SIREDON. At the end of last century specimens of a kind of branchiate tailed Amphibian were brought to Europe from the lakes of Mexico; they were examined by the zoologists of Paris and described by Cuvier in Humboldt's Recueil d'Observations de Zoologie, vol. i., and by Daudin in Hist. des Reptiles (Paris, 1802-1804), under their native name of "Axolotls." The animals were named Siren pisciformis by Shaw (Zool., vol. iii.). Wagler, in his Natürliches System der Amphibien (Stuttgart, 1828-1833), separated the axolotl from the Linnaean genus Siren and called it Siredon axolotl, and later writers have often referred to the animal under the name Siredon pisciforme, Shaw.
The axolotl of Mexico is about 6 or 7 inches in length; it has four pairs of gill-slits and three pairs of long feather-like external branchiae. The branchial apertures are between the hyoid arch and the first branchial arch, and between the first-second, second-third, and third-fourth branchial arches. The branchiae are attached to the first, second, and third branchial arches. The body is cylindrical, and a median membranous fin extends along the trunk dorsally, is continued along the tail, passes round the end of the latter and terminates ventrally at the anus. It has four limbs, which are short and somewhat stout; the anterior terminate in four and the posterior in five digits. The colour of the axolotl is a uniform black.
The animal is therefore, except in size, very similar to the aquatic larva of Triton, or other Salamandroid, and Cuvier expressed the opinion that it was a larval form which for some unknown reason was unable to attain the adult condition. That it could not be considered simply as the larva of an unknown species of Salamandroid was evident from the fact that it possessed fully developed sexual organs in both sexes. There was every reason to believe that it bred freely in the branchiate condition in which it was discovered. The animal is so common in the lakes near the city of Mexico that it is brought regularly to market and used largely by the Mexicans as food (9) [96-1].
If nothing more than the above were known about the axolotl it would be classed among the Perennibranchiata, in the family Proteida, having its nearest ally in the genus Menobranchus. Up till the year 1865 no actual observations had been made by zoologists on the breeding of the axolotl: all that was known was that the genital organs in many of the specimens examined were in perfectly mature condition. In that year, on January 18, 6 axolotls, 5 males and 1 female,which had been living for a year in the menagerie of reptiles of the Musee d'Histoire Naturelle at Paris,began to breed, and the deposition and hatching of the eggs was carefully studied by Prof. A. Dumeril (1) [96-1]. The eggs were 2 mm. in diameter, and the period of development within the egg was 28 to 30 days ; the larva? were hatched in February, and were 14 mm. to 16 mm. in length. In the beginning of September, when the larvae had almost reached the size of the parents, it was noticed that one of them was undergoing a metamorphosis similar to that of the larval Triton to the adult. In a short time yellow spots appeared on the skin, the branchiae disappeared, the gill-slits closed up, the median fin disappeared, the animal began to breathe air and permanently quitted the water. The same process of metamorphosis was repeated by several of the larvae, until finally out of several hundred about thirty reached the salamandroid condition. The parents in the meantime were still alive, and had undergone no change. When the structure of the transformed specimens was examined, they were found to resemble in all generic characters the genus Amblystoma, of which several species were known, inhabiting various parts of North America. [96-2] The consideration of Dumeril's discovery gives rise to several perplexing questions, which have been discussed by many zoologists experienced in the study of the Amphibia, and even now can scarcely be said to be completely settled. The first question is To what species of Amblystoma did the transformed axolotls of Dumeril belong? Dumeril himself, in the full account (2) [96-1] which he published concerning the animals and their metamorphosis, was unable to give a decided opinion concerning the identification of the species of his Amblystoma, but on a subsequent occasion he confirmed the suggestion of Prof. E. D. Cope (10) [96-1] that the specific characters were those of A. mavortium, Cope (described in Proc. Ac. Philad., 1867).
The publication of Dumeril's discovery excited a great deal of interest among European naturalists, and for a time experiments and observations on axolotls in captivity were carried on with great earnestness. The metamorphosis in the ease of Dumeril's specimens had taken place quite unexpectedly, but the case seemed to offer an opportunity for ascertaining the action of definite conditions in producing definite processes of growth. Marie von Chauvin (6) [96-1], at Freiburg, at the instigation of Prof. Weissmann, attempted, and with perfect success, to transform young axolotls into the Amblystoma form by gradually bringing the animals from water into air.
