Abstract
Changes to host behaviour induced by some trematode species, as a means of increased trophic transmission, represents one of the seminal examples of host manipulation by a parasite. The amphipod Echinogammarus marinus (Leach, 1815) is infected with a previously undescribed parasite, with infected individuals displaying positive phototaxic and negative geotaxic behaviour. This study reveals that the unknown parasite encysts in the brain, nerve cord and the body cavity of E. marinus, and belongs to the Microphallidae family. An 18 month population study revealed that host abundance significantly and negatively correlated with parasite prevalence. Investigation of the trematode's influence at the transcriptomic level revealed genes with putative neurological functions, such as serotonin receptor 1A, an inebriated-like neurotransmitter, tryptophan hydroxylase and amino acid decarboxylase, present consistent altered expression in infected animals. Therefore, this study provides one of the first transcriptomic insights into the neuronal gene pathways altered in amphipods infected with a trematode parasite associated with changes to its host's behaviour and population structure.
Highlights
This study aims to identify the parasite species infecting E. marinus by comparing the parasite ribosomal RNA gene sequences with sequences deposited in public databases and those isolated from M. papillorobustus
Those that were found infecting the brain were found to make up approximately 7–9% of the total population (∼20% of the infected portion) with similar infection between males and females The 18S, 28S and Internal Transcribed Spacer (ITS) regions of the parasite ribosomal RNA gene was obtained and used in conjunction with previously published sequences to perform a phylogenetic analysis
This reveals that the unknown parasite within the E. marinus population at Langstone Harbour is a trematode belonging to the family Microphallidae (Fig. 2)
Summary
Certain parasite species are known to induce changes to host behaviour as a means of increased trophic transmission, a process that represents one of the seminal examples of parasite-induced behavioural manipulation (Pearson, 1972; Bethel and Holmes, 1973; Camp and Huizinga, 1979; Helluy, 1983a, b; Combes, 1991; Combes et al 1994; Haas, 1994; Esch et al 2002; Mouritsen and Poulin, 2002). The discovery that E. marinus can be infected with a parasite apparently capable of manipulating host behaviour (Guler and Ford, 2010) has never been further investigated In this case, infected specimens of the normally evasive E. marinus were found to spend significantly more time in the light and higher in the water column than uninfected individuals, a behaviour consistent with a parasitic strategy to increase trophic transmission. Such manipulative parasites are not unknown in amphipods, as the behaviour of infected E. marinus is similar to that observed in Gammarus insensibilis infected by the trematode Microphallus papillorobustus (Helluy, 1983a, b).
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