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A pronounced change in both the internal and external morphology of an animal that takes place in a short amount of time, triggered by some combination of external and internal cues. The extent of morphological change varies considerably among species. Even when morphological changes are relatively slight, metamorphosis typically brings about a pronounced shift in habitat and lifestyle. The precise morphological, physiological, and biochemical changes that constitute metamorphosis; the neural, hormonal, and genetic mechanisms through which those changes are controlled; and the ecological consequences of those changes and when they take place continue to be studied in a wide variety of animals. The hormonal and genetic control of metamorphosis has been best examined in a few species of insect, amphibian, and fish (such as flounder), but other aspects of metamorphosis have been investigated for other insect, amphibian, and fish species as well as for marine invertebrates and, indeed, representatives of essentially every animal phylum.
Amphibians exhibit extensive tissue remodeling during metamorphosis, including resorption of the tail musculature and skeletal system; major reconstruction of the digestive tract; degeneration of larval skin and pronounced alteration in skin chemical composition; growth of the hind and fore limbs; degeneration of the gills and associated support structures; shifts in mode of nitrogen excretion, from ammonia to urea; alteration in visual system biochemistry; replacement of larval hemoglobin with adult hemoglobin; and differential growth of the cerebellum. See Amphibia
Metamorphosis among insects is associated primarily with wing development. Bristletails and other species that do not develop wings and are not descended from winged ancestors exhibit no pronounced metamorphosis. Metamorphosis is most dramatic among holometabolous species, which pass through a distinctive and largely inactive pupal stage; in such species, all of the transformations separating the larval morphology and physiology from that of the adult take place in the pupa. Wings, compound eyes, external reproductive parts, and thoracic walking legs develop from discrete infolded pockets of tissue (imaginal discs) that form during larval development. See Insecta
The most dramatic metamorphic changes in fish are seen among flounder and other flatfish: in such species, during metamorphosis a symmetrical fish larva becomes an asymmetrical adult, with both eyes displaced to the dorsal surface. The transformation of leptocephalus larvae into juvenile eels is also dramatic; such transformation includes a shift in the position of the urinary and digestive tracts from posterior to anterior.
The control of metamorphosis among crabs, barnacles, gastropods, bivalves, bryozoans, echinoderms, sea squirts, and other marine invertebrates is poorly understood, partly due to the very small size of the larvae—they rarely exceed 1 mm in length, and most are less than 0.5 mm. The larvae of some marine invertebrate species are triggered to metamorphose by specific substances associated with adults of the same species, or with the algae or animals on which they prey. See Annelida, Bivalvia, Echinodermata, Gastropoda, Mollusca
Among insects, the timing of metamorphosis is influenced by environmental factors such as temperature, humidity, photoperiod, pheromone production by neighboring individuals, and the nutritional quality of the diet. In a number of species, larvae can undergo developmental arrest (a diapause) in response to unfavorable environmental conditions, so that metamorphosis can be delayed for many months or even years. The hormonal basis for such effects has been at least partly worked out for a number of insect species.
Among marine invertebrates and in at least some fish species, there is also considerable flexibility in the timing of metamorphosis. At some point in the development of marine invertebrates and apparently also in the development of some coral reef fishes, individual larvae become “competent” to metamorphose. It is not yet clear what makes larvae competent; the development of external receptor cells, or the completion of specific neural pathways, or the activation of hormonal systems or their receptors are likely possibilities. See Endocrine system (invertebrate)
a change in the organic structure of plants and animals.
Plants. Metamorphosis in plants is modification in the principal plant organs that is usually associated with a change in their functions or environment. It, is ontogenetic and consists of a change in an individual organ’s development, which has been altered and established in the evolutionary process.
Metamorphosis usually affects the shoot and the leaf (as a lateral organ) because of various environmental conditions. Metamorphosis of an aboveground shoot with green leaves is most often provoked by a lack of moisture and is observed in regions subject to droughts. Thus, most succulents, such as cacti and African spurges, have fleshy stems that store water and become organs of photosynthesis. In the axils of underdeveloped leaves, their stems develop short shoots with clusters of spines. Owing to the absence of leaves, the evaporative surface area of the shoot sharply decreases. Such a decrease in surface is also observed in the metamorphosis of such aboveground organs as cladophylls (for example, in asparagus) and phylloclades (for example, in butcher’s-broom). In plants with such organs, photosynthesis is performed by the rough, dry stem, which often becomes flattened and leaflike in appearance.