The transformed axolotls observed by Dumeril were kept alive in the Paris Museum, and for ten years showed no symptoms of breeding or sexual activity. It was currently believed that the Amblystoma derived from the metamorphosis of Siredon was sterile. This belief ultimately proved erroneous. In the autumn of 1874 the animals in the menagerie of reptiles wore transferred to new premises, where they were all placed in more healthy conditions. Immediately after this the Amblystoma deposited fertilized eggs, and the fact was reported by M. Blanchard to the Academie des Sciences (4) [96-1], with the comment that the Amblystoma was thus shown to be similar to other cold-blooded animals which were capable of reproducing in both the young and the adult condition.
Although at first Duméril believed and stated that his specimens of axolotl belonged to the species which bears that name in Mexico, he afterwards, in his more detailed work on the subject (2), explained that the grounds for his first opinion had been insufficient. American zoologists, especially Baird and Cope, had distinguished several species of Siredon, and Baird had separated the Mexican species, which alone was originally called axolotl, as Siredon mcxicanus. Duméril came to the conclusion that the axolotls in the Paris Museum were identical with Siredon lichenoides, Baird (described in Stansburg, Exped. Gr. Salt Lake, Utah). All the axolotls which were kept and studied and subjected to experiment by naturalists on the Continent after Dumeril's discovery were descendants cf the Paris specimens, so that the results obtained really did not necessarily prove anything with regard to the true Mexican axolotl, Siredon mexicanus, if that were really a distinct species. There is no evidence in literature to show whence the first axolotls in the Paris Museum were obtained. It was evident that Siredon lichenoides was capable of breeding in both the larval and the salamandroid condition, and that its metamorphosis in captivity in Europe was rare and to a certain extent controlled by definite external conditions. Prof. O. C. Marsh has recorded his experience of the metamorphosis of S. lichenoides. He obtained several specimens from alpine lakes 7000 feet above the sea in Wyoming Territory, and some of these metamorphosed into Amblystoma mavortium, Cope. Marsh does not say if the larvae he obtained were sexually mature, nor did he ascertain if breeding of the species in the larval condition took place at all in the lakes he visited; he thinks it probable that the metamorphosis in that region was rare in the natural conditions.
The metamorphosis of the true axolotl, undoubtedly obtained from the Lake of Mexico, seems to have been observed only once namely, by Tegetmeier in London. That naturalist had 5 specimens, and one of them underwent the metamorphosis. In 1871 Cope (10) [96-1] stated that no one had seen the metamorphosis of the true siredon, Siredon mexicanus, Baird, and that no Amblystomae had been obtained from Mexico south of the Tropic of Cancer, while the true axolotl is found south of that line. He was unaware of Tegetmeier's observation. He further declared that Prof. Baird was aware of the metamorphosis of all the North American species of Siredon so-called, excluding S. mexicanus, years before the observation of it by Duméril, though he had at first named one of them Siredon lichenoides, in the belief that it was adult. Cope considered the observation of Duméril important, as showing that siredons reproduced as such.
Finally, according to Boulenger (7) [96-1], the S. lichenoides and mexicanus of Baird are synonymous, the Paris axolotl is identical with the same species, and the perfect form into which it changed is identical with A. tigrinum, mexicanum, and mavortium of Cope, obscurum of Baird, while the form named Siredon gracilis by Baird is probably the larva of Amblystoma tenebrosum. Boulenger adopts the name A. tigrinum of the synonyms given above, and gives as the distribution United States and Mexico ; the specific diagnosis isseries of palatine teeth extending to external fissure of choanae; plicae of tongue radiating from behind; costal grooves twelve; head large; brown or blackish, with yellow markings.
