Host–parasite interactions and competition between tubificid species in a benthic community

1. Parasites can be important determinants of host community structure while host community structure can influence the success of parasites, although both are often overlooked. In two laboratory experiments, we examined interactions among Myxobolus cerebralis syn Myxosoma cerebralis Höfer, the myxozoan parasite that causes salmonid whirling disease, and two coexisting tubificid species: Tubifex tubifex (Müller), which is the alternate host of the parasite, and Limnodrilus hoffmeisteri Claparède, which is not susceptible. In the first experiment, we examined T. tubifex infection prevalence when exposed to nine doses of spores. In the second experiment, we examined tubificid and parasite success under three spore doses when tubificids were combined in a response surface experimental design used to detect interactions among species. 2. The outcomes of interactions between tubificid species were complex. The number and biomass of offspring of both tubificid species were density dependent when in monoculture or in combination with the other species. Adult growth of T. tubifex was also density dependent in monoculture, but when L. hoffmeisteri replaced one‐half of the T. tubifex in the high‐density treatment, adult growth of T. tubifex was higher than in monoculture. Adult growth of L. hoffmeisteri was always density independent. Whether T. tubifex was exposed to the parasite or not did not change the outcome of these interactions. However, adult growth of T. tubifex, but not L. hoffmeisteri, was highest when M. cerebralis was present. 3. Infection prevalence of T. tubifex increased with increasing spore dose. Infection prevalence was lowest in the high‐density T. tubifex monoculture and highest in the low‐density T. tubifex monoculture and when T. tubifex was in combination with L. hoffmeisteri. 4. Both intraspecific and interspecific competition influenced tubificid success, but T. tubifex gained some competitive advantage through increased adult growth when in combination with L. hoffmeisteri. Whether T. tubifex was exposed to the parasite or not did not change the outcome of the interactions between the tubificid species. 5. The presence of L. hoffmeisteri did not decrease the prevalence of infection in T. tubifex, suggesting that parasite success was unaltered by the presence of this non‐susceptible species.     

A comparison of two tubificid oligochaete species as candidates for sublethal bioassay tests relevant to subtropical and tropical regions

Two tubificid oligochaetes, Branchiura sowerbyi and Tubifex tubifex, were evaluated as potential test organisms for sediment bioassays. We attempt to reproduce the sediment bioassay proposed by Reynoldson et al. (1991) using his strain of Tubifex tubifex and his reference sediment and to compare this technique using Branchiura sowerbyi. This species was chosen because it is more common and dominant in tropical and subtropical environments than Tubifex tubifex. Data on survival and reproduction were obtained for both species, and growth estimates were obtained for B. sowerbyi. The sublethal bioassay with T. tubifex confirmed earlier estimates of a test duration of 4 weeks at 22.5 °C. B. sowerbyi cultures can produce usable estimates in 21 days at 30 °C.     

Ecology and production of Salvinia natans Hoffim; in Kashmir

Summary Ecological problems arising as a result of rapid growth of S. natans Hoffim in some lakes of Kashmir have been studied. High viability of spores coupled with rapid vegetative propagation seem to be responsible for the spread of the weed. Lake channels protected from wind by a canopy of trees are the most favourable situations in which Salvinia produces a thick mat. Water in these channels is rich in nutrients. Oxygen poor layers of water beneath the mat are detrimental for insect and fish life. Besides, the changing light conditions have profound influence on the submerged vegetation. The most common associates of Salvinia are Hydrocharis dubia and Ceratophyllum demersum.Maximum plant biomass is recorded during August–September with higher efficiency at partially shaded sites. Correlation exists between caloric values and dry matter production and between caloric value and per cent organic carbon.Under laboratory conditions, water of a pH between 4.6–6.0, sodium chloride concentrations higher than 1 % and continuous submergence under water are harmful to the growth of the weed. Partial shady situations seem to be quite beneficial for the growth. Harvesting would probably be the most economical method of control.     

