Gammarid Gammarus Research Articles

Predatory preferences of a non-indigenous crab do not depend on prey invasion scenarios

AbstractPredatory non-indigenous species (NIS) have profound impacts on global ecosystems, potentially leading to native prey extinction and reshaping community dynamics. Among mechanisms potentially mediating predator impacts and prey invasion success are predator preferences between native vs. non-indigenous prey, a topic still underexplored. Using functional response and prey preference experiments, this study focused on the predation by the non-indigenous Japanese brush-clawed shore crab, Hemigrapsus takanoi, between the native gammarid Gammarus duebeni and the analogous non-indigenous Gammarus tigrinus. Although H. takanoi showed subtle differences in its functional response type between the two prey species, its preferences across their environmental frequencies were not strongly influenced by the prey invasion scenario. The findings highlight the need for a comprehensive understanding of interactions in ecosystems with multiple NIS, offering fresh insights into complex feeding interactions within marine environments.

Distribution of crustaceans (Malacostraca, Arthropoda) in rivers of the semi-desert zone (with the example of the Volgograd reservoir tributes)

As a result of our studies of the macro- and nectozoobenthos of the rivers of the semidesert zone (the Volgograd reservoir basin), 16 Malacostraca species were recorded, most of which are representatives of the Ponto-Caspian complex. Compared to the Volgograd reservoir, there is an increase in the quantitative indicators of mysids and isopods and a decrease in the number of species, abundance and biomass of amphipods and cumaceans in the semi-desert zone rivers. The average abundance and biomass of crustaceans in the surveyed rivers are approximately 2.5 times lower than in the Volgograd Reservoir. Crustaceans of the Ponto-Caspian and PontoAzov complexes, as well as the isopods Asellus aquaticus (Linné, 1758) inhabit fresh and oligohaline waters, while the gammarids Gammarus lacustris (Sars, 1863) live in oligohaline and polyhaline waters. Amphipods are characterized by the largest number of species, which, taking into account their morphology and the type of substrate which they live on, are assigned to three ecomorphs, namely: clingers, crawlers, and diggers. No symbionts in the Yeruslan basin rivers were found. The noted amphipod ecomorphs are distributed relatively evenly with a slight dominance of crawlers (45%). It has been revealed that two types of crustacean cenoses are formed in the surveyed rivers. The first type is formed mainly by consortium interactions of the edificatory mollusks Dreissena with amphipods Chelicorophium curvispinum Sars, 1895, Dikerogammarus haemobaphes (Eichwald, 1841), mysids Paramysis ullskyi Czerniavsky, 1882, P. lacustris (Czerniavsky, 1882), and associated species. The second cenosis in the rivers is represented by a complex of small coastal mysid species Katamysis warpachowskyi Czerniavsky, 1882, Limnomysis benedeni Sars, 1893, and amphipods Chaetogammarus warpachowsky (Sars, 1894), the relationships within which are based on common biotopic conditions and feeding patterns.

Effects of diet and temperature on the fatty acid composition of the gammarid Gammarus locusta fed alternative terrestrial feeds

The fast and remarkable growth of global aquaculture in recent years has created new challenges, such as guaranteeing a sustainable supply of raw materials used for aquafeed formulation. Gammarids are low-trophic crustaceans with an increasing interest in aquaculture due to their high nutritional profiles and their capacity to grow under high-density conditions. Moreover, gammarids have the ability to thrive on a wide range of sidestreams while accumulating relatively high levels of long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA). In the present study, juveniles of the marine gammarid Gammarus locusta were cultured at four different temperatures (5°C, 10°C, 15°C, and 20°C) for 21 days and fed three diets, including the seaweed Fucus sp. as control, and carrot leaves and coconut flesh representing two agri-food industry sidestreams. Our results indicate that both the survival and biomass of G. locusta were highly affected by diet, with coconut showing the lowest growth performance. The temperature had no effect on biomass, although high temperature (20°C) resulted in a decrease in survival. The effects of temperature on the gammarid fatty acids were not evident, with diet being the main modulator of the profiles. Furthermore, the results also reveal that the Fucus sp. diet was associated with relatively high percentages of n-3 and n-6 LC-PUFA. Interestingly, essential LC-PUFA such as eicosapentaenoic (20:5n-3, EPA) and docosahexaenoic (22:6n-3, DHA) acids were detected in gammarids fed on either Fucus sp. or any of the sidestreams irrespectively of their presence in the diets. These results suggest an ability of G. locusta for LC-PUFA biosynthesis (trophic upgrading) and/or retention, making this species a promising candidate for the production of high-value ingredients for aquafeeds.

