Amphidromus is a genus of tropical air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Camaenidae.
The shells of Amphidromus are relatively large, from to in maximum dimension, and particularly colorful. During the 18th century, they were among the first Indonesian land snail shells brought to Europe by travelers and explorers. Since then, the genus has been extensively studied: several comprehensive monographs and catalogs were authored by naturalists and zoologists during the time period from the early 19th to the mid 20th centuries. Modern studies have focused on better understanding the evolutionary relationships within the group, as well as solving taxonomic problems.
The genus Amphidromus is unusual in that it includes species that have dextral shell-coiling and species that have sinistral shell-coiling. In addition, some species within this genus (such as Amphidromus janus (L. Pfeiffer, 1854))are particularly notable because their populations simultaneously include individuals with left-handed and right-handed shell-coiling. This is an extremely rare phenomenon, and very interesting to biologists. Studies focused on the soft anatomy of Amphidromus are scattered and fragmentary. Information on the internal anatomy is known only from a few species, and no larger, comparative morphological study has ever been carried out.
Species in the genus Amphidromus are arboreal — in other words, they are tree snails. However, more detailed information on their habits is still lacking. The general feeding habits of these snails are unknown, but a few species are known to feed on microscopic fungi, lichens or terrestrial algae. Amphidromus themselves are preyed upon by birds, snakes, and probably also by smaller mammals such as rats.
Taxonomy and history
The generic name is derived from the ancient Greek words (), meaning "on both sides", and (), meaning "running", alluding to the different chiralities of the shells. The shells of Amphidromus are relatively large, and quite colorful; considerable numbers of them were among the first Indonesian land snail shells brought back to Europe by travelers and explorers during the 18th century. Comparatively speaking, malacologists have gathered a much smaller number of specimens.
Several species and forms were described before 1800, most of them with inadequate locality data. At least two names — Amphidromus laevus (Müller, 1774) and the form A. perversus f. aureus Martyn, 1784 – still (as of 2017) have not yet been reported from a precise locality. During the first half of the nineteenth century, many species and varieties were named, again usually with poor locality data. Not until Eduard von Martens (1867) published his monograph Martens E. von (1867). Die Preussische Expedition nach Ost-Asien. Zoologischer Theil. 2: xii, 447 pp., 22 plates. was there an attempt to cover the entire complex of species within this genus. The 1867 monograph contained considerable information both on the variation within the genus, and on the problems of the geographic distribution of the species. Many concepts that originated with von Martens are still (as of 2017) in use.
In 1896, Hugh FultonFulton H. (1896). "A list of the species of Amphidromus, Albers, with critical notes and descriptions of some hitherto undescribed species and varieties". Annals and Magazine of Natural History (6)17: 66–94, plates 5–7. organized 142 specific and varietal names into eighteen species groups containing a total of 64 species. When Henry Augustus Pilsbry's 1900 monograph Manual of Conchology Pilsbry H. A. (1900). Manual of Conchology, structural and systematic, with illustrations of the species. Second series: Pulmonata. Volume 13. Australasian Bulimulidae: Bothriembryon, Placostylus. Helicidae: Amphidromus. 253 pp., 72 plates, page 184. appeared, the number of species in the genus Amphidromus had increased to 81, and these were placed in nineteen groups. Pilsbry's study has remained the only illustrated monograph of the genus, and it is still considered indispensable for any serious study of the genus.
