Leeches Baicalobdella torquata feed on hemolymph but have a low effect on the cellular immune response of amphipod Eulimnogammarus verrucosus from Lake Baikal

Lake Baikal is one of the largest and oldest freshwater reservoirs on the planet with a huge endemic diversity of amphipods (Amphipoda, Crustacea). These crustaceans have various symbiotic relationships, including the rarely described phenomenon of leech parasitism on amphipods. It is known that leeches feeding on hemolymph of crustacean hosts can influence their physiology, especially under stressful conditions. Here we show that leeches Baicalobdella torquata (Grube, 1871) found on gills of Eulimnogammarus verrucosus (Gerstfeldt, 1858), one of the most abundant amphipods in the Baikal littoral zone, indeed feed on the hemolymph of their host. However, the leech infection had no effect on immune parameters such as hemocyte concentration or phenoloxidase activity and also did not affect glycogen content. The intensity of hemocyte reaction to foreign bodies in a primary culture was identical between leech-free and leech-infected animals. Artificial infection with leeches also had only a subtle effect on the course of a model microbial infection in terms of hemocyte concentration and composition. Despite we cannot fully exclude deleterious effects of the parasites, our study indicates a low influence of a few leeches on E. verrucosus and shows that leech-infected amphipods can be used at least for some types of ecophysiological experiments.

Cellular Immune Response of an Endemic Lake Baikal Amphipod to Indigenous Pseudomonas sp

Studies of invertebrates have shown that the internal environment of crustaceans is not always sterile in normal conditions, and in many species, it can be populated by microorganisms even in the absence of any visible pathological processes in the body. This observation raises the question of whether genetically modified indigenous hemolymph microorganisms can be used for biotechnological purposes inside the crustacean either as local producers of some compounds or as sensors to physiological parameters. In this study, we tested the ability of the bacteria isolated from the hemolymph of the amphipod Eulimnogammarus verrucosus to hide from the cellular immune response of the host as the most important feature for their potential long-term application in vivo. 16S rDNA amplicon sequencing revealed five common bacterial genera in all analyzed samples of the amphipod hemolymph, among which Pseudomonas is most easily subjected to genome modification and, thus, the most prospective for biotechnological application. Cultivation of Pseudomonas gave us a number of strains undoubtedly derived from the amphipod hemolymph, and one of them (belonging to the Pseudomonas fluorescens group) was chosen for further tests. The primary culture of amphipod hemocytes was used to analyze the immunogenicity of the strain and showed a pronounced reaction of the immune cells to a high amount of the bacteria within six hours. This result indicates that modulation of cellular immune response to metabolically active bacterial cells is not mandatory for the survival and wide distribution of these microorganisms in the hemolymph of numerous amphipod individuals.

Variation in the immune state of Gammarus pulex (Crustacea, Amphipoda) according to temperature: Are extreme temperatures a stress?

Temperature is known to impact host-parasite interactions in various ways. Such effects are often regarded as the consequence of the increased metabolism of parasites with increasing temperature. However, the effect of temperature on hosts' immune system could also be a determinant. Here we assessed the influence of temperature on the immunocompetence of the crustacean amphipod Gammarus pulex. Amphipods play a key ecological role in freshwater ecosystems that can be altered by several parasites. We investigated the consequences of three weeks of acclimatization at four temperatures (from 9 °C to 17 °C) on different immunological parameters. Temperature influenced both hemocyte concentration and active phenoloxidase enzymatic activity, with lower values at intermediate temperatures, while total phenoloxidase activity was not affected. In addition, the ability of gammarids to clear a bacterial infection was at the highest at intermediate temperatures. These results suggest a dysregulation of the immune system of gammarids in response to stress induced by extreme temperature.

Limiting immunopathology: Interaction between carotenoids and enzymatic antioxidant defences

The release of reactive oxygen and nitrogen species (ROS and RNS) during the inflammatory response generates damages to host tissues, referred to as immunopathology, and is an important factor in ecological immunology. The integrated antioxidant system, comprising endogenous antioxidant enzymes (e.g. superoxide dismutase SOD, and catalase CAT) and dietary antioxidants (e.g. carotenoids), helps to cope with immune-mediated oxidative stress. Crustaceans store large amounts of dietary carotenoids for yet unclear reasons. While being immunostimulants and antioxidants, the interaction of these pigments with antioxidant enzymes remains unclear. Here, we tested the interaction between dietary supplementation with carotenoids and immune challenge on immune defences and the activity of the antioxidant enzymes SOD and CAT, in the amphipod crustacean Gammarus pulex. Dietary supplementation increased the concentrations of circulating carotenoids and haemocytes in the haemolymph, while the immune response induced the consumption of circulating carotenoids and a drop of haemocyte density. Interestingly, supplemented gammarids exhibited down-regulated SOD activity but high CAT activity compared to control ones. Our study reveals specific interactions of dietary carotenoids with endogenous antioxidant enzymes, and further underlines the potential importance of carotenoids in the evolution of immunity and/or of antioxidant mechanisms in crustaceans.

