The amphibian larva represents a morphologically distinct stage between the embryo and adult. The larva is a free-living embryo. It must find food, avoid predators, and participate in all other aspects of free-living existence while it completes its embryonic development and growth. Salamander and caecilian larvae are carnivorous, and they have a morphology more like their respective adult forms than do anuran larvae. Not long after emerging from their egg capsules, larval salamanders, which have four fully developed limbs, start to feed on small aquatic invertebrates. The salamander larvae are smaller versions of adults, although they differ from their adult counterparts by the presence of external gills, a tailfin, distinctive larval dentition, a rudimentary tongue, and the absence of eyelids. Larval caecilians, also smaller models of adults, have external gills, a lateral-line system (a group of epidermal sense organs located over the head and along the side of the body), and a thin skin.

In anurans, tadpoles are fishlike when they hatch. They have short, generally ovoid bodies and long, laterally compressed tails that are composed of a central axis of musculature with dorsal and ventral fins. The mouth is located terminally (recessed), ringed with an oral disk that is often fringed by papillae and bears many rows of denticles made of keratin. Tadpoles often have horny beaks. Their gills are internal and covered by an operculum. Water taken in through the mouth passes over the gills and is expelled through one or more spiracular openings on the side of an opercular chamber. Anuran larvae are microphagous and thus feed largely on bacteria and algae that coat aquatic plants and debris.

Salamander larvae usually reach full size within two to four months, although they may remain larvae for two to three years before metamorphosis occurs. Some large aquatic species, such as the hellbender (Cryptobranchus alleganiensis) and the mud puppy (Necturus maculosus), never fully metamorphose and retain larval characteristics as adults (see below heterochrony). Tadpole development varies in length between species. Some anuran species living in xeric (dry) habitats, in which ephemeral ponds may exist for only a few weeks, develop and metamorphose within two to three weeks; however, most species require at least two months. Species living in cold mountain streams or lakes often require much more time. For example, the development of the tailed frog (Ascaphus truei) takes three years to complete.

Metamorphosis

Metamorphosis entails an abrupt and thorough change in an animal’s physiology and biochemistry, with concomitant structural and behavioral modifications. These changes mark the transformation from embryo to juvenile and the completion of development. Hormones ultimately control all events of larval growth and metamorphosis, and in many instances, development is accompanied by a shift from a fully aquatic life to a semiaquatic or fully terrestrial one.

Although salamanders undergo many structural modifications, these changes are not dramatic. The skin thickens as dermal glands develop and the caudal fin is resorbed. Gills are resorbed and gill slits close as lungs develop and branchial (gill) circulation is modified. Eyelids, tongue, and a maxillary bone are formed, and teeth develop on the maxillary and parasphenoid bones. Changes that occur in caecilians—the closure of the gill slit, the degeneration of the caudal fin, and the development of a tentacle and skin glands—are also minor.

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Skeletal changes are much more dramatic in anurans because tadpoles make an abrupt and radical transition to their adult form. Limbs complete their development, and the forelimbs break through the opercular wall, early in metamorphosis. The tail shrinks as it is resorbed by the body, dermal glands develop, and the skin becomes thicker. As lungs and pulmonary ventilation develop, gills and their associated blood circulation disappear. Adult mouthparts replace their degenerating larval equivalents, and hyolaryngeal structures develop. All anurans except pipids (family Pipidae) develop a tongue. In the newly differentiated digestive tract, the intestine is shortened. The eyes become larger and are structurally altered; eyelids appear. These extreme changes of anuran metamorphosis clearly demarcate the shift from an aquatic to a terrestrial mode of life. Other less obvious yet nonetheless radical modifications of the larval skull and hyobranchial apparatus (that is, the part of the skeleton that serves as base for the tongue on the floor of the mouth) occur to make room for newly developed sense organs. These modifications also facilitate the transition from larval modes of feeding and respiration to those of the adult.

During metamorphosis, the urogenital system of all amphibians is also modified. A mesonephric or opisthonephric kidney—which uses nephrons located either in the middle or at the end of the nephric ridge in the developing embryo—replaces the degenerating rudimentary pronephric kidney. This transition is linked to the shift from production of a large volume of dilute ammonia to a small amount of concentrated urea. Gonads and associated ducts also appear and begin their maturation.

Heterochrony

Neoteny, once a widely used label for the condition of sexually mature larvae, has been discontinued by biologists and replaced by the concept of heterochrony. Heterochrony refers to the change in the timing and rate of developmental events, and it is a widespread feature in amphibian evolution, particularly in salamanders. During development, a structure can begin to develop sooner (predisplacement) or later (postdisplacement) in an organism than it occurred in the ancestral species or parents. Also, a structure may continue to develop beyond the previous embryological sequence (hypermorphosis) or the developmental sequence can stop earlier than normal (progenesis or hypomorphosis). Each of these heterochronic events can produce a structurally or functionally different organism.

The classical “neotenic” salamander, the axolotl (Ambystoma mexicanum), is a paedomorphic species (that is, a species that retains aspects of its juvenile form during its adult phase); it retains its larval gills. In the mole salamander (Ambystoma talpoideum), some populations also display hypomorphic development in which the development of several larval traits to the adult condition is delayed. Since the gonads mature, a population of sexually mature salamanders with a larval morphology is produced. Heterochrony also explains the presence of larval traits in adults of the salamander families Cryptobranchidae (hellbenders) and Proteidae (olms and mud puppies).

Heterochrony is not confined to salamanders. The different sized eardrums in the American bullfrog (Lithobates catesbeianus) are examples of hypermorphism in male bullfrogs. The development of the eardrums in the male extends beyond that of the female.