Natural history ( Amphibian )
Tagged: animal, cat, fish, invertebrateLife cycle
Most amphibians have a biphasic life cycle involving aquatic eggs and larvae that metamorphose into terrestrial adults. They deposit large numbers of eggs in water; clutches may exceed 5,000 eggs in the tiger salamander (Ambystoma tigrinum) and 45,000 eggs in large bullfrogs (Rana catesbeiana). Egg size and water temperature influence the length of time required by embryos to develop in the egg before hatching; many anuran eggs laid in warm water require only one or two days to develop, whereas eggs deposited in cold mountain lakes or streams may not hatch for 30 to 40 days. Development of salamander eggs usually requires more time, with hatching occurring 20 to 270 days after fertilization.
Reproduction
The three living groups of amphibians have distinct evolutionary lineages and exhibit a diverse range of life strategies. The breeding behaviour of each order will be discussed in its individual section below. One similar tendency among amphibians has been the evolution in some species of direct development of young in terrestrial eggs, thus eliminating the aquatic egg and larval stage. The majority of species of the largest salamander family (Plethodontidae), some caecilians, and many species of anurans deposit eggs on land; these eggs develop directly and hatch as miniature adults. Numerous caecilians and a few species of anurans and salamanders are viviparous; i.e., they give birth to live young. Anurans present a gamut of life history variations. Centrolenids and phyllomedusine hylids deposit eggs on vegetation above streams or ponds, from which hatchling tadpoles (anuran larvae) drop into the water where they continue to develop throughout their larval stage. Some species from the families Leptodactylidae and Rhacophoridae create foam nests for their eggs in aquatic, terrestrial, or arboreal habitats; after hatching, tadpoles usually develop in water. Dendrobatids and other anurans deposit their eggs on land and transport them to water. Female hylid marsupial frogs are so called because they carry their eggs in a pouch on their backs; they are sometimes not transported within the pouch but are exposed on the back. In some species when tadpoles emerge, the female places them in a pond.
Embryonic stage
Within the egg a series of semipermeable gelatinous capsules enclose the embryo suspended in fluid surrounding the yolk (perivitelline fluid). These capsules can be eventually digested by hatching enzymes that are produced by frontal glands on the snouts of embryos. All nutrients necessary for egg development within the ovary are supplied by the yolk in the egg capsule, but various stages utilize different nutrients. In early development, fats are the necessary energy source, followed by an increasing reliance on carbohydrates during gastrulation (early developmental stage in which the embryo consists of two cell layers). After gastrulation, a return to fat utilization occurs. During the latter developmental stages, when morphological structures form, proteins are the primary energy source. By the neurula stage (embryonic stage in which nervous tissue develops), cilia appear on the embryo. The fluid movement of these hairlike structures rotates the embryo within the perivitelline fluid. Caecilians, salamanders, and some anurans have external gills that press against the inner wall of the egg capsule, permitting exchange of gases (oxygen and carbon dioxide) with maternal tissues. Ammonia is the principal form of nitrogenous waste and is diluted by a constant diffusion of water in the perivitelline fluid.
The embryos of aquatic salamanders develop limbs in an anteroposterior wave, and digits appear sequentially on both sets of limbs. Salamanders that deposit their eggs in streams produce embryos that develop both sets of limbs before they hatch, but salamanders that deposit their eggs in still water have embryos that develop only forelimbs before hatching. In contrast, the limbs of anurans do not appear until after hatching. Soon after the appearance of forelimbs, most pond-dwelling salamanders develop an ectodermal projection known as a balancer on each side of the head. These rodlike structures arise from the mandibular arch, contain nerves and capillaries, and produce a sticky secretion. They keep newly hatched larvae from sinking into the sediment and aid the salamander in maintaining its balance before forelimbs develop. After the forelimbs appear, the balancers degenerate.
Paired adhesive organs arise from the hyoid arch, located at the base of the tongue, during the embryonic and early larval stages in anurans. The sticky mucus they secrete can form a threadlike attachment between a newly hatched tadpole and the egg capsule or vegetation. Thus the tadpole that is still developing can remain in a stable position until it is capable of swimming and feeding on its own, after which the adhesive organs degenerate.
