A hypothesis on the evolution of the redspotted newt, Notophthalmus viridescens

Amphibians are often said to have two lives, but Notophthalmus viridescens has three. This newt, called the Common, Eastern, or Redspotted Newt, begins life as a yellowish-green larva; it then undergoes metamorphosis to become a small red terrestrial "eft." After a few years on land, it returns to the water to change back to a green aquatic form, and to breed (efts are not sexually mature). It remains in this green form throughout adulthood.

The larvae hatch, from eggs laid on submerged vegetation in ponds, at a length of about 7 mm. They are a greenish yellow and have the external gills, laterally compressed tail, and dorsal keel typical of salamander larvae. In 2-3 months, once they reach a length of 35-40 mm, they undergo metamorphosis. They are then called efts; they are red (ranging from a dull orange-brown to the more common bright red) and have granular skin with toxin-producing glands; the red color is thus thought to be aposematic. The efts spend 1-3 years on land, although one source (Eastern Newt) gives an upper limit of seven years. However, dry conditions can keep efts from undergoing their second metamorphosis, which may have been the cause of the seven-year case(s) (the author does not say where this figure comes from). Usually, though, the second metamorphosis occurs within about three years (Handbook, 99); efts use two biological compasses to find their natal pond: a light-dependent magnetic compass and an intensity detector sensitive to the polarity of the magnetic field. Adults lose the poisonous, granular skin and regain the compressed tail (males grow a dorsal keel in the breeding season), and become green and yellow instead of red. Adults spend nearly all of their time in the water, as do the larvae.

Notophthalmus viridescens was first described by Rafinesque and is in the family Salamandridae. It was once classified in the genus Triturus (as were several other salamandridae that now have their own genera) and is occasionally referred to in older books as Diemyctylus viridescens. The range of N. viridescens includes most of the Eastern half of the United States. There are four subspecies: N. v. viridescens, the redspotted newt; N. v. dorsalis, the broken-stripe newt; N. v. louisianensis, the central newt; N. v. piaropicola, the peninsula newt. The striped newt, N. perstriatus, also has an eft stage (Audubon, 276).

Also of interest are the Triturus newts of Europe; most of the twelve species of this genus have a similar life cycle, with an eft stage that lasts for 2-5 years. Efts may return to the water every spring, not to breed or to metamorphose but to take advantage of the higher availability of food there. Here, too, the young are more toxic than the aquatic adults (Triturus).

Although a terrestrial stage is typical of N. viridescens, it may be skipped; populations in Long Island, NY, and Woods Hole, MA, are neotenic, usually retaining their gills. Although many salamanders are facultatively neotenic, this situation is entirely logical for N. viridescens in light of the similarity of the adult to the larva: they are the same color and have the same type of skin; they eat similar foods, are both non-toxic, have laterally compressed tails, and are aquatic.

While neoteny is obligate in some salamanders, many more have facultative neoteny, in which only a few populations in the species exhibit neoteny. Its frequency suggests that it either is easily evolved and has done so independently many times, or that a common ancestor of Hynobiidae, Salamandridae, Ambystomatidae, and Plethodontidae (Duellman & Trueb, 195) had the ability to become neotenic given the right circumstances, and passed the trait to these salamanders. Either way, the presence of facultative neoteny in N. viridescens poses no special problem in the evolution of this species. Apparently, the eft stage is not necessary for individuals in the Long Island and Woods Hole populations: "As noted by Wilbur & Collins (1973), teleologically the decision to initiate metamorphosis is an educated guess that the risk of metamorphosis is less than the risk of remaining in the aquatic habitat" (Duellman & Trueb, 195). Larvae may be 'forced' to metamorphose if there is a lack of food in the aquatic habitat or if that habitat is in danger of desiccation (Duellman & Trueb, 195).

Neoteny is most often found in populations inhabiting permanent aquatic habitats where fishes are rare or absent (Duellman & Trueb, 194); the habitat of N. viridescens consists of ponds and small pools in streams where submerged vegetation is present. Fish often live in the same ponds, and will eat viridescens larvae. Presumably they will also eat larger neotenic viridescens. I propose that N. viridescens leaves the water (where it does) to escape an aquatic predator, probably a fish, and that neotenic populations do not live with predatory fish (I was not able to find out whether fish are present in the habitats of the Long Island and Woods Hole populations). The adults, then, are large enough for the predators to be a small or nonexistent threat.

