Migration and ultrastructure of the acanthocephalan Echinorhynchus gadi Zoega in Müller, 1776 in intermediate host under experimental conditions

Amphipods Eogammarus tiuschovi were experimentally infected by the acanthocephalan Echinorhynchus gadi (Acanthocephala: Echinorhynchidae). Within the first four days post-infection, acanthors of the acanthocephalan caused the cellular response of the host, which ended with their complete encapsulation on day 4 post-infection. The acanthors obtained through the experiment were examined ultrastructurally. Two syncytia (frontal and epidermal) and a central nuclear mass are found in the acanthor's body. The frontal syncytium has 3-4 nuclei and contains secretory granules with homogeneous, electron-dense contents. Since the secretory granules occupy only the anterior one-third of this syncytium, it is suggested that the contents of these granules are involved in the acanthor's migration through the gut wall of the amphipod. The central nuclear mass consists of an aggregation of fibrillar bodies and a few electron-light nuclei distributed on the periphery. Some of these nuclei, located near the central nuclear mass, are assumed to be a source of the acanthocephalan's internal organs. The epidermal syncytium surrounds the frontal syncytium and the central nuclear mass. It is represented by a superficial cytoplasmic layer, but the bulk of the cytoplasm is concentrated in the posterior one-third of the acanthor's body. Syncytial nuclei are evenly distributed throughout the cytoplasm. The muscular system of the acanthors consists of 10 longitudinal muscle fibers located below the superficial cytoplasmic layer and two muscle retractors crossing the frontal syncytium.

Pathogens and other symbionts of the Amphipoda: taxonomic diversity and pathological significance

With over 10000 species of Amphipoda currently described, this order is one of the most diverse groups of freshwater and marine Crustacea. Members of this group are globally distributed, and many are keystone species and ecosystem engineers within their respective ecologies. As with most organisms, disease is a key factor that can alter population size, behaviour, survival, invasion potential and physiology of amphipod hosts. This review explores symbiont diversity and pathology in amphipods by coalescing a range of current and historical literature to provide the first full review of our understanding of amphipod disease. The review is broken into 2 parts. The first half explores amphipod microparasites, which include data pertaining to viruses, bacteria, fungi, oomycetes, microsporidians, dinoflagellates, myxozoans, ascetosporeans, mesomycetozoeans, apicomplexans and ciliophorans. The second half reports the metazoan macroparasites of Amphipoda, including rotifers, trematodes, acanthocephalans, nematodes, cestodes and parasitic Crustacea. In all cases we have endeavoured to provide a complete list of known species that cause disease in amphipods, while also exploring the effects of parasitism. Although our understanding of disease in amphipods requires greater research efforts to better define taxonomic diversity and host effects of amphipod symbionts, research to date has made huge progress in cataloguing and experimentally determining the effects of disease upon amphipods. For the future, we suggest a greater focus on developing model systems that use readily available amphipods and diseases, which can be comparable to the diseases in other Crustacea that are endangered, economically important or difficult to house.

Encapsulation of the Acanthocephalan Corynosoma strumosum (Rudolphi, 1802) LÜHE, 1904, in the Intermediate Host Spinulogammarus ochotensis

We describe the thin and ultra-thin structures of the envelopes surrounding the cystacanth of Corynosoma strumosum (Rudolphi, 1802) Lühe, 1904, in its intermediate host. A total of 4,357 amphipods from 2 species were examined: Locustogammarus locustoides (Brandt, 1851) and Spinulogammarus ochotensis (Brandt, 1851). Eleven corynosome cystacanths were found in 6 S. ochotensis specimens. Three were enclosed in acellular cysts originating from the parasite. Three other cystacanths were also encysted and were surrounded by a lighter capsule consisting of the host's hemocytes. Five cystacanths were enclosed in a cyst and a darker capsule, in which both the acanthocephalans and their surrounding envelopes were destroyed. We suggest that the cystacanth's cyst is a protective barrier against the host's cellular response, while the lighter and darker capsules represent different stages of parasite degeneration.

A minimalist macroparasite diversity in the round goby of the Upper Rhine reduced to an exotic acanthocephalan lineage

The round goby, Neogobius melanostomus, is a Ponto-Caspian fish considered as an invasive species in a wide range of aquatic ecosystems. To understand the role that parasites may play in its successful invasion across Western Europe, we investigated the parasitic diversity of the round goby along its invasion corridor, from the Danube to the Upper Rhine rivers, using data from literature and a molecular barcoding approach, respectively. Among 1666 parasites extracted from 179 gobies of the Upper Rhine, all of the 248 parasites barcoded on the c oxidase subunit I gene were identified as Pomphorhynchus laevis. This lack of macroparasite diversity was interpreted as a loss of parasites along its invasion corridor without spillback compensation. The genetic diversity of P. laevis was represented by 33 haplotypes corresponding to a haplotype diversity of 0·65 ± 0·032, but a weak nucleotide diversity of 0·0018 ± 0·00015. Eight of these haplotypes were found in 88·4% of the 248 parasites. These haplotypes belong to a single lineage so far restricted to the Danube, Vistula and Volga rivers (Eastern Europe). This result underlines the exotic status of this Ponto-Caspian lineage in the Upper Rhine, putatively disseminated by the round goby along its invasion corridor.