Larval stage
The amphibian larva represents a morphologically distinct stage between the embryo and adult. Most aquatic larvae depend on the environment for their food supply. Salamander and caecilian larvae remain more like their respective adults in structure, function, and nutritional requirements than do anuran larvae. Not long after emerging from their shells, larval salamanders, which have four fully developed limbs, start to feed on small aquatic invertebrates. The salamander larvae are smaller versions of adults, from which they differ by the presence of external gills, a tail fin, distinctive larval dentition, and a rudimentary tongue as well as by the absence of eyelids. Larval caecilians, also smaller models of adults, have thin skin, external gills, and a lateral-line system (a group of epidermal sense organs located over the head and along the side of the body). Anuran larvae resemble fish 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. Their mouths, located terminally (recessed), are ringed with fringing papillae and have many rows of keratinized (i.e., infused with a tough, horny material: keratin) denticles on the lips as well as horny jaws. Their gills are internal, and water taken in through the mouth passes over them and is expelled through one or more spiracular openings on the side of the head. Salamander and caecilian larvae are carnivorous and have digestive systems similar to those of adult forms, but the herbivorous anuran larvae have long coiled intestines and no actual stomachs.
Larvae of salamanders usually reach full size within two or three months, although some large aquatic species such as the hellbender (Cryptobranchus alleganiensis) and the mud puppy (Necturus maculosus) require up to five years to mature. Tadpole development varies in length between species. Some anuran species living in xeric habitats in which ephemeral ponds may exist for only a few weeks are fully developed in two weeks. Most species, however, need at least a month, and species living in cold mountain streams or lakes often require much more time such as the tailed frog (Ascaphus truei), whose development takes three years.
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 hatchling to adult, the most obvious of which are the development of completely new structures and functions. Other changes include modification of larval structures and regression of structures and functions useful only to the larvae. Hormones ultimately control all events of larval growth and metamorphosis. Changes that occur in the caecilian closure of the gill slit, degeneration of the caudal fin, and development of a tentacle and skin glands are minor. 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 also resorbed and gill slits close as lungs develop and branchial circulation is modified. Eyelids, tongue, and a maxillary bone are formed, and teeth develop on the maxillary and parasphenoid bones.
Tadpoles make an abrupt and radical transition to their adult form. Skeletal changes are much more dramatic in anurans than they are in salamanders and caecilians. Their hindlimbs mature and forelimbs form in the branchial chambers from which they emerge late in metamorphosis. The tail is resorbed; dermal glands develop and the skin becomes thicker. As lungs and pulmonary ventilation develop, internal gills and associated blood vessels degenerate. The adult mouthparts replace the degenerating larval mouthparts, and hyolaryngeal structures develop. All anurans except pipids 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 that the anuran undergoes 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 occur to make room for newly developed sense organs and also to allow the transition to be made from larval feeding and respiration modes to those of the adult.
During metamorphosis the urogenital system of all amphibians is modified. An opisthonephric or mesonephric kidney 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.
Neoteny
Neoteny entails the maturation of a larva’s reproductive capabilities without the concomitant development of its external morphological features; this phenomenon occurs in some aquatic salamanders and is due to delayed somatic development. Different patterns of partial metamorphosis, ranging from a lack of development in jaw and cranial elements to a retention of larval skin structure, are exhibited by all four families of salamanders in which neoteny is a necessary developmental stage (i.e., the obligate neotenes). These families Amphiumidae, Cryptobranchidae, Proteidae, and Sirenidae) consist of 15 species. Five species of Ambystoma and nine species of hemidactyline plethodontids also are obligate neotenes, and 15 species of four families are facultative neotenes (i.e., able to adopt a neotenic strategy when expedient). The most renowned neotenic salamander is the Mexican axolotl (Ambystoma mexicanum). Investigations of this species have shown that embryonic development and metamorphosis are under endocrine control, especially by the hormone thyroxine.
Comments
Got something to say?