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Free keywords:
complex life cycle; food web; Nematoda; skin
penetration; trophic level; vector transmission
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.