It is therefore very probable that the Paris specimens were really Mexican axolotls, and there is no doubt that these animals do in captivity undergo metamorphosis. So far as is known, they never do so in their natural conditions. But the animals are specifically identical with A. tigrinum, which is found in many parts of the United States, from New Jersey to California, and normally breeds in the salamandroid condition. It is not known at present whether the larva of A. tigrinum ever attains sexual maturity in other regions where the species occurs besides Mexico. It is not improbable that it does so. De Filippi (8) [96-1] found in a marsh on the shores of the Lago Maggiore 48 larvae of Triton alpestris in the branchiate condition, which contained fully developed ova and spermatozoa, so that the occurrence of sexual maturity in the larvae of Amblystoma is not unique. Prof. August Weissmann (5) [96-1] has discussed at considerable length and with much thoughtfulness the true significance of the phenomena exhibited by the axolotl, and has concluded that its ancestors passed through the normal life-history of Amblystoma, the climate of the Mexican tableland having been at one time moist enough to permit of the existence of a terrestrial Salamandroid ; that the climate has now became so dry and unfavourable to vegetation that no amphibian can live in it except in water; and that Amblystoma has become adapted to these conditions by ceasing to pass through its metamorphosis, and breeding entirely in the branchiate condition. Thus the metamorphosis which takes place occasionally in captivity is a case of what has been called since Darwin's epoch atavism; its peculiarity consists in the fact that the evolution of the animal has resulted in the arrest of development at a larval stage, and the occasional reversion is the continuation of the development to the higher condition of the ancestor. Atavism is the occasional resemblance of one individual to some remote ancestors instead of to its immediate parents. Another possible way of explaining the axolotl is to suppose that it has remained in the perennibranchiate condition while other members of the same species elsewhere have developed into the salamandroid condition. This explanation cannot be the true one. It would necessitate the belief that a metamorphosis lasting a few days or weeks, and induced often by the gradual removal of the animal from water into air, could produce the same specific characters as a gradual development which has occupied a great number of generations. The axolotl is an example of one of the most curious and interesting modes by which animals may be adapted to their conditions, and two species formed out of one. At present the disappearance of the metamorphosis from the life-history of the axolotl has taken place so recently that not even specific differences exist, according to some observers, between the metamorphosed axolotl and the natural Amblystoma tigrinum. At some future time slight differences are almost sure to occur, and then there will be two species or the tendency to metamorphosis in the axolotl will be lost. In the latter case some slight differences will probably be developed between the axolotl and the branchiate larva of A. tigrinum in other parts of America; and then the axolotl and A. tigrinum will be two species. Finally, it may be pointed out that it is possible that the axolotl could have reached its present locality and conditions without any change in the climate of Mexico. The lakes in the arid district might somehow occasionally be visited by breeding A. tigrinum, and of the larvae so produced in them some might become sexually mature before metamorphosing, and so give rise to the present axolotls.
There is some reason to believe, according to the American zoologist Prof. Cope, that the perennibranchiate Menobranchus lateralis, Tschudi, of the Mississippi, which when full grown is over a foot in length, and has four branchial apertures, stands in the same relation to the genus Batrachoseps, Bonap., as Siredon to Amblystoma.
Literature.(1) A. Duméril, Comptes Rendus, vol. lx., 1865, p. 765; (2) A. Duméril, Nouv. Arch. Mus., ii, 1866; (3) A. Duméril, Comptes Rendus, vol. lxi. p. 775; (4) M. Blanchard, ibid., vol. lxxxii., 1876, p. 716; (5) A. Weissmann, Zeitschr. f. wiss. Zool., xxv. p. 297 ; (6) M. von Chauvin, ibid., xxvii. p. 522; (7) G. A. Boulenger, Brit. Mus. Cat.Batrachia gradieniia, &c., 1882; (8) De Filippi, Archivio per la Zoologia, 1861 ; (9) De Saussure, Verhandl. d. Schweiz. naturforsch. Gesellsch. Einsiedeln, 1868; (10) E. D. Cope, '' Metamorphosis of Axolotl," Amer. Journal, 1871; (11) O. C. Marsh, Amer. Jour., , xlvi. p. 364; (12) Tegetmeier, Proc. Zool. Soc., 1870. (J. T. C.)
96-1 These numerals refer to the "Literature" (infra).
96-2 The generic characters of Amblystoma, Tschudi, are, according to Boulenger -- tongue, subcircular or oval, with radiating plicae, lateral borders free, anterior border slightly free; two traverse series of palatine teeth in same straight line, not separated by a wide interspace in the middle; otes five; tail more or less compressed.
The above article was written by: J. T. Cunningham, B.A., Fellow of University College, Oxford.