Organ-specific accumulation of cadmium and zinc in Gammarus fossarum exposed to environmentally relevant metal concentrations

One of the best approaches for improving the assessment of metal toxicity in aquatic organisms is to study their organotropism (i.e., the distribution of metals among organs) through a dynamical approach (i.e., via kinetic experiments of metal bioaccumulation), to identify the tissues/organs that play a key role in metal regulation (e.g., storage or excretion). This study aims at comparing the organ-specific metal accumulation of a non-essential (Cd) and an essential metal (Zn), at their environmentally relevant exposure concentrations, in the gammarid Gammarus fossarum. Gammarids were exposed for 7 days to 109Cd- or 65Zn-radiolabeled water at a concentration of 52.1 and 416 ng.L−1 (stable equivalent), respectively, and then placed in clean water for 21 days. At different time intervals, the target organs (i.e., caeca, cephalons, intestines, gills, and remaining tissues) were collected and 109Cd or 65Zn contents were quantified by gamma-spectrometry. A one-compartment toxicokinetic (TK) model was fitted by Bayesian inference to each organ/metal dataset in order to establish TK parameters. Our results indicate: i) a contrasting distribution pattern of concentrations at the end of the accumulation phase (7th day): gills > caeca ≈ intestines > cephalons > remaining tissues for Cd and intestines > caeca > gills > cephalons > remaining tissues for Zn; ii) a slower elimination of Cd than of Zn by all organs, especially in the gills in which the Cd concentration remained constant during the 21-day depuration phase, whereas Zn concentrations decreased sharply in all organs after 24 h in the depuration phase; iii) a major role of intestines in the uptake of waterborne Cd and Zn at environmentally relevant concentrations.

Morphology and zoogeography of the burrower-like gammarid Gammarus koshovi (Bazikalova, 1946) (Crustacea, Amphipoda, Gammaridae) – An overlooked and poorly known species in the Siberian fauna

A comprehensive survey of several lakes in the Tuva Republic of Russia yielded, besides the ubiquitous Gammarus lacustris, the discovery of a small freshwater, lacustrine amphipod not previously recorded in this area. A comparative study of the Tore-Khol Lake gammarid population, probably conspecific with Gammarus koshovi (Bazikalova, 1946) originating from Khubsugul Lake, was conducted. The species, G. koshovi, is characterized by a specific habitus: a small-sized compact body, all limbs shortened, carpi of pereopods (PIII and PIV) reduced, coxal plates broad, and pereopod dactyli sturdy. It has been suggested that juveniles of the euryoecious G. lacustris or other large species could be confused with the relatively small G. koshovi. Consequently, we decided to present the distribution of gammarid species throughout south Siberia and Mongolia, referring to the sequences of works primarily by Soviet authors, which may be hard to access by international readers. We discuss affinity with related groups, distribution, and ecology of G. koshovi to better understand their evolution. Additionally, the zoobenthic species diversity of widely represented groups in the ecosystems of Tore-Khol Lake is briefly reviewed. An identification key for the Siberian Gammarus with 10 species is provided.

Microsporidian infections in the species complex Gammarus roeselii (Amphipoda) over its geographical range: evidence for both host\u2013parasite co-diversification and recent host shifts

