Infection dynamics of Cichlidogyrus tilapiae and C. sclerosus (Monogenea, Ancyrocephalinae) in Nile tilapia (Oreochromis niloticus L.) from Uganda.

The infection dynamics of the gill monogeneans Cichlidogyrus tilapiae and C. sclerosus on Oreochromis niloticus with respect to habitat type (reservoir, stream, ponds and cages), host sex, size and seasons was determined between January and November 2008. During the study period, 45.2% of the 650 fish examined were infected with Cichlidogyrus spp. The infected hosts harboured an average of 8.6 ± 3.4 parasites/fish. Across habitat types, the proportion of infected fish was not statistically different. In contrast, the number of parasites recorded on infected fish from different habitat types differed significantly. The highest parasite number was recorded in reservoir-dwelling fish and lowest in stream-dwelling hosts. Concerning sex, more female O. niloticus were infected and harboured a high number of parasites than male and sexually undifferentiated fish. A weak negative relationship was found between rainfall and monthly parasite infections. However, a higher number of parasites and proportion of infected hosts were found during dry than in wet seasons, except in ponds. Results of this study show that differential exposure due to changes in fish behaviour associated with habitat modification and sex may account for the infection difference across the sampled sites. Meanwhile, rainfall and the associated hydrological events are important factors regulating monogenean infections in tropical aquatic environments. The continuous presence of Cichlidogyrus spp. in fish provides evidence of possible parasite outbreaks, indicating the application of biosecurity measures as crucial for the success of intensive fish farming.

Niche dimensions in fishes: an integrative view.

Current shifts in ecosystem composition and function emphasize the need for an understanding of the links between environmental factors and organism fitness and tolerance. The examples discussed here illustrate how recent progress in the field of comparative physiology may provide a better mechanistic understanding of the ecological concepts of the fundamental and realized niches and thus provide insights into the impacts of anthropogenic disturbance. Here we argue that, as a link between physiological and ecological indicators of organismal performance, the mechanisms shaping aerobic scope and passive tolerance set the dimensions of an animal's niche, here defined as its capacity to survive, grow, behave, and interact with other species. We demonstrate how comparative studies of cod or killifish populations in a latitudinal cline have unraveled mitochondrial mechanisms involved in establishing a species' niche, performance, and energy budget. Riverine fish exemplify how the performance windows of various developmental stages follow the dynamic regimes of both seasonal temperatures and river hydrodynamics, as synergistic challenges. Finally, studies of species in extreme environments, such as the tilapia of Lake Magadi, illustrate how on evolutionary timescales functional and morphological shifts can occur, associated with new specializations. We conclude that research on the processes and time course of adaptations suitable to overcome current niche limits is urgently needed to assess the resilience of species and ecosystems to human impact, including the challenges of global climate change.

Dynamics and predicted decline of Anguillicola crassus infection in European eels, Anguilla anguilla, in Neusiedler See, Austria.

The eel population in Neusiedler See has been maintained by regular massive stocking since 1958. After the establishment of the National Park Neusiedler See-Seewinkel in 1993, eel stocking was prohibited and the population, together with the specific parasites of eels, was predicted to decline to extinction within 10 years. This investigation was undertaken to document the decline and extinction of the Anguillicola crassus population in eels. From 1994 to 2001, 720 eels were collected from two sites in the lake. Prevalence and abundance of A. crassus were lower in spring than in summer and autumn and larger eels harboured more parasites than smaller ones. Neither year of study nor sampling site were correlated with parasite infection levels. No significant trend in the population parameters of A. crassus was detected over the 8 years of the survey. This suggested that there had been no significant decline in the eel population. This suggestion was confirmed by investigations of the fishery, which also found evidence of regular illegal stocking. The stability of the A. crassus population over the past decade seems to reflect the lack of change in eel population density. No mass mortalities of eels occurred over the period despite the many similarities between Neusiedler See and Lake Balaton in Hungary. Differences in eel size, eel diet and the lack of large-scale insecticide use are discussed as possible explanations for the absence of eel mass mortalities in Neusiedler See.

Long-term investigation of the composition and richness of intestinal helminth communities in the stocked population of eel, Anguilla anguilla, in Neusiedler See, Austria.

