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Free keywords:
cestode; complex life cycle; life-history theory; phylogenetic analysis; size at maturity
Abstract:
Organisms with complex life cycles occupy distinct
niches as larvae and adults. One presumed advantage of this is the
ability to exploit different resources successively throughout ontogeny.
Various taxa, however, have evolved nonfeeding, nongrowing
adult stages. We show theoretically that this counterintuitive nogrowth
strategy is favored when the optimal larval size is greater
than or equal to the optimal adult size for reproduction. We empirically
investigated this in a group of parasitic worms (helminths).
Helminths are transmitted trophically between hosts before reproducing
in large, high-trophic-level hosts, and most undergo considerable
growth as adults in their final host. Some well-studied tapeworm
species (Schistocephalus, Ligula, and Digramma species) are
notable exceptions; they reproduce semelparously without any
growth in their final habitat (the gut of piscivorous birds). Using
cross-species comparative analyses, we show that these tapeworms
that do not grow in their final host (1) attain larval sizes in their
last intermediate host (fishes) that are comparable to or larger than
the adult sizes reached by tapeworms that do grow in the same adult
niche (also piscivorous birds) and (2) are large, even as larvae, relative
to the mass of their final hosts. These results are consistent with the
idea that a massive larval size can make adult growth superfluous,
and we discuss whether this likely applies to other complex life cycle
taxa with nonfeeding, nongrowing adults.