BackgroundMicrosporidians are obligate endoparasites infecting taxonomically diverse hosts. Both vertical (from mother to eggs) and horizontal (between conspecifics or between species) transmission routes are known. While the former may promote co-speciation and host-specificity, the latter may promote shifts between host species. Among aquatic arthropods, freshwater amphipod crustaceans are hosts for many microsporidian species. However, despite numerous studies, no general pattern emerged about host specificity and co-diversification. In south-eastern Europe, the gammarid Gammarus roeselii is composed of 13 cryptic lineages of Miocene to Pleistocene age but few genotypes of one lineage have spread postglacially throughout north-western Europe. Based on nearly 100 sampling sites covering its entire range, we aim to: (i) explore the microsporidian diversity present in G. roeselii and their phylogenetic relationships, especially in relation to the parasites infecting other Gammaridae; (ii) test if the host phylogeographical history might have impacted host–parasite association (e.g. co-diversifications or recent host shifts from local fauna).MethodsWe used part of the small subunit rRNA gene as source of sequences to identify and determine the phylogenetic position of the microsporidian taxa infecting G. roeselii.ResultsMicrosporidian diversity was high in G. roeselii with 24 detected haplogroups, clustered into 18 species-level taxa. Ten microsporidian species were rare, infecting a few individual hosts in a few populations, and were mostly phylogenetically related to parasites from other amphipods or various crustaceans. Other microsporidians were represented by widespread genera with high prevalence: Nosema, Cucumispora and Dictyocoela. Two contrasting host association patterns could be observed. First, two vertically transmitted microsporidian species, Nosema granulosis and Dictyocoela roeselum, share the pattern of infecting G. roeselii over most of its range and are specific to this host suggesting the co-diversification scenario. This pattern contrasted with that of Dictyocoela muelleri, the three species of Cucumispora, and the rare parasites, present only in the recently colonised region by the host. These patterns suggest recent acquisitions from local host species, after the recent spread of G. roeselii.ConclusionsMicrosporidians infecting G. roeselii revealed two scenarios of host–parasite associations: (i) ancient associations with vertically transmitted parasites that probably co-diversified with their hosts, and (ii) host shifts from local host species, after the postglacial spread of G. roeselii.

Evaluation of oxidative stress biomarkers in the freshwater gammarid Gammarus dulensis exposed to trout farm outputs

The current study reports a combined seasonal monitoring of the Crnica River watercourse using physico-chemical parameters and biomarkers measured in Gammarus dulensis with the aim of correlating the effects of pollution with the level of oxidative stress biomarkers in this amphipod. The pollution source is identified as a trout farm in the upper part of the Crnica River (Eastern Serbia). Physico-chemical water parameters and sediment trace element concentrations were measured in the Crnica River during three seasons. Three localities downstream [CR2, CR3, and CR4 (20 m, 400 m, and 1.3 km from the trout farm discharge, respectively)] were assessed in comparison with a reference location (CR1), 250 m upstream from the trout aquaculture in the Crnica River. Antioxidant biomarkers including the enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), glutathione reductase (GR), and glutathione-S-transferase (GST) were determined in pooled samples of G. dulensis. Our findings showed statistically different antioxidant enzyme levels in gammarids from the first downstream locality (CR2) compared to those from other three localities. The results indicate that oxidative stress occurred to some extent despite the assumed antioxidant induction as a result of exposure to elevated levels of some elements and changed water parameters, the effects seeming in part to be mediated by suppression of SOD and CAT (during summer and autumn); and by suppression of GPX and GR (during autumn) at the CR2 locality. The activity of GST in all seasons was higher at the CR2 locality than at the other localities, suggesting the presence of some permanent xenobiotic next to the aquaculture output. Seasonal difference in the physical and chemical water parameters and sediment trace element concentrations were observed. Overall, our results indicate a locality- and season-dependent antioxidant enzymes response in G. dulensis, pointing at progressively reduced deleterious effect of trout farm outputs with movement further downstream.

Relative benefit of the invasive Echinogammarus berilloni (Catta, 1878) over native gammarids under fish predation (Gasterosteus aculeatus Linnaeus, 1758)

Running waters of Central Europe are progressively invaded by the southwest European amphipod Echinogammarus berilloni—as currently observed in the karstic upper River Lippe catchment (Paderborn Plateau, northwest Germany). We tested experimentally whether the succession of the invader is favored by lower predation intensity of three-spined sticklebacks (Gasterosteus aculeatus) compared to predation on the native gammarids Gammarus pulex and G. fossarum. Predation experiments were combined with two modifiers: species interaction (separate exposure of each species vs. joint exposure of each native species with the invader) and habitat structure (absence vs. availability of refuge structures as hideouts against predation). When the species were exposed separately to the predator and no refuges were offered, predation rates did not differ significantly. When refuges were present, predation success generally decreased, but was relatively lower on the invader E. berilloni than on the native species. When native and the invasive species were exposed jointly to predation, predation success decreased further in both species combinations. However, under both conditions, with and without refuge structures, survival of E. berilloni was significantly greater than in G. pulex. The findings corroborate our assumption that the invader E. berilloni profits from sympatric occurrence with native gammarid species, especially when hideout options are available. Whether this effect was caused by competition for refuges or simply by a relatively lower activity level of E. berilloni and smaller predator stimulus for the fish remains to be cleared. Without doubt, the difference in predation is a factor that contributes to the invader’s success.

