METAMORPHOSIS VS NEOTENY (PAEDOMORPHOSIS) IN SALAMANDERS (CAUDATA )

Robin R. Kiyonaga

Introduction

Metamorphosis

NEOTENY (PAEDOMORPHOSIS)

BIOLOGICAL SIGNIFICANCE OF METAMORPHOSIS VS NEOTENY

Conclusion

References

INTRODUCTION

     Shrouded by mystery, the order caudata (or 'tailed one' if you like) has always enticed the imagination of the human race. It was once believed that the European black and yellow fire salamander could walk through fire without harm, giving rise to the name 'salamander', an Arab-Persian word meaning 'lives in fire' (Asay). In 1725 the first fossil salamander was discovered in Germany, but was misidentified at the time as the skeleton of a human who had perished in Noah's Flood (Flank). Today is no different. The mystery of the salmander is still among us except now we have the tool of science, not myth to help us understand these creatures.
 
     Of these mysteries, possibly the most intriguing of present relates with salamander metamorphosis. The occurance of metamorphosis has been known for ages, but the phenomenon of attaining reproductive maturity while retaining larval external morphology has not. Given many lables from neoteny to paedomorphism this occurrence has only been recognized since the middle of the last century. Since that time, many questions have arisen dealing with larval reproduction. How does a salamander transform from an aquatic larvae to a terrestrial adult? Do all salamanders go through metamorphosis? If not, then why? Within that same time, people have thought of answers to these questions. This essay will provide you some of those answers and hopefully some insight into this topic based on past studies. With that in mind, let us begin on the road to knowledge beginning with metamorphosis.

METAMORPHOSIS

     Metamorphosis defined is: A series of abrupt postembryonic changes involving structural, physiological, biochemical, and behavioral transformations (Duellman & Trueb, 1986). In amphibians, these changes are of three main types: (1) regression of structures and functions that are significant only to the larvae; (2) transformation of larval structures into a form suitable for adult use; and (3) development of structures and functions de novo that are essential to the adult (Dodd & Dodd, 1976).
 
     Metamorphosis is also often referenced to in three main stages: (1) premetamorphosis, found only in anurians, which is characterized by considerable growth and development of larval structures but not metamorphic changes; (2) prometamorphosis, a period of continued growth, especially of limbs, and initiation of minor metamorphic changes; and (3) climax, the period of radical changes that culminate in the loss of most larval characteristics. It is the initiation of tail regression which marks the beginning of this period, and complete resorption of the tail which marks the end (Etkin 1932, 1955 as cited by Dodd & Dodd, 1976). Ultimately this is all controlled by hormones.

Endocrinology

     The endocrinology of any organism is huge. If I were to include everything dealing with salamander metamorphosis, this would be a book, not an essay, so for your sake (and especially mine) I have tried to keep this as short and simple as possible. Keep in mind that this is a very simplified overview of this topic. Not all salamanders metamorphose in the same way.
 
     As written by Duellamn & Trueb (1986), during the stages of metamorphosis there is a finely tuned integration among the endocrine glands which influence morphological and physiological changes. What could be considered the center of this integration is the hypothalamo-pituitary-thyroid axis. This consists of the hypothalumus, the pituitary gland, and (what is considered the keystone of amphibian metamorphosis) the thyroid gland.

     Metamorphosis begins when the hypothalumus of the salamander brain sends a signal to the pituitary gland to release its hormones. These hormones of the pituitary then stimulate the thyroid to begin its own release of hormones to the rest of the organism. During development, the sensitivity of tissues to thyroid hormones change, and at metamorphic climax there is an apparent surge in the production of thyroid hormones. Depending upon the response of the organism to these factors, metamorphosis may or may not occur. Instead neoteny (paedomorphism) may be the end result.
 
     There is no clear reason as to why tissues respond to the various thyroid hormones in different ways and at different times (Duellman & Trueb, 1986). Duellman and Trueb (1986) suggest that perhaps insight into the control of metamorphosis may be obtained by better understanding how cells selectively bind certain thyroid hormones to intracellular receptor molecules. Maybe along these lines an answer may not be far away.

Morphology

     Well now that we know something about the hormones involved during metamorphosis in salamanders, let's talk a little about what kinds of changes are caused by them. If all goes well, locomotion, sensory, respiratory, and feeding structures are altered to prepare the organism for life on land. Specific features of this involve caudal fin resorbtion, thickening of the skin, and the development of dermal glands. Also prevalent are the resorbtion of gills, closing of the gill slits, development of eyelids, formation of a mouth with a wider gape, development of teeth, and the development of a tongue (Duellman & Trueb, 1986). Sexual maturity is also obtained during this time in most salamanders.
 