Data from a long-term study of the intestinal helminth parasite community of eels, Anguilla anguilla, stocked into the shallow eutrophic Neusiedler See, Austria, were collected over an 8 year period (1994-2001). In total, 720 eels from 2 sampling sites were examined. The parasite community showed characteristics similar to those in the natural eel populations in rivers of the UK and mainland Europe: it was species poor, with only 5 species (Acanthocephalus lucii, Acanthocephalus anguillae, Raphidascaris acus, Proteocephalus macrocephalus, Bothriocephalus claviceps) comprising the component community and a maximum infracommunity richness of 4 species. Over the period, the intestinal parasite community of the sampling site in Illmitz, which was originally dominated by A. lucii, changed. As levels of A. anguillae increased to a point at which it dominated the community, diversity increased whilst dominance of a single species decreased. By contrast the community in the southern sampling site remained rather constant with a continuous high infection level of A. anguillae and low abundance of A. lucii. Both acanthocephalan species exhibited higher infection levels in larger eels and in different seasons of the year and the infection parameters were significantly different between the years of study. The significant differences in the infection levels of the 2 acanthocephalan species at the 2 sampling sites were surprising as both acanthocephalan species use the same intermediate host, Asellus aquaticus, and the sampling sites were in close proximity and were similar in terms of water quality, host size and invertebrate abundance. Differences in the fish communities of the 2 sampling sites and eel movements rather than interspecific competition are discussed as possible explanations for the differences in the parasite communities of the 2 sampling sites.

Fish macroparasites as indicators of heavy metal pollution in river sites in Austria.

This paper describes two approaches to evaluate the use of fish macroparasites as bioindicators of heavy metal pollution at selected river stretches in Austria. Firstly changes in the diversity and richness of endoparasites of the cyprinid barbel, Barbrus barbuls (L.), were tested in relation to heavy metal contents in the aquatic system. Secondly, the bioaccumulation potential of cadmium, lead and zinc was assessed in the acanthocephalan, Pomphorhynchus laevis (Miller, 1776), and compared with that in the muscle, liver and intestine of its barbel host. The present results indicated that in order to validate the role of parasite community patterns related to heavy metal pollution, more investigations on food web dynamics, interelationships between parasites and the presence/absence of intermediate hosts will be essential. Heavy metal concentrations differed significantly between the organs of barbel and P. laevis (P=0.001) with levels up to 2860 fold in the parasite. The high level of heavy metal accumulation in P. laevis compared with that in its barbel host, suggests that despite variability in the parasite infrapopulation, host mobility and feeding behaviour, P. laevis is a most sensitive indicator of heavy metals in aquatic ecosystems.

Endoparasitic helminths of the European eel, Anguilla anguilla, from four disconnected meanders from the rivers Leie and Scheldt in western Flanders, Belgium.

The endoparasitic helminth communities of the European eel, Anguilla anguilla (L.), were investigated in four meanders, cut off from the rivers Leie and Scheldt in western Flanders, Belgium. Six species of helminths (2 cestodes, 2 nematodes and 2 acanthocephalans) were found. The dominant parasite species was the nematode Anguillicola crassus (Kuwahara, Niimi et Itagaki, 1974) infecting 79% of the eel population with intensities up to 112 specimens per fish. At two localities no acanthocephalans could be found, whereas these parasites were very common at the other sites. The prevalence, mean intensity, intensity and abundance, their correlation to the body length, and the frequency distributions were analysed. The site selection of parasites is in relation to food composition and feeding habits of eels, physiological and structural differences in the intestine and possible interspecific competition were discussed.

Food dependence and energetics of freeliving nematodes : II. Life history parameters of Caenorhabditis briggsae (Nematoda) at different levels of food supply.

The food dependence of larval duration, fecundity and the intrinsic rate of natural increase follow a hyperbolic form, which can for the former be described by the Michaelis-Menten function.Maximal larval duration at 20° C is 62 h, maximal fecundity is 153 eggs per female and r max is 1.136 per day. The lower food threshold is 108 E. coli cells·ml-1 (=0.06 mg dry weight·ml-1) for larval growth and 2·108 cells·ml-1 for reproduction and "r". 50% of maximal performances (K s ) are attained at 5·108 and 7.5·108 cells·ml-1 respectively.Reproductive effort at dense food is highest immediately after maturation (e.g. 50% of the total eggs produced by a female are laid within 2 days after onset of egg production). At lower food densities the reproductive effort is delayed.Larval mortality increases strongly below 109 cells·ml-1.The results reported sofar were obtained with E. coli cells which were harvested at the phase of decreasing population growth in batch cultures. With cells from the exponential and the stationary phase, performances are increased and decreased respectively. This is partly due to differences in bacterial biomass per unit cell, partly an expression of the change of nutritive value of bacterial cells with growth phases.

Food dependence and energetics of freeliving nematodes : I. Respiration, growth and reproduction of Caenorhabditis briggsae (Nematoda) at different levels of food supply.