Countering morphological ambiguities: development of a PCR assay to assist the identification of <Emphasis Type="Italic">Tubifex tubifex</Emphasis> oligochaetes

The freshwater oligochaete, Tubifex tubifex, is a common resident of organic-rich sediments worldwide. Although it is a familiar species to fish enthusiasts, toxicologists and parasitologists, T. tubifex often confounds definitive identification due to the similarity that immature specimens bear to several other common oligochaetes, and given the degree of plasticity of key morphological characters due to environmental conditions and/or age of specimen. To solve this identity crisis, we used a polymerase chain reaction based molecular approach and developed a T. tubifex specific assay with primers that amplify a 192 bp fragment of the internal transcribed spacer region 1 ribosomal DNA. We tested these primers on four T. tubifex mitochondrial genotypes, and on other oligochaete species from nine genera. The primers amplified all specimens identified morphologically asT. tubifex. They did not amplify any other species, including morphologically similar worms possessing hair chaetae (Dero digitata, Ilyodrilus templetoni, Tubifex ignotus or Rhyacodrilus spp.) or other oligochaetes often found with T. tubifex (Lumbriculus variegates, Limnodrilus hoffmeisteri, Stylodrilus heringianus or Trichodrilus sp.). This technique should remove the uncertainties all too often associated with identification of T. tubifex.     

A morphological reappraisal of Tubifex blanchardi Vejdovský, 1891 (Clitellata: Tubificidae)

Tubifex blanchardi Vejdovský, 1891 is a freshwater tubificid, often living in sympatry with Tubifex tubifex (Müller 1774). Although considered from its discovery as a species on its own, its biological status is debated. During the early seventies T. blanchardi was reduced to a mere form of T. tubifex, as a particular case of polymorphism in chaetal pattern. Using classical histological techniques, microdissections of portions of the male genital apparatus and phalloidin staining of dissected copulatory organs we investigated 163 mixed individuals of T. blanchardi and T. tubifex belonging to sympatric populations from the Lambro River (Milan, Northern Italy). The internal morphology of T. blanchardi is described for the first time. Our results show that T. tubifex and T. blanchardi differ in several characters concerning both their external and internal morphology, and in the fine organization of their copulatory organs. Several independent character sets support the separation of T. blanchardi from T. tubifex, suggesting that it is an independent species. This study also supports the idea that T. blanchardi and T. bergi (Hrabě, 1935), another species closely related to T. tubifex, are not conspecific. The observed morphological differences between allopatric populations of T. tubifex are discussed.     

Physiological response of Spisula subtruncata (da Costa, 1778) to different seston quantity and quality

Individuals of the bivalve Spisula subtruncata were fed a mixed diet comprising of sea water enriched with the diatom Phaeodactylum tricornutum and ashed silt within a range of concentrations, simulating natural conditions above pseudofaeces threshold. The designed ranges for total particulate matter were between 10 and 30 mg l^–1and organic content of seston 15–40%. Filtration rate, rejection rate, ingestion rate and absorption rate were measured at those different conditions. Filtration rate and rejection rate were significantly correlated to total particulate matter and percentage of organic matter, with higher rates at higher values of total particulate matter and lower values of percentage organic matter. Ingestion rate was maintained at similar levels in all the treatments and organic enrichment of the ingested food occurred due to preingestive selection of the filtered material. A differential absorption rate occurred at different levels of organic matter in the diet with high rates at high values of the organic content of the diet. S. subtruncata showed different physiological responses to changes of the food conditions: (1) Increase of pseudofaeces production at increasing levels of particulate matter, (2) preingestive selection of organic material which enriched the organic fraction of ingested food, (3) stabilized ingestion rate and (4) increase of the absorption rate at high organic levels of the seston.     

Observations on cohorts of Tubifex tubifex cultured at different food levels,using cellulose substrate

In order to clarify the relationship between food availability and population dynamics in aquatic oligochaetes, short term cultures and cohort cultures of Tubifex tubifex were reared using substrates with different food concentrations, obtained by mixing sand with different amounts of cellulose powder.In short term experiments, T. tubifex seemed capable of utilizing cellulose substrates for growth or egg production. Yet, in the cohort experiment, newly hatched worms could not grow on cellulose substrates. Survivorship, however, appears to be influenced by cellulose concentration giving an indication that this material, although inadequate, is somehow utilized.     