     Strangely in middle of all this, the one thing that is not affected is limb kinematics. This was clearly shown by Ashley-Ross (1994), in her study on this subject. Anyway, now that we have a background in metamorphosis we can now move on to neoteny (paedomorphosis), the alternative to metamorphosis.

NEOTENY (PAEDOMORPHOSIS)

     Neoteny defined is: The phenomenon of attaining reproductive maturity while retaining the larval external morphology (Duellman & Trueb, 1986). Unfortunately, neoteny is only one word out of a multitude of terminology used to describe this feature. Referencing aspects of delayed somatic maturity and precocious reproductive maturity has caused controversy as to what is the right word to use. Other terms such as paedogenesis and paedomorphosis have been applied and constantly redefined with the addition of new terms (progenesis, parthenopaedogenesis) only to hamper effective communication in this area (Duellman & Trueb, 1986). This is not to say that neoteny is more right than any of the other terminology, but for the sake of simplicity it is what will be used to communicate the idea of 'larval reproduction' in this essay.

Endocrinology

     Based on the endocrinology of metamorphosis, neoteny is thought to be the result of dysfunction at one or more of a number of levels in the endocrine system (Dodd & Dodd, 1976). Test results have shown that the insensitivity of tissues to thyroid hormones is what causes metamorphic cessation in obligate neotenes (Harris, 1956, as cited by Dodd & Dodd, 1976), while in facultative neotenes the low level of activity in the hypothalamo-pituitary-thyroid axis is responsible for the phenomenon (Duellman & Trueb, 1986). It is not known here whether external or internal cues which activate the axis are present, or if there is some other genetic reason individuals are insensitive, but it is thought that natural selection may be operating to select genotypes for neoteny in areas of harsh terrestrial environments (Duellman & Trueb, 1986).

Morphology

     From what you have read about neoteny so far, you can imagine that there in not much change in morphology. Salamanders which go through this process may obtain varying degrees of partial metamorphosis, but the only significant change occurs in the gonadal region of the salamander. This allows them the capability to reproduce.

BIOLOGICAL SIGNIFICANCE OF METAMORPHOSIS VS NEOTENY

     Metamorphosis in amphibians is a widespread phenomenon. Because of this, many attempts have been made to assess its biological significance (Dodd & Dodd, 1976). According to Dodd and Dodd (1976), it enables predominantly terrestrial forms to lay their eggs in water, which is considered an advantage. It also enables them to exploit two different food resources in a lifetime. From this viewpoint, it seems metamorphosis would be the ideal option for any amphibian. So why do some salamanders select neoteny instead?

     As stated by Duellman and Trueb (1986); All members of four families of salamander are neotenic, and at least some populations of salamanders in all other families have neotenic individuals. This would suggests neoteny as a universal occurrence among salamanders, therefore it must have some biological significance. Based on several studies addressing this problem, neoteny basically enables salamanders to avoid the terrestrial environment if its conditions are too inhospitable. In the case of predation, if there are a lot of predators living on land which feed on a particular salamander species, it would make sense for the salamanders to spend as much time as possible in an aquatic environment.

     Of course being neotenic has its drawbacks as well. Genetic diversity may become a problem because of a lack of gene flow (Titus & Gaines, 1991). Neotenic salamanders are restricted to the gene pool of the aquatic environment in which they live unless they metamorphose and travel to a new pool, or vice versa.

CONCLUSION

     I hope you learned a lot about metamorphosis and neoteny in salamanders from this paper, and I hope even more that it will inspire you to read further on this subject matter. Much of the mystery still remains, which means there is much more to discover. What is written here is only a scratch on the surface of these topics. There is a lot more going on than meets the eye, some that we know about, and a lot we don't. Metamorphosis and neoteny are just the beginning when dealing with salamander growth and development. There are other roads which may be traveled. Some species such as Plethodon dunni have no free-living larval stage, and are therefore born fully formed juvenile adults (Leonard & Associates, 1996). Other salamanders labeled Newts actually go through a reverse metamorphosis phase, where a terrestrial salamander returns to its aquatic stage of life. This makes you wonder if 'Amphibian' (meaning 'double life') really does a good job of describing the order Caudata .

References

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Duellman, William E. & Trueb, Linda. Biology of Amphibians. San Francisco: McGraw-Hill Book Company,      1986.

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