Activity level and aggregation behavior in the crustacean gammarid Gammarus insensibilis parasitized by the manipulative trematode Microphallus papillorobustus

Hosts manipulated by parasites are profoundly altered organisms exhibiting a broad range of potential modifications. Exploring this multidimensionality is an emerging field. Previous studies have shown that the bird trematode Microphallus papillorobustus induces several behavioural changes in the gammarid Gammarus insensibilis. Knowing that aggregation behaviour and reduced activity levels are strategies that limit predation in other species of amphipods, we explored in this study these behavioural responses for infected and uninfected G. insensibilis in the presence of host and non-host predator olfactory cues (bird faeces and fish mucus). While uninfected individuals reduced their activity level in the presence of predator cues, infected individuals did not change their activity level in presence of aquatic bird faeces. We also studied the gammarid aggregation behaviour. Uninfected gammarids in clean water spent significantly more time in aggregates than did infected individuals. Among the uninfected individuals, the aggregation level tended to increase when bird faeces and fish mucus were added, but the difference was not significant. Among infected individuals, the level of aggregation was significantly increased only with the bird faeces. We discussed our results in the context of the literature on multidimensional manipulations, acknowledging that subtle differences between unparasitized and parasitized gammarids can also be by-products of manipulation on other traits.

Feeding choice and predation pressure of two invasive gammarids, Gammarus tigrinus and Dikerogammarus villosus, under increasing temperature

In most European freshwater ecosystems, the invasive gammarids Gammarus tigrinus and Dikerogammarus villosus strongly impair recipient communities through predation of a wide range of native invertebrates. Due to the effects of temperature on both the physiology and the behaviour of such ectotherms, understanding how global warming may influence their ecological impact is a research priority. These species were therefore exposed to three different food types to determine their detritivorous, herbivorous and carnivorous characters, and predation was measured characterizing the Holling’s functional response. The effect of increasing water temperatures (15, 20, 25°C) on both the food choice and predatory activities was investigated. Both species showed a significant preference for animal tissues at all temperatures. The total food intake increased with temperature for G. tigrinus but did not change for D. villosus, which may result from specific species differences in metabolic requirements. The consumption of live prey strongly increased with temperature. The main differences were an increased searching efficiency in G. tigrinus and a decreased handling time in D. villosus as temperature increased, which may result from differences in foraging strategies. These results suggest that climate change is likely to increase the predation pressure of both invasive gammarids on prey species.

Juvenile growth rate equation for the freshwater gammarid Gammarus roeseli

Juvenile growth rate equation for the freshwater gammarid Gammarus roeseli

Dietary supply with essential lipids affects growth and survival of the amphipod Gammarus roeselii

Growth and survival of benthic macroinvertebrates depend on the availability and the quality of potential food sources. The significance of essential biochemical nutrients, such as sterols and polyunsaturated fatty acids (PUFAs), for benthic invertebrates has been insufficiently studied. We investigated the effects of these essential lipids on growth and survival of the benthic gammarid Gammarus roeselii , a widespread species in streams, rivers and lentic waters of Central Europe, in standardized feeding experiments. Juvenile gammarids were fed a mixture of three cyanobacteria with no evidence of toxin production, either unsupplemented or supplemented with cholesterol or the long-chain PUFA docosahexaenoic acid (DHA) using bovine serum albumin (BSA) to load algal or cyanobacterial cells with single lipid, and a mixture of three eukaryotic algae containing various sterols and long-chain PUFAs. Our results revealed that growth and especially survival of gammarids on the cyanobacterial diet significantly increased upon supplementation with cholesterol and DHA, indicating that the nutritional inadequacy of cyanobacteria for gammarids and potentially other benthic invertebrates is at least partially due to a deficiency in these essential lipids. We propose that the expected increase in the frequency of pelagic cyanobacterial mass developments as a consequence of global warming will also affect benthic food web processes to an as-yet-unknown magnitude.

