persistence of multiple enemies and in particular vertically transmitted parasites (VTPs) is of considerable importance. Here a model that allows a virulent VTP to be maintained in a system containing a host and a horizontally transmitted parasite (HTP) is analysed. The method of persistence relies on the VTP offering the host a level of protection from the HTP. The VTP is assumed to reduce the HTPs ability to transmit to the host through ecological interference. We show that VTPs are more likely to persist with HTPs that prevent host reproduction than with those that allow it. The VTP persists more easily in r-selected hosts and with highly transmittable HTPs. As the level of protection through interference increases the densities of the host also increase. We also show that VTPs when they do persist tend to stabilise the host population cycles produced by free-living HTPs. The study raised questions about persistence of diseases through interactions with others, and also the stabilising effects of VTPs on dynamical systems in a biological control context.     

Evolution of early male-killing in horizontally transmitted parasites

Early male-killing (MK) bacteria are vertically transmitted reproductive parasites which kill male offspring that inherit them. Whereas their incidence is well documented, characteristics allowing originally non-MK bacteria to gradually evolve MK ability remain unclear. We show that horizontal transmission is a mechanism enabling vertically transmitted bacteria to evolve fully efficient MK under a wide range of host and parasite characteristics, especially when the efficacy of vertical transmission is high. We also show that an almost 100% vertically transmitted and 100% effective male-killer may evolve from a purely horizontally transmitted non-MK ancestor, and that a 100% efficient male-killer can form a stable coexistence only with a non-MK bacterial strain. Our findings are in line with the empirical evidence on current MK bacteria, explain their high efficacy in killing infected male embryos and their variability within and across insect taxa, and suggest that they may have evolved independently in phylogenetically distinct species.     

The spread of fecally transmitted parasites in socially-structured populations

Mammals are infected by a wide array of gastrointestinal parasites, including parasites that also infect humans and domesticated animals. Many of these parasites are acquired through contact with infectious stages present in soil, feces or vegetation, suggesting that ranging behavior will have a major impact on their spread. We developed an individual-based spatial simulation model to investigate how range use intensity, home range overlap, and defecation rate impact the spread of fecally transmitted parasites in a population composed of social groups (i.e., a socially structured population). We also investigated the effects of epidemiological parameters involving host and parasite mortality rates, transmissibility, disease-related mortality, and group size. The model was spatially explicit and involved the spillover of a gastrointestinal parasite from a reservoir population along the edge of a simulated reserve, which was designed to mimic the introduction pathogens into protected areas. Animals ranged randomly within a "core" area, with biased movement toward the range center when outside the core. We systematically varied model parameters using a Latin hypercube sampling design. Analyses of simulation output revealed a strong positive association between range use intensity and the prevalence of infection. Moreover, the effects of range use intensity were similar in magnitude to effects of group size, mortality rates, and the per-contact probability of transmission. Defecation rate covaried positively with gastrointestinal parasite prevalence. Greater home range overlap had no positive effects on prevalence, with a smaller core resulting in less range overlap yet more intensive use of the home range and higher prevalence. Collectively, our results reveal that parasites with fecal-oral transmission spread effectively in socially structured populations. Future application should focus on parameterizing the model with empirically derived ranging behavior for different species or populations and data on transmission characteristics of different infectious organisms.     

Sexually transmitted parasites and host mating behavior in the decorated cricket

Sexually transmitted diseases play a potentially important rolein the ecology and evolution of host mating behavior. Here,we use a sexually transmitted nematode-cricket (Mehdinema alii–Gryllodessigillatus) system to examine the effects of parasitism on hostmating activity and female choice. Previous work has shown thatinfected male crickets produce a significantly smaller nuptialgift (spermatophylax) than uninfected males. This is expectedto result in reduced spermatophylax feeding duration and earlyampulla removal. Here, we hypothesize that the parasite-mediatedreduction in spermatophylax size will consequently shorten femaleintercopulatory interval. We predict that females mated to infectedmales will exhibit a shorter intercopulatory interval than femalesmated to uninfected males. To test this hypothesis, we experimentallymeasured the behavioral responses of females mated to uninfectedand infected males. We found no significant difference betweenfemale handling of the spermatophylax and ampulla from infectedversus uninfected males. Although the duration of spermatophylaxconsumption is positively correlated with the duration of ampullaattachment, neither of these variables is correlated with femaleintercopulatory interval. Intercopulatory intervals for femalespreviously mated with uninfected versus infected males are notstatistically different. We conclude that parasitism in maleG. sigillatus does not influence female intercopulatory intervalor male mating success. We found no evidence for female matechoice based on male infection status. The lack of female choiceis consistent with theoretical predictions involving parasitesthat are sexually transmitted.     

Requirements for carotenoids in fish diets

Carotenoids are among the most widespread and important pigment classes in living organisms, and in marine animals, astaxanthin is the most commonly occurring red carotenoid. Carotenoids are vitamin A precursors and are fundamental in photosynthesis and light protection in plants. Increasing attention has been drawn to a possible light protection, cancer prevention and immune enhancement by carotenoids in mammals. Reported functions in fish range from a general enhancement of performance to specific functions in reproduction and metabolism. In this paper, we show that astaxanthin is essential for growth and survival of fish and crawfish, and discuss this fact in relation to the use of purified and semipurified diets in nutritional studies. The similarity in action of astaxanthin and canthaxanthin compared to α-tocopherol (vitamin E) and retinol (vitamin A) suggests that these two carotenoids should be listed among the fat soluble vitamins.