Polymorphus minutus affects antitoxic responses of Gammarus roeseli exposed to cadmium

The acanthocephalan parasite Polymorphus minutus is a manipulator of its intermediate host Gammarus roeseli, which favours its transmission to the final host, a water bird. In contaminated environments, G. roeseli have to cope with two stresses, i.e. P. minutus infection and pollutants. As P. minutus survival relies on its host's survival, we investigated the influence of P. minutus on the antitoxic defence capacities and the energy reserves of G. roeseli females after cadmium exposure. In parallel, malondialdehyde, a toxic effect biomarker, was measured in G. roeseli females and in P. minutus. The results revealed that infected females displayed higher cell damage than uninfected ones, despite an apparent increase in reduced glutathione and metallothionein production. In fact, the increase of these antitoxic systems could be counterbalanced by carotenoid intake by the parasite, so that the overall defence system seemed less efficient in infected females than in uninfected ones. In addition, we demonstrated that cadmium induced cell damage in P. minutus, probably linked with cadmium accumulation in the parasite. Altogether, we observed a paradoxical pattern of responses suggesting that P. minutus increases cadmium toxicity in G. roeseli females although (i) it tends to increase several host antitoxic defence capacities and (ii) it bears part of the pollutant, as reflected by cell damage in the parasite.

Low dose TBT exposure decreases amphipod immunocompetence and reproductive fitness

The antifouling agent tributyltin (TBT) is a highly toxic pollutant present in many aquatic ecosystems. Despite of regulations on the usage of TBT, it remains in high concentrations in sediments both in harbors and in off-shore sites. The toxicity of TBT in mollusks is well documented. However, adverse effects in other aquatic organisms, such as crustaceans, are less well known. This study is an effort to assess the effects of environmentally realistic concentrations of TBT on an ecologically important species in Swedish fresh and brackish water ecosystems, the benthic amphipod Monoporeia affinis. Field collected animals were exposed during gonad maturation to TBT (70 and 170 ng/g sediment d wt) for five weeks in static microcosms with natural sediment. Exposure concentrations were chosen to reflect effects at concentrations found in Swedish coastal sediment, but below expected effects on survival. TBT exposure resulted in a statistically significant adverse effect on oocyte viability and a doubling of the prevalence of microsporidian parasites in females, from 17% in the control to 34% in the 170 ng TBT/g sediment d wt exposure. No effects on survival were observed. Borderline significant effects were observed on male sexual maturation in the 70 ng TBT/g d wt exposure and on ecdysteroid levels in the 170 ng/g sediment d wt exposure. Both reproduction and parasite infection effects are of ecological importance since they have the potential to affect population viability in the field. This study gives further evidence to the connection between low dose contaminant exposure and increases in microsporidian parasite infection.

Is there a role for antioxidant carotenoids in limiting self-harming immune response in invertebrates?

Innate immunity relies on effectors, which produce cytotoxic molecules that have not only the advantage of killing pathogens but also the disadvantage of harming host tissues and organs. Although the role of dietary antioxidants in invertebrate immunity is still unknown, it has been shown in vertebrates that carotenoids scavenge cytotoxic radicals generated during the immune response. Carotenoids may consequently decrease the self-harming cost of immunity. A positive relationship between the levels of innate immune defence and circulating carotenoid might therefore be expected. Consistent with this hypothesis, we show that the maintenance and use of the prophenoloxidase system strongly correlate with carotenoid concentration in haemolymph within and among natural populations of the crustacean Gammarus pulex.

Allergenicity and allergens of amphipods found in nori (dried laver)

Gammaridean and caprellid amphipods, crustaceans of the order Amphipoda, inhabit laver culture platforms and, hence, are occasionally found in nori (dried laver) sheets. Amphipods mixed in nori may cause allergic reactions in sensitized patients, as is the case with other crustaceans, such as shrimp and crab, members of the order Decapoda. In this study, dried samples of amphipods (unidentified) found in nori and fresh samples of gammaridean amphipod (Gammarus sp., not accurately identified) and caprellid amphipod (Caprella equilibra) were examined for allergenicity and allergens using two species of decapods (black tiger prawn and spiny lobster) as references. When analyzed by ELISA, sera from crustacean-allergic patients reacted to extracts from amphipod samples, although less potently than to the extracts from decapods. In IgE-immunoblotting, a 37-kDa protein was found to be the major allergen in amphipods. Based on the molecular mass and the cross-reactivity with decapod tropomyosin evidenced by inhibition ELISA and inhibition immunoblotting, the 37-kDa protein was identified as amphipod tropomyosin.