Seasonal variability of antioxidant biomarkers and energy reserves in the freshwater gammarid Gammarus roeseli

In gammarids, behavioural and biochemical biomarkers are commonly used in ecotoxicological studies. In our study, we have investigated seasonal variations of several biochemical biomarkers in Gammarus roeseli, a freshwater species. Organisms were sampled monthly over a 1-year period. Gender was distinguished to measure antioxidant enzyme activities like total glutathione peroxidase (GPxtot), selenium-dependent glutathione peroxidase (SeGPx) and catalase enzymes, lipoperoxidation end-product (malondialdehyde, MDA), and energy reserves with protein and lipid contents. In the same time, usual water physico-chemical parameters were measured at the sampling site. A based-gender difference was observed for parameters related to oxidative stress. Females showed higher antioxidant enzyme activities and lower MDA level than males. Parameters related to oxidative stress and energy reserves appeared correlated with temperature and physiological status of organisms. Females GPx activities were lower in autumn and winter when no breeding occurred. In both gender, MDA levels were correlated with temperature with an increase of lipoperoxidation in summer. Lipid contents were the lowest in summer and the highest in winter, probably due to the reproductive status of organisms and their feeding behaviour. Gender-based differences of biochemical parameters suggest a specific sensitivity of males and females in ecotoxicological experiments. Moreover, organisms could be more vulnerable in summer when MDA levels are high and energy reserves low. Deleterious effect of xenobiotics would be different with gender and season.

Consequences of introducing the invasive amphipod Gmelinoides fasciatus into large shallow Lake Peipsi: present distribution and possible effects on fish food

The objective of the study was to assess the effect of the Baikalian amphipod Gmelinoides fasciatus, a successful invader into Europe, as a fish food source in Lake Peipsi to where it was introduced in the 1970s. In 2002–2006, the littoral macrozoobenthos was studied along 17 transects (0.1–4 m depth, 384 quantitative samples) to determine the recent distribution of G. fasciatus and its share in the benthic community. Earlier records on the littoral macrozoobenthos from the years 1970, 1980, 1990 and 2000 were also used for comparison. Our results indicate that the invasive G. fasciatus is the dominant species (about 43% of total macrozoobenthos abundance) in the littoral zone of Lake Peipsi, while the native gammarids Gammarus lacustris and Pallasea quadrispinosa appear to be extinct and oligochaetes seem to have declined. G. fasciatus preferred hard substrates and it was mainly distributed close to water shallow beach areas (water depth <0.2 m), where its abundance reached locally up to 29 000 ind. m−2 (97% of total macrozoobenthos abundance). This distribution pattern of the invader in summer makes it mostly inaccessible for adult fish and increases its survival rate. Being highly mobile, G. fasciatus is not sensitive to water level fluctuations in shallow lakes. These behavioural characteristic increase its population success.

A local gammarid uses kairomones to avoid predation by the invasive crustaceans Dikerogammarus villosus and Orconectes limosus

Since the 1980s the Ponto-Caspian gammarid Dikerogammarus villosus has spread throughout Europe while displacing native species and is predicted to invade further continents. After it was introduced into Europe in the 1890s the North American crayfish Orconectes limosus spread throughout Europe and served as a vector to displace native crayfish as well. In Lake Constance (Germany) the previously dominant gammarid Gammarus roeselii is subjected to both of these invasive crustaceans. In our experiments both species placed predation pressure on G. roeselii. Kairomone perception tests in a Y-maze revealed the capability of the gammarids G. roeselii and D. villosus to perceive and avoid the scent of the predator crayfish O. limosus. Both species also avoided the kairomone of the other gammarid, but did not avoid the scent of their own species. This taxa specific behavior suggests that taxa specific signals are used. This behavior can help the gammarids avoid shelters previously occupied by predators.

Effect of parasite-induced behavioral alterations on juvenile development

Like many trophically transmitted parasites, the trematode Microphallus papillorobustus alters the behavior of its intermediate host, the crustacean gammarid Gammarus insensibilis, in a way that favors its vulnerability to definitive hosts (aquatic birds). Parasitized females still produce eggs, but because juvenile development occurs inside the female marsupial brood pouch, young gammarids are subject to the same risk of predation as their mothers until they exit the marsupium. We explored the idea that developing juveniles can adjust their developmental schedule in a state-dependent manner according to the parasitic status of the mother. We predicted that juveniles from parasitized females would accelerate their development, or exit the marsupium at an earlier stage, to avoid predation by birds. Contrary to our expectations, we observed the opposite, that is, juveniles from parasitized females exited the marsupial brood pouch significantly later than those from uninfected mothers. We discuss these results in relation to current ideas on host manipulation by parasites in ecosystems. Copyright 2009, Oxford University Press.

