Metamorphosis
Metamorphosis, or a change in form, in biology means the transition from a larval stage to an adult stage. In most animals, embryonic development leads to the formation of a larval stage with very different characteristics than the adult stage. Larval morphology, or form, may be specialized for some function such as growth (which requires feeding and associated structures) or dispersal. Some larval forms, called exotrophic, feed, while others, called endotrophic, are nonfeeding. Juvenile and adult forms often live in very different environments.
Cnidarians have varying types of metamorphosis. Some species have three distinct life history stages: the planula, medusa, and polyp. The planula stage is the free-living larval stage. The medusa stage involves a single individual or a colony of individuals that act as a single free-swimming organism (examples include jellyfish and man-o-war). The polyp stage is sessile (adhered to the substrate) and may involve a single individual or a colony of individuals (examples include sea anemones and corals). Some species lack the free-swimming medusa stage. In others, the medusa is the dominant life history stage and the polyp stage is lacking completely.
Molting and metamorphosis in arthropods is controlled by environment and hormones. Insects experience no size increase in the egg, pupal (the thirdstage in the life of an insect that undergoes complete metamorphosis), or adult stages. All growth occurs during the intermediate larval or nymphal stages. Anametabolous (without change) metamorphosis occurs in the primitive insect groups Colembola, Diplura, Protura, and Thysanura. Juveniles change little except in size and proportion from egg to adult. After reaching adulthood, defined as sexual maturity, they continue to molt, adding antennal segments.
Many insects (including dragonflies, grasshoppers, and cockroaches) and crustaceans (crawfish and crabs) develop through hemimetabolous (incomplete or gradual) metamorphosis. In hemimetabolous metamorphosis, the insect egg hatches into a nymph. The nymph is similar to the adult in general morphology, only smaller. The nymph is an actively feeding stage, and as it grows it must shed its exoskeleton and produce a new, larger one. This process is called molting. In insects with hemimetabolous metamorphosis, the gonads do not mature until after the final molt.
Some insects (including flies, butterflies, wasps, and bees) have holometabolous metamorphosis (they undergo a complete metamorphosis,having distinct larval and pupal stages). There are four distinct stages in the life cycle of holometabolous insects: egg, larva, pupa, and imago (adult). The larva is segmented and wormlike. The larval stage is a feeding stage and consists of several subdivisions called instars. Each instar ends in molting, which allows the larva to grow.
The life cycle of the leopard frog (Rana pipiens). Metamorphosis occurs when the tadpole grows legs, and ends with the resorption of the tail.The final instar ends with pupation. Prior to pupation, the animal stops feeding and the cuticle hardens and darkens to form the puparium (pupal chamber), where metamorphosis will take place. The pupa begins to darken just prior to the emergence of the imago. In the larvae of these organisms, imaginal disks (clusters of cells carried with a larva that will develop into different adult body parts) are formed. These disks will produce adult organs, but they remain quiescent, or inactive, in the embryo until the appropriate time.
Most, but not all, amphibians have a biphasic (two-phase) life history with an aquatic larval stage that metamorphoses to become an adult, a process known as indirect development. Many frogs have a free-living, aquatic larval stage as a tadpole. Near the end of the larval stage, many larval structures are reabsorbed or remodeled and adult structures begin to form.
During metamorphosis bones begin to ossify, the tail is reabsorbed, limbs form, and larval respiratory and feeding structures (including gills and a beak with keratinized—covered with a tough protein like our fingernails— mouth parts) are replaced by adult structures (including lungs and movable jaws). The digestive system is remodeled to accommodate a transition from a largely herbivorous diet to one that is strictly carnivorous.
In salamanders, the larval stage is more similar to the adult stage than is the case with frogs. Metamorphosis usually involves the replacement of larval gills with lungs; ossification of the skull, vertebral column, and limbs; and the remodeling of the tail and feeding apparatus to conform to the requirements of life on land.
Although this familiar mode of development is common among amphibians, some salamanders, caecilians, and frogs lack a free-living larval stage. In many species with a monphasic life history, a miniature version of the adult is hatched directly from the egg (direct development) in what is called ovoviparity, or birthed by the female, in what is known as viviparity.
The loss of a free-swimming larval stage has been hypothesized to release a major limit on morphological diversification in some groups of direct-developing frogs, because the pre-pattern established by larval structures is no longer present. Evidence of this morphological release can be seen in the great diversity of species and morphologies attained by some amphibian groups that have lost the free-living tadpole stage.
Bibliography
Brusca, Richard G., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.
Gilbert, Scott F. Developmental Biology, 5th ed. Sunderland, MA: Sinauer Associates, 1997.
Gould, Stephen Jay. Ontogeny and Phylogeny. Cambridge, MA: Harvard University Press, Belknap Press, 1977.
Kalthoff, Klaus. Analysis of Biological Development, 2nd ed. Boston: McGraw-Hill, Inc., 2001.
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