title: The trophic vacuum and the evolution of complex life cycles in trophically transmitted helminths.
abstract: Parasitic worms (helminths) frequently have complex life cycles in which they are transmitted trophically between two or more successive hosts. Sexual reproduction often takes place in high trophic-level (TL) vertebrates, where parasites can grow to large sizes with high fecundity. Direct infection of high TL hosts, while advantageous, may be unachievable for parasites constrained to transmit trophically, because helminth propagules are unlikely to be ingested by large predators. Lack of niche overlap between propagule and definitive host (the trophic transmission vacuum) may explain the origin and/or maintenance of intermediate hosts, which overcome this transmission barrier. We show that nematodes infecting high TL definitive hosts tend to have more successive hosts in their life cycles. This relationship was modest, though, driven mainly by the minimum TL of hosts, suggesting that the shortest trophic chains leading to a host define the boundaries of the transmission vacuum. We also show that alternative modes of transmission, like host penetration, allow nematodes to reach high TLs without intermediate hosts. We suggest that widespread omnivory as well as parasite adaptations to increase transmission probably reduce, but do not eliminate, the barriers to the transmission of helminths through the food web.
authors.affiliation: Marine Science Institute, University of California, Santa Barbara, CA 93106-6150, USA Department of Evolutionary Ecology, Max Planck Institute for Evolutionary Biology, August-Thienemann-Strasse 2, 24306 Plön, Germany daniel.benesh@lifesci.ucsb.edu.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Benesh
authors.affiliation: Department of Evolution, Ecology and Behaviour, Institute of Integrative Biology, University of Liverpool, Liverpool L69 7ZB, UK.
authors.forename: James C
authors.initials: JC
authors.lastname: Chubb
authors.affiliation: Department of Evolution, Ecology and Behaviour, Institute of Integrative Biology, University of Liverpool, Liverpool L69 7ZB, UK.
authors.forename: Geoff A
authors.initials: GA
authors.lastname: Parker
mesh_terms: D000818:Animals; D005075:Biological Evolution; D017753:Ecosystem; D020387:Food Chain; D006790:Host-Parasite Interactions; D007448:Invertebrates / Q000469:parasitology; D008018:Life Cycle Stages; D009348:Nematoda / Q000254:growth & development / Q000502:physiology*; D012098:Reproduction; D014714:Vertebrates / Q000469:parasitology
keywords: Nematoda; complex life cycle; food web; skin penetration; trophic level; vector transmission