Mutualistic relationships between cohabiting tubificid species

Results from experiments with the three species constituting the dominant tubificid community of eutrophic lakes in Scandinavia, i.e. Tubifex tubifex, Limnodrilus hoffmeisteri and Potamothrix hammoniensis, suggested the involvement of mutualistic mechanisms. Previously Wavre & Brinkhurst, 1971; Brinkhurst, 1974 and McMurtry et al., 1983, described phenomena of this kind within a North American species complex involving T. tubifex and L. hoffmeisteri, as well.In mixed cultures, in comparison with monocultures of the respective species, I found that growth rate increased in the order of 50% and sexual maturity was reached earlier. Especially the three-species combinations rapidly increased in weight. Experiments with choice chambers, containing sediments enriched with the faecal pellets of each species alone or in species combinations, revealed specific reactions by the worms. As an average of three replicates, each starting with 45 recently hatched worms in equal proportions of the three species mentioned above, 14 chose the faeces of L. hoffmeisteri, 11 chose P. hammoniensis, 2 chose T. tubifex, 4 chose a combination of T. tubifex and L. hoffmeisteri and 4 chose blanks. Another 10 worms did not succeed in entering the choice chambers from the sandy substrate into which all worms were introduced and rapidly died off.The results seem to support the hypothesis that increased growth rate achieved in mixed cultures is due to each species selectively grazing concentrations of bacteria associated with the faeces of another species in the community. Earlier findings have shown that each species characteristically tends to cultivate only one species-specific bacterial strain at a time in its intestines (Wavre & Brinkhurst, 1971).Since the above species combination is by far the most frequently occurring oligochaete community in eutrophic lakes in Scandinavia and monocultures of any of the species involved are fairly uncommon under natural conditions, one may speculate that mixed cultures better utilize common available resources in accordance with experimental results.     

Preliminary investigations on the utilization of tubificid worms by postlarvae of Macrobrachium lanchesteri (de Man)

Postlarvae of Macrobrachium lanchesteri were stocked in confined waters. The larvae were fed on an ad libitum food of Tubifex tubifex worms for 30 days and their rearing has been established. 1 Kg of initially stocked postlarvae need 4.5 Kg of live T. tubifex to produce 0.537 Kg of new flesh. The worms were found to increase the crude protein content of larval flesh by 17% of the dry matter.     

The influence of substrate organic content on the growth of a stream chironomid

Stream detritus and Tipula (Diptera) feces were wet-sieved to a 63–250 µm particle size range and mixed in three proportions with ashed sand of the same size to yield food substrates of varying quality. Growth of fourth-instar Stictochironomus annulicrus (Townes) (Diptera: Chironomidae) larvae was used to indicate whether the more nutritious particles were selected from these mixtures. Respective mean relative growth rates on the substrates, in order of decreasing percentage organic matter (i.e. food quality), were: 0.0229, 0.0104 and 0.0004 mg·mg^–1d^–1. Replicates of the relatively high quality substrate consistently elicited the highest growth rates; animals grew at intermediate rates on medium quality, and slowest on low quality substrates. Thus, this collector does not appear to have selected for food quality under the described experimental conditions.     