The absence of salamanders of eft size in ponds that also contain adult N. viridescens would confirm this hypothesis (presence of predatory fish would also be helpful as data, but I was not able to find this information). A survey was made of the amphibians and reptiles of Allegany State Park that listed nine species of salamanders (one of which was then classified as a subspecies of another). Besides N. viridescens itself, they included two large burrowing salamanders (Ambystoma maculatum and Plethodon glutinosus), two aquatic salamanders substantially larger than efts (Gyrinophilus porphyriticus and Pseudotriton ruber), and four salamanders of a size similar to efts. Of these, one (Plethodon cinereus) was entirely terrestrial, and the other three (Desmognathus fuscus, Desmognathus ocrophaeus, and Eurycea bislineata) inhabit rock-bottomed streams rather than the slower water with submerged vegetation that N. viridescens prefers. Furthermore, both Desmognathus are nocturnal; N. viridescens is diurnal.

Neoteny is regarded as a specialized condition, with neotenic salamanders (both facultative and obligate) arising from non-neotenic ancestors. Also, among the Salamandridae the only genera to exhibit any neoteny are Notophthalmus and Triturus (which is itself polyphyletic); the phylogenetic separation of the three groups can be seen in the accompanying chart. Because neotenic populations are known to evolve from non-neotenic but not vice-versa, I would consider the ancestor of N. viridescens to have been capable of metamorphosis, with the neotenic populations of viridescens (and of Triturus) evolving later.

All members of Salamandridae for which data is available are known to have toxic and/or noxious skin secretions(Duellman & Trueb, 250). This is true of the eft but not of the adult; thus the ancestor of N. viridescens probably had a land stage similar to the eft, and probably had a green larva and a red, toxic, terrestrial adult phase (many salamanders change coloration upon metamorphosis). Larvae do not remain larvae for very long: they undergo their first metamorphosis at 2-3 months of age (many salamanders take more time than this, but few take less). The onset of metamorphosis could probably not come any sooner because there is usually a minimum size, below which the newt is unlikely to survive as an adult due to the stresses of metamorphosis. It seems that metamorphosis comes as soon as it easily can.

After the eft has attained a certain size (around 75 mm) it is probably safe to return to the water. The geographical variation in the length of the eft stage seems to pertain to the size of the eft: in regions where temperature and especially food availability as affected by climate allow the efts to grow quickly, they remain efts for a much shorter time than efts in harsher, usually more northern areas (the pattern is similar to that for bullfrogs, who spend a few years as tadpoles in the northern US but may metamorphose the same summer they hatched in southern populations). Many salamanders exhibit changes such as a laterally flattened tail and smoother skin when they return to the water briefly to breed, apparently for better swimming ability; it seems Triturus and Notophthalmus initially underwent similar temporary changes, which eventually became permanent. Reversion to green coloration was probably for purposes of camouflage, as in the larva. Adults do not, however, reestablish external gills (individuals in neotenic populations usually keep their gills), so apparently not all larval characteristics are attainable by a formerly terrestrial individual. I was not able to find out about other morphological and physiological differences between adult and larval N. viridescens. Presumably the reason the adults return to the water rather than continue to live on the land is the increased availability of food; even the efts of Triturus enter the water occasionally during the spring for the greater amount of food available there.

N. viridescens, then, evolved from a newt with an eft stage but without the stage that we now refer to as the adult. Their aquatic habitat had abundant enough food for them to become neotenic, but the younger newts did not survive to maturity. Neoteny was therefore selected against, except for a few populations in which neotenics not only are present but also are closer to eft than to aquatic adult size: one population of neotenics was found to consist of individuals 53-67 mm long (compare to 73-98 and 77-110 mm for adults of two more typical populations) (Handbook, 99). Among the terrestrial newts, though, several were able to survive in the water indefinitely after returning there to breed, and secondarily took on traits that are normally characteristic of larvae or of neotenics to allow them to do so.

Works Cited

Anonymous. Eastern Newt: Notophthalmus viridescens. http://www.npwrc.usgs.gov/narcam/idguide/rsnewt.htm

Anonymous. Triturus. http://www.darkwave.org.uk/~caleb/triturus.html

Behler, John L., and King, F. Wayne. National Audubon Society Field Guide to Reptiles and Amphibians. New York: Alfred A. Knopf, 1979.

Bishop, Sherman C. Handbook of Salamanders. Ithaca, NY: Comstock Pub. Associates, 1967.

Bishop, Sherman C. The Amphibians and Reptiles of Allegany State Park. Albany: University of the State of New York, 1927.

Duellman, William E., and Trueb, Linda. Biology of Amphibians. Baltimore: Johns Hopkins University Press, 1994.

Larson, Allan. Salamandridae: the newts and true salamanders. http://biodec.wustl.edu/~larsontl/salamandridae.html

Phillips, John R., and Borland, Chris S. Use Of A Specialized Magnetoreception System For Homing By The Eastern Red-Spotted Newt Notophthalmus viridescens. http://www.biologists.com/JEB/188/01/jeb9188.html