Sometimes only part of a shoot undergoes metamorphosis, for example, the formation of woody, leafless spines in hawthorn and honey locust. Lianas, which grow under conditions of high moisture and reduced light, are sometimes characterized by aboveground shoots that have been modified into climbing organs, or tendrils; for example, in passion flowers and grapes, the tendrils are modified inflorescences. Not uncommonly, only the leaves undergo metamorphosis (for example, the spines on barberry stems and the tendrils of legumes). Tendrils may be formed from the entire leaf blade (as in certain vetchlings) or from only a leaflet of a compound leaf (in peas). The leaves of insectivorous plants are modified into unique traps. In some acacias the leaf blades may not develop at all, and photosynthesis is performed by stiff, flattened petioles, or phyllodes.
In perennial, primarily herbaceous plants, metamorphosis of underground shoots is common to ensure survival through unfavorable periods, renewal of growth, and vegetative propagation. These underground organs, which serve a: storage function, lack green leaves but have buds; they include rhizomes, tubers, bulbs, and corms. Root metamorphosis is usually associated with hypertrophy of the storage function (for example, the,formation of edible roots) or with a specific root function above ground (for example, the aerial roots of epiphytes and the pneumatophores of mangroves).
A modified shoot adapted for seed propagation may arise in a flower: the sepals, petals, stamens, and carpels may correspond, according to their origin, to the leaves, and the flower receptacles, to the stem. This modification is confirmed in plants whose flowers sprout, or proliferate (for example, roses and avens).
Ideas concerning metamorphosis in plant organs developed primarily in connection with efforts to understand flowers. Such attempts can be traced to the Italian botanist Cesalpino in the 16th century and to the German botanist J. Jung in the 17th century. The term “metamorphosis” was introduced into scientific circles by C. Linnaeus in 1755, who mistakenly thought that various flower parts formed as a result of the metamorphosis of certain stem tissues. In 1759, K. F. Wolff first described the formation of leaf buds and floral parts on the growing point of the shoot, thereby showing their homology. The theory of metamorphosis was formulated in 1790 by J. W. von Goethe, who understood the process to be one of leaf transformation during the plant’s ontogeny. Goethe’s ideas were used to explain the formation of metamorphic organs in the phylogeny of various taxonomic groups of plants.
Metamorphosis may occur at various stages in the development of organs. Many herbaceous plants form shoots that first appear on the soil’s surface and have green assimilative leaves. These leaves are subsequently lost, and secondary roots develop that gradually burrow into the earth, where they become underground storage organs, or rhizomes. Thus, true metamorphosis occurs as one organ is transformed into another with a change of form and function. In most cases, however, it is not the mature organs but their embryos that undergo metamorphosis. According to the Soviet physiologist D. A. Sabinin, the process of determination, defining the final form of an organ’s embryo and tracing various stages in its development, is related to the accumulation of specific physiologically active substances and depends on a series of external and internal factors.
REFERENCESSerebriakov, I. G. Morfologiia vegetativnykh organov vysshikh rastenii. Moscow, 1952.
Fedorov, A. A., M. E. Kirpichnikov, and Z. T. Artiushenko. Atlas po opisaternoi morfologii vysshikh rastenii, vols. 1–2. Moscow-Leningrad, 1956–62.
Goethe, J. W. Izbrannye sochineniia po estestvoznaniiu. Moscow, 1957.
Sabinin, D. A. Fiziologiia razvitiia rastenii. Moscow, 1963.
Pervukhina, N. V. Problemy morfologii i biologii tsvetka. Leningrad, 1970.
T. I. SEREBRIAKOVA,
Animals Animal metamorphosis, or metabolism, consists of significant changes in the organic structure in postembryonic development. It is usually related to an abrupt change in the environment and the way of life of the animal during its individual development, or ontogeny. It may be a change from free-swimming life to an attached existence or from aquatic life to life on land or in the air. Thus, in the life cycle of animals that undergo metamorphosis, there is at least one larval stage in which the organism differs markedly from its adult form. In developing through metamorphosis, animals in one stage or another of their ontogeny perform functions that facilitate the preservation and furthering of the species.