Growth Rate of the Microalga Tetraselmis suecica Changes the Biochemical Composition of Artemia Species

The impact of different microalgal semicontinuous cultures on growth and biochemical composition in the next link of the food chain was tested using the filter feeder Artemia species as a model. The marine microalga Tetraselmis suecica was cultured semicontinuously with renewal rates between 10% and 50% and used to feed Artemia. Microalgal cultures maintained with a low renewal rate that had biochemical composition similar to that of the stationary-phase cultures commonly used in aquaculture produced poor growth and survival and low food-conversion efficiency compared to cultures maintained with a high renewal rate. Changes in the renewal rate in microalgal cultures also resulted in important changes in the gross biochemical composition of the filter feeder. The gross biochemical composition of the Artemia resembled that of the microalgae used as food except for total lipid content. The percentage of protein in the organic fraction of Artemia increased from 45% to 65% of the organic weight with increasing renewal rates in the microalgal cultures, while the carbohydrate percentage decreased under the same conditions. Higher renewal rates resulted in higher lipid percentages in the microalga, but in Artemia the percentage of lipids decreased from 19% of the organic weight with a renewal rate of 10%, to 13% with a renewal rate of 50%. The percentage of all polyunsaturated fatty acids in Artemia, including 20:5n-3, increased slightly with increasing renewal rates in the microalgal cultures. Results emphasize the importance of controlling microalgal nutritional value for the success of aquaculture food chains in which filter feeders are involved. Received October 15, 2000; accepted December 29, 2000.     

The occurrence of oligochaetes in lake chains receiving pulp mill waste and their relation to eutrophication on the trophic scale

Earlier conceptions of oligochaetes as indicators of trophy were somewhat enlarged to include more littoral, meiofaunal and naidid species. The amplitudes of different species on the oligotrophy-eutrophy axis seem to be generally large. Certain species which probably originally occur mainly in very humic environments seem also to be able to endure very strong organic loading caused by pulp mills. Species of this kind seem to be Dero digitata, Specaria josinae, Slavina appendiculata and Aulodrilus pluriseta. Chaetogaster langi and Arcteonais lomondi are also species which live in areas receiving pulp mill wastes. A. lomondi and D. digitata seem to be species living in greater depths than the naidids generally. Tubifex tubifex densities as high as 600000 ind · m^-2 were found near a pulp mill when the meiofauna method was used. A third of the T. tubifex were found deeper than 5 cm from the sediment surface. S. josinae and S. appendiculata may be found down to depths between 5 and 10 cm.     

The Influence of Environmental Factors and Microhabitat Availability on the Distribution of an Aquatic Oligochaete Assemblage in a Mediterranean River Basin

Some aquatic oligochaete species, mainly certain naidids and tubificids, are often associated with polluted environments, with high levels of organic matter and oxygen deficit. However, in the stations sampled in the Guadaira river basin, which can be arranged on an axis of organic pollution, the main aquatic oligochaete species (Nais elinguis and Tubifex tubifex) inhabiting these environments do not show a distribution pattern significantly related to the water physico-chemical characteristics measured. Nor, have we registered any association between these species and any kind of microhabitat, defined here: water current, presence of different types of vegetation, substrate coarseness and presence of anoxic sediments. Paranais litoralis is the exception, being related to areas without water current, located at higher altitudes, with high levels of dissolved organic matter and an anoxic substrate made up of fine particles. These results can be explained by the highly fluctuating nature of Mediterranean ecosystems and by the marked generalist character and ubiquity of the oligochaete assemblage species, able to colonize any kind of environment.     

Tubifex tubifex as a link in food chain transfer of hexachlorobenzene from contaminated sediment to fish

Sediments contaminated with poorly water-soluble organic chemicals pose a risk to aquatic food chains. Sediment-associated chemicals can be accumulated by endobenthic, sediment-ingesting invertebrates. Some tubificid species – or other benthic annelids – serve as food for benthivorous fish, which thereby ingest the sediment-borne chemicals and may accumulate contaminant concentrations far higher than from water exposure only, and transfer them to organisms of higher trophic levels. For measurement of biomagnification, a sediment based food chain was developed and established in the laboratory. The two-step food chain included the sediment-dwelling freshwater oligochaete Tubifex tubifex (Müller) as a representative species of benthic infauna. The three-spined stickleback (Gasterosteus aculeatus, Linné), a small teleost fish which often feeds primarily on benthic invertebrates, served as a model predator. Spiked artificial sediment and reconstituted water as the overlying medium were used. Experiments were performed using ¹⁴C-labelled hexachlorobenzene, a hydrophobic pollutant as a model compound. To examine the influence of benthic prey on the bioaccumulation of the test substance in the predator, fish were exposed to spiked water, spiked sediment, pre-contaminated prey organisms, or combinations of these exposure routes. The results of these experiments indicate that for hexachlorobenzene, the presence of contaminated Tubifex tubifex as a food source in combined exposure leads to significantly higher accumulation in fish than exposure to single pathways.     