The impact of the invasive Ponto-Caspian amphipod Pontogammarus robustoides on littoral communities in Lithuanian lakes

The intentionally introduced Pontogammarus robustoides is the most successful amphipod invader of Lithuanian inland waters and has become established in large lakes. Its impact on littoral invertebrate communities was studied by comparing similar habitats across lakes that harbour or are devoid of the invader. In habitats where P. robustoides is well established and numerous, it significantly reduces species richness and community diversity. Moderate pontogammarid density in habitats that can sustain the native gammarid Gammarus lacustris, however, revealed no negative impact on diversity metrics. Among the lakes studied, the benthic biomass did not differ in invaded and uninvaded habitats. The biomass of indigenous invertebrates (excluding chironomids, which exhibited high lake-specific biomass variation) was lower in the places with well-established P. robustoides. A detrimental impact was observed upon the native isopod Asellus aquaticus and a negative correlation with most of the higher taxa of native invertebrates. In the invaded lake habitats that favour P. robustoides, a change in community structure and a decrease in diversity up to twofold or more are to be expected.

Stream ecosystem response to multiple experimental floods from a reservoir

AbstractThe effects of multiple experimental floods in the same year on the ecology of a river (River Spöl) below a large reservoir were investigated. In the flood year (2000), three floods of different magnitude (10 m3/s on 15 June, 25 m3/s on 5 July, and 10 m3/s on 10 August) were implemented, each lasting 7–8 h. Regulated baseflow discharge was maintained at less than 2.5 m3/s. Samples of macroinvertebrates, periphyton, water chemistry, and seston were collected for one year (1999) before and then immediately before and after each flood in the Spöl and in an adjacent reference tributary (Val da l'Aqua). Samples also were collected periodically between floods to evaluate temporal response patterns. During two of the floods, suspended sediments and seston were measured at 30 min to 1 h intervals. Suspended sediment and seston reached peak concentrations within the first hour of each flood, then decreased within about 2 h. Peak seston and suspended sediment concentrations during each flood decreased by an order of magnitude from the July flood to the August flood. Two‐way ANOVA indicated significant flood effects on periphyton standing crops expressed as either chlorophyll a or ash‐free dry mass (AFDM). Recovery of periphyton following each flood appeared to be progressively slower after the later (July and August) floods. Seston chlorophyll levels also were lower following each flood, being significantly correlated to periphyton biomass. Two‐way ANOVA indicated that the floods significantly decreased macroinvertebrate taxonomic richness, density, and biomass. Taxa that decreased from the floods included the turbellarian Crenobia alpina and the gammarid Gammarus fossarum. Taxa that showed a fast recovery to the floods included the Chironomidae, Simuliidae, and Baetidae. Our data indicate a strong temporal effect of flood disturbance on benthic assemblages (periphyton and macroinvertebrates) below reservoirs that partly reflects species‐specific life histories and traits, and the cumulative effects of earlier floods. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Field evidence for density-dependent effects in the trematode Microphallus papillorobustus in its manipulated host, Gammarus insensibilis.

Numerous studies have demonstrated that parasites with complex life cycles frequently manipulate the phenotype of their hosts to increase their transmission rate. Little is known, however, concerning density-dependent processes within infrapopulations of manipulative parasites--whether parasites cooperate to manipulate the host, whether competition counteracts with these potential cooperative benefits, or both. Here we explored these ideas, focusing on the association between the manipulative trematode Microphallus papillorobustus and its second intermediate host, the gammarid Gammarus insensibilis. From the data collected in the field, we found no evidence that co-occurring M. papillorobustus individuals benefit from the presence of conspecifics; instead, individuals in larger infrapopulations suffered reduced size and fecundity. Thus, the net effect of increasing density suggests that competition rather than cooperation is the dominant force in infrapopulations of M. papillorobustus.

Egg production by metacercariae of Microphallus papillorobustus: a reproductive insurance?

This study aimed to determine whether metacercariae of Microphallus papillorobustus (Trematoda: Microphallidae) produce eggs by self-fertilization when their intermediate host, the gammarid Gammarus insensibilis, dies before being eaten by predatory definitive hosts. Such a mechanism would allow the parasite to reproduce even if transmission fails. The percentage of metacercariae that produced eggs and the mean number of eggs were significantly higher in metacercariae placed in a Ringer's solution compared with those in salt water or metacercariae remaining inside the dead gammarid. The lack of egg production in metacercariae removed from dead gammarids in the field supported the idea that self-fertilization is not, for this parasite, a mechanism of reproductive insurance when establishment in the definitive hosts fails.