A Lack of Crowding? Body Size Does Not Decrease with Density for Two Behavior-Manipulating Parasites

Abstract

For trophically transmitted parasites that manipulate the phenotype of their hosts, whether the parasites do or do not experience resource competition depends on such factors as the size of the parasites relative to their hosts, the intensity of infection, the extent to which parasites share the cost of defending against the host's immune system or manipulating their host, and the extent to which parasites share transmission goals. Despite theoretical expectations for situations in which either no, or positive, or negative density-dependence should be observed, most studies document only negative density-dependence for trophically transmitted parasites. However, this trend may be an artifact of most studies having focused on systems in which parasites are large relative to their hosts. Yet, systems are common where parasites are small relative to their hosts, and these trophically transmitted parasites may be less likely to experience resource limitation. We looked for signs of density-dependence in Euhaplorchis californiensis (EUHA) and Renicola buchanani (RENB), two manipulative trematode parasites infecting wild-caught California killifish (Fundulus parvipinnis). These parasites are small relative to killifish (suggesting resources are not limiting), and are associated with changes in killifish behavior that are dependent on parasite-intensity and that increase predation rates by the parasites' shared final host (indicating the possibility for cost sharing). We did not observe negative density-dependence in either species, indicating that resources are not limiting. In fact, observed patterns indicate possible mild positive density-dependence for EUHA. Although experimental confirmation is required, our findings suggest that some behavior-manipulating parasites suffer no reduction in size, and may even benefit when "crowded" by conspecifics.

The relationship between handling time and cortisol release rates changes as a function of brain parasite densities in California killifish Fundulus parvipinnis

“…transmission to the same definitive hosts), and show no signs of competition in their F . parvipinnis host (Weinersmith et al , ).…”

“…Lacking any information about how RENB influences F. parvipinnis physiology it was predicted that changes in hormone levels would be in the same direction as those induced by EUHA because the two parasites work towards a common goal (i.e. transmission to the same definitive hosts), and show no signs of competition in their F. parvipinnis host (Weinersmith et al, 2014).…”

Evolution of complex life cycles in trophically transmitted helminths.II. How do life‐history stages adapt to their hosts?

“…Although reported mainly in adults, resource limitation may often be greater in intermediate hosts, where parasite/host size ratios tends to be higher than in definitive hosts (Kuris & Lafferty, ; Lafferty & Kuris, ; Poulin, ), and crowding effects have been observed in larvae that are large relative to their hosts, for example Schistocephalus solidus , in both first (Michaud et al ., ) and second intermediate hosts (Heins et al ., ). Such effects were not found in trematodes Euhaplorchis californiensis and Renicola buchanani with small larvae relative to their intermediate host (California killifish), where resources may not be limiting (Weinersmith et al ., ).…”

Invisible Designers: Brain Evolution Through the Lens of Parasite Manipulation

“…In response to escalating costs, parasites have two main options. First, they can infect the host in larger numbers to share the additional metabolic costs (Weinersmith et al 2014;Gopko et al 2017). Second, they can evolve indirect manipulation mechanisms that act upstream or downstream of the expensive signal.…”