Dietary restriction in two rotifer species: the effect of the length of food deprivation on life span and reproduction

According to resource allocation theory, animals face a trade off between the allocation of resources into reproduction and into individual growth/maintenance. This trade off is reinforced when food conditions decline. It is well established in biological research that many animals increase their life span when food is in suboptimal supply for growth and/or reproduction. Such a situation of reduced food availability is called dietary restriction. An increase in life span under dietary restricted conditions is seen as a strategy to tolerate periods of food shortage so that the animals can start reproduction again when food is in greater supply. In this study, the effect of dietary restriction on life span and reproduction in two rotifer species, Cephalodella sp. and Elosa worallii, was investigated using life table experiments. The food concentration under dietary restricted conditions was below the threshold for population growth. It was (1) tested whether the rotifers start reproduction again after food replenishment, and (2) estimated whether the time scale of dietary restricted conditions is relevant for the persistence of a population in the field. Only E. worallii responded to dietary restriction with an increase in life span at the expense of reproduction. After replenishment of food, E. worallii started to reproduce again within 1 day. With an increase in the duration of dietary restricted conditions of up to 15 days, which is longer than the median life span of E. worallii under food saturation, the life span increased and the life time reproduction decreased. These results suggest that in a temporally (or spatially) variable environment, some rotifer populations can persist even during long periods of severe food deprivation.     

Sublittoral and profundal Oligochaeta fauna of the Lake Constance (Bodensee-Obersee)

Fourteen tubificid species and one lumbriculid make up the oligochaete population of the northern half of Lake Constance, a meso- to mesoeutrophic lake. Tubifex tubifex makes up 72% of the total abundance, the clean water species Stylodrilus heringianus only 0.1%. No correlation exists between total worm abundance and organic carbon content of the sediment, only 4% of the organic carbon being used by the worms. A highly significant correlation between particulate organic matter loading and its organic carbon content from 10 tributaries and total oligochaete abundance is demonstrated. The ecological index I_PA is defined and used to assess the biologically relevant trophic status of the lake bed. This integrates information on the percent representation of three groups of worm species classified on atrophic basis and total abundance of the worms. Despite control of the increase in phosphorus loadings progressive eutrophication of sediments was observed after an 8 year interval.     

Feeding and digestion by the mussel Mytilus edulis L. (Bivalvia: Mollusca) in mixtures of silt and algal cells at low concentrations

Mussels Mytilus edulis L. from two populations were exposed for 2 days or 2 wk to mixtures of silt and the diatom Phaeodactylum tricomutum Bohlin before measuring rates of feeding, the passage time for food in the gut, absorption efficiency and metabolic rate. Experimental diets were set up to span the range of organic content in seston from the natural habitats (% organic matter by weight: 7–55%), but were less than the total levels of natural seston. Absorption efficiencies were adequately modelled in an exponential relationship to food quality and to gut passage time, although at high proportional silt concentrations metabolic faecal losses led to negative net rates of absorption. Over short-term exposures the scope for growth was a simple function of food quality, and the nutritional quality of the diet was best expressed as organic content per unit volume of particles. Over a period of 2 wk physiological acclimation occurred, across all levels of experimental food quality (which were as low as 10% organic matter by weight), resulting in positive growth potential. Relevant mechanisms of compensation include increased rates of ingestion, increased absorption efficiency and an apparent increase in digestive capacity, estimated here as gut fullness. In experiments in which natural diets are simulated by adding silt to phytoplankton cells, the consequences for net rates of absorption depend on the balance between mean particle size and organic content per unit volume. Calculations show how, in some circumstances, growth may be enhanced by the addition of small silt particles to living phytoplankton cells.     