Behavioural and physiological effects of the trophically transmitted cestode parasite, Cyathocephalus truncatus, on its intermediate host, Gammarus pulex

Some parasites with complex life-cycles are able to manipulate the behaviour of their intermediate hosts in a way that increases their transmission to the next host. Gammarids infected by the tapeworm Cyathocephalus truncatus (Cestoda: Spathebothriidea) are known to be more predated by fish than uninfected ones, but potential behavioural manipulation by the parasite has never been investigated. In this study, we tested the hypothesis that C. truncatus is able to manipulate the behaviour of one of its intermediate hosts, Gammarus pulex (Crustacea: Amphipoda). To assess if any behavioural change was linked to other phenotypic alterations, we also measured the immunity of infected and uninfected individuals and investigated the pathogenic effects of the parasite. Infected gammarids were significantly less photophobic than uninfected ones, but no effect of infection on the level of immune defence was found. The results on survival, swimming activity and oxygen consumption suggest that the parasite also has various pathogenic effects. However, the alteration in host phototaxis was not correlated to some of these pathogenic effects. Therefore, we propose that the modification in host reaction to light is a behavioural manipulation, explaining the previously observed increase of gammarid predation rate.

Microphallids in Gammarus insensibilis Stock, 1966 from a Black Sea lagoon: host response to infection

We examined the patterns of parasite melanization in Gammarus insensibilis using data on microphallids from Pomorie Lagoon (Black Sea) in the light of 3 predictions associated with host survival: (i) hosts invest more in defence in an environment where the likelihood for infection is higher; (ii) multiple immune challenges exhaust host reserves and result in decreased melanization rates in older hosts; (iii) host immune response is directed against the cerebral metacercariae of Microphallus papillorobustus that alter amphipod behaviour and are most detrimental to the host. G. insensibilis was capable of melanizing the metacercariae of all four species of trematodes found to be hosted by the amphipods. The frequency of melanization and mean abundance of melanized metacercariae were substantially higher than those observed in the same amphipod-gammarid system on the French Mediterranean coast. However, the rate of melanization was low and showed a significant decrease with amphipod size. Although the 4 species were differentially melanized, the host response was largely directed against Microphallus hoffmanni and M. subdolum. We suggest that (i) the lower melanization efficiency with age is due to the mode of infection, probably leading to loss of haemolymph and monopolization of the defence resources for wound healing and (ii) in the French system, host response focuses on the most prevalent and abundant species.

Allergy to freshwater shrimp (Gammarus)

We report three new cases of allergy to Gammarus, two of them involving cutaneous symptoms, and the third one with occupational asthma. The results showed exposure to feed containing Gammarus shrimp to be the main cause of the allergic symptoms in the three patients. In all cases the intervention of IgE was demonstrated, with the absence of cross-reactions with other common allergenic arthropods.

Effects of Microphallus papillorobustus (Platyhelminthes: Trematoda) on serotonergic immunoreactivity and neuronal architecture in the brain of Gammarus insensibilis (Crustacea: Amphipoda)

The larval flatworm Microphallus papillorobustus encysts in the protocerebrum of its intermediate host, Gammarus insensibilis, and changes the gammarid's responses to mechanical and photic stimuli. The resulting aberrant escape behaviour renders infected gammarids more susceptible to predation by birds, the definitive hosts of the parasite. We used immunocytochemical methods to explore the mechanisms underlying these subtle behavioural modifications. Whole mounts of gammarid brains were labelled with fluorescent anti-serotonin and anti-synapsin antibodies and viewed using confocal microscopy. Two types of change were observed in infected brains: the intensity of the serotonergic label was altered in specific regions of the brain, and the architecture of some serotonergic tracts and neurons was affected. A morphometric analysis of the distribution of the label showed that serotonergic immunoreactivity was decreased significantly (by 62%) in the optic neuropils, but not in the olfactory lobes, in the presence of the parasite. In addition, the optic tracts and the tritocerebral giant neurons were stunted in parasitized individuals. Published evidence demonstrates changes in serotonin levels in hosts ranging from crustaceans to mammals infected by parasites as diverse as protozoans and helminths. The present study suggests that the degeneration of discrete sets of serotonergic neurons might underlie the serotonergic imbalance and thus contribute to host manipulation.