Even the simplest animals, such as Suctoria, undergo some form of metamorphosis: the new individuals that bud off are covered with cilia and swim; they subsequently lose their cilia and become anchored life forms, feeding by means of extended tubes. For lower invertebrates, such as sponges and coelenterates, metamorphosis is typical, enabling the free-swimming larvae, such as parenchymula, amphiblastula, and planula, to fulfill the function of species distribution. In many cases this type of metamorphosis is complicated by the alternation of generations, by which reproduction alternates from asexual to sexual (as in the scyphozoans and many flatworms). Nemerteans undergo a distinctive, so-called necrotic metamorphosis, by which the future adult develops within the larva, while the basic mass of the larva atrophies. In many invertebrates whose metamorphosis does not involve alternation of generations, a larva emerges from the egg and performs a distributive function (for example, the trochophore of polychaetes, the veliger of marine mollusks). The mature adult has larval segments (remaining from the primary larva) and postlarval segments (which form later). For example, the antennules, antennae, and mandibles of crustaceans develop from appendages of the nauplius and correspond to larval segments.
The transition to freshwater and terrestrial life led to the disappearance of larval stages of development in many animals. The development of the Roman snail is characterized by the emergence from the egg of an individual similar to the mature adult; in the egg the organism passes through a stage resembling the veliger of marine forms. This type of development is called cryptometabolism. Numerous myriapods and lower proturans undergo anamorphosis in the postembryonic stage of development; this process consists of changes involving increases in the number of segments and joints of the tendrils.
For most Apterygota insects, ametabolism, or development without fundamental changes, is characteristic. The development of wings in many insects brought various changes in their ontogeny. If the way of life of early postembryonic stages and the adult form is similar, a nymph that resembles the mature insect emerges from the egg. Organic changes are accompanied by the gradual growth of wing primordia (hemimetabolism, epimorphosis). Animals with a life cycle that has a sharp division of basic functions (feeding in the larval stage, migration and reproduction in the adult stage) are said to undergo holometabolism. In this type of development the vermicular larva bears no resemblance to the adult form. The transition of the larva into the adult form is accompanied by abrupt changes in the organism and occurs in the pupal stage, during which the pupa does not eat and seldom moves; the larval tissues are broken down in the pupa, and the organs of the adult insect (such as the wings) are formed. Echinodermatous larvae (dipleurula, bipinnaria, and pluteus) and coelenterate larvae (tornaria and the tailed ascidian larva) swim freely and, thereby, serve to distribute the species.
Among vertebrates, metamorphosis characterizes lampreys, whose larvae, or ammocoetes, live in the soil and whose adults parasitize fishes. Many fishes, such as the dipnoans, have larvae with external gills and adult forms with gills in a special cavity and lungs. Larval amphibians, or tadpoles, emerge from eggs and resemble small fishes; they can live only in water. Depending on the extent of metamorphosis, the organs of the larval stage disappear and are replaced by those of the adult animal. A little frog with the remnant of a tail emerges onto dry land and soon takes on the form of a mature frog. Metamorphosis is controlled by hormones. The hormone of insects from the prothoracic glands was identified in 1954 and synthesized in 1966. This hormone, ecdysone, also regulates molting. Retardation of metamorphosis is caused by juvenile hormone, which is released by nearby glands. In amphibians, metamorphosis is regulated by hormones of the thyroid gland.
REFERENCESEzhikov, I. I. Metamorfoz nasekomykh. Moscow, 1929.
Gilyarov, M. S. “Vliianie kharaktera rasseleniia na khod ontogeneza nasekomykh.” In Zhurnal obshchei biologii, 1945, vol. 6, no. 1.
Ivanov, P. P. Rukovodstvo po obshchei i sravniteVnoi embriologii Lenin-grad, 1945.
Novak, V. J. A. Insect Hormones, 3rd ed. London, 1966.
M. S. GILIAROV