Cannibalistic phenomena in salamandra salamandra larvae in certain water bodies and under experimental conditions

Salamandra salamandra larvae originating from a single batch (layed by one female) were raised at different temperatures and on ion-rich or distilled water. Temperature, but not the presence or absence of ions affected growth. Larvae originating from one batch or from several batches born on the same day, were kept without food for long periods of several months in order to observe cannibalistic predation under such conditions. This phenomenon is known in natural populations of salamanders inhabitating either rain pools in rocks or permanent springs poor in aquatic life. It was found that cannibalism increased under experimental conditions proportionally to the increase in size between the larvae.     

Sludge Reduction by Predatory Activity of Aquatic Oligochaetes in Wastewater Treatment Plants: Science or Fiction? A Review

Biological aerobic wastewater treatment plants (WWTPs) produce a lot of excess sludge. The costs for handling this residual product are increasing, so the search for alternative techniques to reduce the amount of sludge has to be continued. Activated sludge consists of inorganic and organic substances, bacteria, protozoa and metazoa. Due to incomplete biomass conversion, sludge consumption yields less oligochaete biomass. From a technological point of view, the application of aquatic oligochaetes to reduce the sludge production offers interesting perspectives. This paper aims to review the feasibility for the reduction of activated sludge in WWTPs by means of aquatic oligochaetes. Also the current techniques concerning sludge reduction are taken into account. Several of the WWTPs relevant parameters, which may influence predatory activity of aquatic oligochaetes, are discussed: particle size, organic content of substrate, bacteria preference, life cycle and population dynamics of aquatic oligochaetes, temperature, pH, oxygen and process conditions. From the literature it appeared that most research has been performed on laboratory scale. Only a few authors mention a significant reduction of the sludge production by ‘sessile’ species such as Lumbriculus. Vermicultures for the reduction of activated sludge are rather common in developing countries. Incidentally large annelid blooms have been noticed in WWTPs. It remains obscure which factors trigger the initiation of annelid blooms in WWTPs and which are of importance to maintain a stable annelid population in WWTPs. The influence of a considerable worm bloom on the waste sludge production is still under investigation.     

Effects of Sewage Sludge on the Growth of Potted Salt Marsh Plants Exposed to Natural Tidal Inundation

Abstract A growth experiment with native plants in pots exposed to natural environmental conditions evaluates the use of sewage sludge as a soil amendment in restoration of a southern California salt marsh. Sludge containing desirable organic matter but also undesirable heavy metal contaminants was mixed with a readily available matrix soil to reduce metal concentrations to levels below legal limits for land applications of sludge. Soil nutrient analysis revealed expected increases in total nitrogen and total phosphorus content with increasing sludge concentration. Soil metals analysis, however, revealed decreases in metal content with increased sludge concentration, a trend evidently caused by higher than expected metal content in the matrix soil. Five artificial soil mixtures ranging from 0 to 70% sludge were accompanied by natural wetland soil controls. Pots containing these soils were placed into a natural salt marsh. The pots were then planted with two native salt marsh plant species, Salicornia virginica and Frankenia grandifolia. Aboveground biomass was harvested after 12 months. Plant growth displayed no obvious change with increasing sludge concentration. Over the concentration ranges used, increased nutrient content did not stimulate plant growth and increased metal content did not inhibit plant growth. Plants grew better in natural wetland soil than in artificial soil mixtures, a trend probably caused by the substantially finer texture and higher organic content of natural soil. All sludge treatments differed more from the natural soil than from each other, implying that within the ranges examined, soil texture and organic content exerted more influence on plant growth than did metal or nutrient concentration. These results suggest that incorporating this sewage sludge in the soil of the restored salt marsh will neither benefit nor harm the plants that will live there and that greatest plant growth will be achieved by mixing the sludge with a fine‐grained matrix soil.