Gammarus Pulex Social Behavior


Parasite-induced alteration of plastic response to predation threat: increased refuge use but lower food intake in Gammarus pulexinfected with the acanothocephalanPomphorhynchus laevis


Larvae of many trophically-transmitted parasites alter the behaviour of their intermediate host in ways
that increase their probability of transmission to the next host in their life cycle. Before reaching a stage
that is infective to the next host, parasite larvae may develop through several larval stages in the intermediate host that are not infective to the definitive host. Early predation at these stages results in parasite death, and it has recently been shown that non-infective larvae of some helminths decrease such risk
by enhancing the anti-predator defences of the host, including decreased activity and increased sheltering. However, these behavioural changes may divert infected hosts from an optimal balance between survival and foraging (either seeking food or a mate). In this study, this hypothesis was tested using the
intermediate host of the acanthocephalan parasite Pomphorhynchus laevis, the freshwater amphipod
Gammarus pulex. We compared activity, refuge use, food foraging and food intake of hosts experimentally
infected with the non-infective stage (acanthella), with that of uninfected gammarids. Behavioural assays
were conducted in four situations varying in predation risk and in food accessibility. Acanthella-infected
amphipods showed an increase in refuge use and a general reduction in activity and food intake. There
was no effect of parasite intensity on these traits. Uninfected individuals showed plastic responses to
water-borne cues from fish by adjusting refuge use, activity and food intake. They also foraged more
when the food was placed outside the refuge. At the intra-individual level, refuge use and food intake
were positively correlated in infected gammarids only. Overall, our findings suggest that uninfected gammarids exhibit risk-sensitive behaviour including increased food intake under predation risk, whereas
gammarids infected with the non-infective larvae ofP. laevisexhibit a lower motivation to feed, irrespective of predation risk and food accessibility

Multidimensionality in host manipulation mimicked by


Manipulative parasites often alter the phenotype of their hosts along multiple
dimensions. ‘Multidimensionality’ in host manipulation could consist in the
simultaneous alteration of several physiological pathways independently of
one another, or proceed from the disruption of some key physiological parameter, followed by a cascade of effects. We compared multidimensionality in
‘host manipulation’ between two closely related amphipods,Gammarus fossarum
andGammarus pulex,naturally and experimentally infected withPomphorhynchus laevis(Acanthocephala), respectively. To that end, we calculated in
each host – parasite association the effect size of the difference between infected
and uninfected individuals for six different traits (activity, phototaxis, geotaxis,
attraction to conspecifics, refuge use and metabolic rate). The effects sizes were
highly correlated between host –parasite associations, providing evidence for a
relatively constant ‘infection syndrome’. Using the same methodology, we compared the extent of phenotypic alterations induced by an experimental injection
of serotonin (5-HT) in uninfectedG. pulexto that induced by experimental or
natural infection withP. laevis. We observed a significant correlation between
effect sizes across the six traits, indicating that injection with 5-HT can faithfully
mimic the ‘infection syndrome’. This is, to our knowledge, the first experimental
evidence that multidimensionality in host manipulation can proceed, at least
partly, from the disruption of some major physiological mechanism.