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.

The life cycle ofSclerocollum saudiiAl-Jahdali, 2010 (Acanthocephala: Palaeacanthocephala: Rhadinorhynchidae) in amphipod and fish hosts from the Red Sea

The rhadinorhynchidSclerocollum saudiiAl-Jahdali, 2010 was found in the intestine of its type host,SiganusrivulatusForsskål & Niebuhr, 1775, a siganid fish permanently resident in a lagoon within the mangrove swamps found on the Egyptian coast of the Gulf of Aqaba (between 28°7′N and 28°18′N). Larval forms of this acanthocephalan (acanthors, acanthellae and cystacanths) were only found inMegaluropus agilisHoek, 1889 (Crustacea: Gammaridae), a benthic amphipod abundant on algae and seagrasses in the lagoon. So, this life cycle ofS. saudiiwas elucidated under semi-natural conditions: embryonated eggs ofS. saudiiwere directly ingested by the amphipod and hatched in its intestine; the released acanthor penetrated the intestinal epithelium in 12–18 h to reach the connective tissue serosa, where it remained for about 3 days, then penetrated the intestinal wall and remained attached to its outer surface for 4 days. It then detached and dropped free in the amphipod haemocoel and transformed into an oval acanthella, growing for 16 days to reach the cystacanth stage. The cystacanth at 46 days post-infection was infective to fish (excysted in its intestine as an active juvenile). Male and female juveniles reached maturity 17 and 23 days post-infection. Recently copulated females first appeared 26 days post-infection and all females seemed to be copulated at 28 days post-infection; partially and fully gravid females first appeared 31 and 35 days post-infection. Mature males and fully gravid females started to die off naturally 31 and 43 days post-infection and were totally expelled from the fish intestine by 42 and 52 days post-infection. The cycle was completed in 89 days and is similar to other known palaeacanthocephalan life cycles, but has its own characteristics.

Parasite-induced alterations of sensorimotor pathways in gammarids: collateral damage of neuroinflammation?

Some larval helminths alter the behavior of their intermediate hosts in ways that favor the predation of infected hosts, thus enhancing trophic transmission. Gammarids (Crustacea: Amphipoda) offer unique advantages for the study of the proximate factors mediating parasite-induced behavioral changes. Indeed, amphipods infected by distantly related worms (acanthocephalans, cestodes and trematodes) encysted in different microhabitats within their hosts (hemocoel, brain) present comparable, chronic, behavioral pathologies. In order to evaluate the potential connection between behavioral disturbances and immune responses in parasitized gammarids, this Review surveys the literature bearing on sensorimotor pathway dysfunctions in infected hosts, on the involvement of the neuromodulator serotonin in altered responses to environmental stimuli, and on systemic and neural innate immunity in arthropods. Hemocyte concentration and phenoloxidase activity associated with melanotic encapsulation are depressed in acanthocephalan-manipulated gammarids. However, other components of the arsenal deployed by crustaceans against pathogens have not yet been investigated in helminth-infected gammarids. Members of the Toll family of receptors, cytokines such as tumor necrosis factors (TNFs), and the free radical nitric oxide are all implicated in neuroimmune responses in crustaceans. Across animal phyla, these molecules and their neuroinflammatory signaling pathways are touted for their dual beneficial and deleterious properties. Thus, it is argued that neuroinflammation might mediate the biochemical events upstream of the serotonergic dysfunction observed in manipulated gammarids – a parsimonious hypothesis that could explain the common behavioral pathology induced by distantly related parasites, both hemocoelian and cerebral.

Biological invasion and parasitism: invaders do not suffer from physiological alterations of the acanthocephalanPomphorhynchus laevis

Biological invasions expose parasites to new invasive hosts in addition to their local hosts. However, local parasites are often less successful in infecting and exploiting their new hosts. This may have major consequences for the competitive ability of hosts, and finally on the fate of the parasite-host community. In Burgundy (Eastern France), the acanthocephalan parasite,Pomphorhynchus laevis, infects 2 amphipod species living in sympatry: the nativeGammarus pulexand the invasiveGammarus roeseli. WhileP. laevisaffects the behaviour and the immunity ofG. pulex,G. roeseliseems unaffected by the infection. In this study, we examined in detail the ability of the parasite to affect the immune system and resource storage of both gammarid species. We found that the infection was associated with a general decrease of the prophenoloxidase activity, haemocyte density, resistance to an artificial bacterial infection and level of sugar reserves inG. pulex, but not inG. roeseli. These results demonstrate a differential ability ofP. laevisto exploit its local and its invasive gammarid hosts. Potential mechanisms of these differential physiological alterations and their potential consequences on the coexistence of both gammarid species in sympatry are discussed.