Metamorphosis

From EvoWiki

Metamorphosis is a fancy term for the changes of form that many organisms undergo as they grow. The two main types are:

• Direct, where immature phases closely resemble mature, reproducing phases (adults)
• Indirect, where immature phases (larvae) often look greatly different from mature phases

Familiar examples of indirect metamorphosis are:

• Frogs: most species have fishlike larvae (tadpoles).
• Four-stage insects (Endopterygota): their larvae are variously called caterpillars, grubs, or maggots, and they have a resting phase (pupa, chrysalis) where they change from larva to adult.
• Ascidians (sea squirts): their tadpole-like larva has a primitive-chordate appearance, while their adult is a sessile filter feeder. The ascidian life cycle has much in common with the career of a tenured professor, an immature phase of feverishly seeking a place to rest followed by one's brain atrophying.
• Many marine invertebrates, notably many mollusks, annelids, echinoderms, and hemichordates, have "maximal indirect development", where the adult develops from a small rudiment of set-aside cells in a planktonic larva, and has little resemblance in body form with that larva.
• Many crustaceans have a larva called a nauplius, which initially has only a few segments, and which adds segments as it grows. Barnacles have filter-feeding sessile adults, and among them, the rhizocephalans (parasitic barnacles) like Sacculina have a blob-shaped adult that sends tendrils into its host.

Metamorphosis can evolve in both directions, from direct to indirect, and from indirect to direct.

Insects have a clear progression from direct to indirect:

• Basal flying insects, like grasshoppers (Orthoptera) and cockroaches (Blattodea): these develop directly, with immature forms (nymphs) having poorly-developed wings.
• Beetles (Coleptera), butterflies and moths (Lepidoptera), etc: larvae have well-defined heads, mouthparts, and legs, though poorly-developed eyes.
• Flies and mosquitoes (Diptera), etc.: larvae have poorly-defined heads and lack mouthparts and legs.

Early land vertebrates (tetrapods) very likely had fishlike larvae; this is inferred from the larval phase of frogs and the embryos of amniotes. This, living on land was originally a feature of adults, whose greater size made them more resistant to drying out.

Barnacles and ascidians have sessile, filter-feeding adults; this is also a special adaptation of adults. Such specialization is taken to an extreme in rhizocephalans, whose adults resemble fungi.

Indirect to direct has happened in amniotes, some frog species, and various marine invertebrates; this happens as a result of the larva-to-adult transition being moved earlier and earlier in time until it is performed inside the egg. This can result in embryos having vestigial larval features, like the gill bars, gill pouches, and aortic arches of amniote embryos.

Perhaps most interesting is the hypothesis that animal development mechanisms have evolved more than once, with the second one being overlaid on the first one. This is the hypothesis of Davidson and others, reported in Knoll and Carroll 1999. According to this hypothesis, maximal indirect development is the ancestral style of bilaterian development. In it, the larva develops by a relatively simple mechanism involving localized cell-cell interactions. The adult develops from part of it by a different mechanism, one that has better scaling to large sizes.

References

Andrew H. Knoll and Sean B. Carroll, "Early Animal Evolution: Emerging Views from Comparative Biology and Geology", Science, 284, 2129 (1999), online as PDF and HTML.