Some bioenergetic parameters of Caenorhabditis briggsae, a saprophagous nematode, were analysed under different conditions of food availability. Respiration (R) and production rates (P) of experimental animals grown on media of defined bacterial concentrations were measured throughout the life cycle of the species at 20°C. Energetics are expressed in the form of instantaneous rates and as cumulative budgets. 1. Food dependence: The food threshold of the species is defined as A (assimilation)=R, P=zero. The respiratory level of the species is generally high compared to other nematode species and increases only weakly with food availability. Starvation (food densities below threshold) is expressed in a strong reduction in metabolism within 48 h. The food dependence of biosynthetic processes (body growth and egg production) follows a hyperbolic form, which can be described by the Michaelis-Menten function. The relationship P:R changes drastically with availability of food, e.g. the production efficiency for the period of maximal reproduction is 0% at 2·108 cells ml-1 (threshold) and 86% at 1010 cells ml-1. 2. R and P follow different forms of size dependence in the course of the life cycle. The relationship between R and body weight (W) can be described by an allometric function, e.g. at high food density, R=2.8 W 0.75 (R in nl O2 ind-1 h-1; W in µg fresh weight). Weight-specific production rates vary considerably during the life cycle: ±constant in the early larval phase (exponential growth, "g"=1.44 day-1 at 1010 cells ml-1); decreasing in the latter larval phase; peak values shortly after onset of reproduction as a result of both body growth and egg production. 3. Differences in resource allocation at varying food densities are also manifest in cumulative energy budgets, e.g. higher R cum is necessary to achieve the same body size at lower food densities. Size at maturation and egg size are reduced to a different degree at low food densities, indicating bioenergetical constraint and trade-off between metabolic processes.

Proteolytic activity in the digestive tract of several species of fish with different feeding habits.

Proteolytic activity in the gut contents of two cichlids and six cyprinids from an artificial basin in Sri Lanka was measured using a simple film strip method. This comparative study contributes to our general knowledge of digestion in herbivorous fish: 1) Specific proteolytic activity (per ml of gut content) is lower in herbivorous than in omnivorous and carnivorous species. 2) Specific proteolytic activity is negatively correlated with the relative length of the gut, but the time of exposure of ingested food to proteolytic enzymes rises with increasing gut length. This results in more intensive proteolytic digestion in herbivorous fish. 3) Proteolytic enzymes seem to be "reabsorbed" in the hindgut of the fish. The effectiveness of this mechanism rises up to a relative gut length of 2.5-3.0. However, the small species Amblypharyngodon melletinus does not fit this relationship.

A bioenergetic study of a benthic nematode, plectus palustris de man 1880, throughout its life cycle : II. Growth, Fecundity and Energy Budgets at Different Densities of Bacterial Food and General Ecological Considerations.

Growth and reproduction of the parthenogenetic freshwater nematode,Plectus palustris, were studied at different controlled levels of food densities at 20° C. A bacteria-sloppy agar mixture was used as substrate and food medium. No growth or reproduction occurred at the lowest food density (8.107 bacterial cells ml-1). At 8.108 cells ml-1, the larval duration was 18.5 days, the instantaneous growth rate (g) of young larvae 0.2 d-1 and the daily fecundity rate during a prolonged period of constant egg production 12.6 eggs·d-1. At a food density of 8.109 cells ml-1, the corresponding values are 12.5 days, 0.4 d-1 and 37.7 eggs d-1.By including the data on respiration from a previous paper (Klekowski et al., 1979), the energetics of the species at different food densities can be discussed: production processes are apparently more dependent on food supply than respiration. However, prolongation of the larval phase in lower food densities greatly increases the cumulated respiratory costs per unit production. A second point is the ability to produce smaller-sized primiparous females in sub-optimal food which shortens the immature life period and serves to reduce the burden of cumulated metabolic costs for attaining sexual maturity.A comparison of the range of food densities used in the experiments with bacterial densities known from lake sediments of different trophic type suggests that food is likely to be the main factor governing the population dynamics of bacterivorous species under field situations.

A bioenergetic study of a benthic nematode, Plectus palustris de man 1880, throughout its life cycle : I. The respiratory metabolism at different densities of bacterial food.

Respiration rates of the bacterivorous freshwater nematode Plectus palustris were measured during the whole life cycle of the species and for animals grown at two food densities. Covariance analysis showed that small, but significant differences exist in the elevation of the respiration rate-body weight regressions (R=aW b, in nl O2/ind·h and µg wet weight) for different food densities. At a food density of 6-9·108 bacterial cells/ml the level of respiration is 14% lower compared to rates of animals cultured at a ten times higher food density. However, the allometric function, R-aW b, adequately describes the relationship of respiration and body weight only during the larval growth phase and for young females, while respiration rates of newly hatched larvae and mature females at maximal egg production have lower metabolic rates. Cumulated metabolic costs to attain a certain age, size and stage of development have been determined and are used in a subsequent paper (Schiemer et al., 1979) to calculate the energy budgets of the species.