Data from: Complex life cycles: why refrain from growth before reproduction in the adult niche?

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 non-feeding, non-growing adult stages. We show theoretically that this counterintuitive 'no-growth' 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 spp.) 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, already 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.

具有复杂生活史的生物在幼虫和成虫阶段占据不同的生态位。该特征的一项推定优势在于，生物可在个体发育过程中依次利用不同的资源。然而，诸多生物类群演化出了非取食、不生长的成虫阶段。我们从理论层面证明，当最优幼虫体型大于或等于用于繁殖的最优成虫体型时，这种反直觉的“不生长”策略将受到选择青睐。我们针对一类寄生蠕虫（helminths）开展了实证研究。寄生蠕虫会通过营养级传递在宿主间传播，随后在大型高营养级宿主内完成繁殖；多数类群在终宿主体内会经历显著的成虫生长阶段。部分被广泛研究的绦虫物种（如Schistocephalus、Ligula和Digramma属物种）是显著的例外：它们在最终栖息环境——食鱼鸟类的肠道内并不会发生生长，却能完成单次生殖。通过跨物种比较分析，我们证实这类在终宿主体内不发生生长的绦虫具备两项特征：其一，它们在末次中间宿主（鱼类）体内达到的幼虫体型，与在相同成虫生态位（同样为食鱼鸟类）内生长的绦虫所达到的成虫体型相当甚至更大；其二，相较于终宿主的体重，它们的幼虫体型已然十分庞大。上述结果与“巨大的幼虫体型可使成虫生长变得多余”这一观点相符，我们还讨论了该结论是否可推广至其他具有非取食、不生长成虫阶段的复杂生活史类群。