Since 1900, the major taxonomic studies on Amphidromus have been faunistic (a study of the fauna of some territory or area) in scope. The papers of American malacologist Paul Bartsch (1917, 1918, 1919) Bartsch P. (1917). "The Philippine land snails of the genus Amphidromus". U. S. Nat. Mus. Bull. 100(1), part 1: 1–47, 22 plates. Bartsch P. (1918). "The land snails of the genus Amphidromus from the islands of the Palawan Passage". Jour. Washington Acad. Sci. 8(11): 361–367. Bartsch P. (1919). "Critical remarks on Philippine Island land shells". Proc. Biol. Soc. Washington 32: 177–184. on the Philippine species, Bernhard Rensch (1932) Rensch B. (1932). "Die Mollusken Fauna der Kleinen Sunda-Inseln, Bali, Lombok, Sumbawa, Flores und Sumba". Zool. Jahrb., Syst. 63: 1–130, 3 plates. on the Lesser Sunda Islands forms, and Tera van Benthem Jutting (1950, 1959)van Benthem Jutting T. (1950). "Critical studies of the Javanese pulmonate land-shells of the families Helicarionidae, Pleurodontidae, Fruticicolidae and Streptaxidae". Treubia 20(3): 381–505, 107 figs.van Benthem Jutting T. (1959). "Catalogue of the non-marine Mollusca of Sumatra and of its satellite islands". Beaufortia 7(83): 41–191, 1 plate, 11 figs. on Javan and Sumatran populations are especially comprehensive. Potentially the most valuable contribution is that of Curt Haniel (1921), Haniel C. (1921). "Variationsstudie an Timoresischen Amphidromus Arten". Zeits. Induct. Abstamm. und Vererbungsl. 25(1–2): 88 pp., 5 plates. who discussed the variation within A. contrarius and A. reflexilabris on Timor; the variations in color and form were well illustrated in a series of color plates.
Literature published after 1900 contains many scattered descriptions of new color forms and subspecies. Of the 309 names in the nomenclatural list, 111 (35.9%) were published after Pilsbry (1900). Adolf Michael Zilch (1953) Zilch A. M. (1953). "Die Typen und Typoide des Natur-Museums Senckenberg. 10: Mollusca, Pleurodontidae (1)". Archiv für Molluskenkunde 82(4–6): 131–140, plates 22–25. listed type specimens in the Senckenberg Museum, and illustrated many previously unfigured species. Frank Fortescue Laidlaw & Alan Solem (1961) recognized 74 species by name, and considered that material from the Banda Islands probably represented an undescribed species. Eleven of the species recognized by Laidlaw & Solem were described after the appearance of Pilsbry's monograph. However, several species recognized by Pilsbry have subsequently been subordinated to subspecific or varietal status, and a few names have been transferred to incertae sedis, since they are based on hundred-year-old references that have not been substantiated by more recent collectors. In fact, the study by Laidlaw & Solem (1961) forms a supplement to Pilsbry's monograph with his extensive plates, and many of Laidlaw & Solem's conclusions concerning the relationships of color forms described as species were taken not so much from new samples, but from the extent of variation that was outlined by Haniel (1921) in his pioneer study.
Characterization Species in the genus Amphidromus usually have smooth, glossy, brightly colored, elongate or conic, dextrally or sinistrally coiled shells. The shells are moderately large, ranging from to in maximum dimension, having from 6 to 8 convex whorls. Their color pattern is usually monochromatic yellowish or greenish, but can be variegated. The aperture is oblique or ovate in shape, without any teeth or folds, and with the aperture height ranging from two-fifths to one-third of total shell height. The peristome is expanded and/or reflected, and is sometimes thickened. The columella may be straight or recurved, and the parietal callus is weak to well-developed, and the umbilicus may be open or closed. The radula is spatulate, has cusped teeth arranged in rows, usually with a monocuspid central tooth and bicuspid or tricuspid lateral teeth. The jaw is thin and weak, with low flat ribs. The pallial region is sigmurethrous, with a very long, narrow kidney. The genitalia are that of typical camaenids, with a long seminal receptacle, a short penis with low insertion of the retractor muscle, and a short or long epiphallic caecum (flagellum and appendix). The spermatophores have a pentagonal outline in cross-section. Amphidromus are typically arboreal animals.
Shell description
The shells of Amphidromus are relatively large, from one to three inches high, and colorful. Amphidromus has an elongate-conic or ovate-conic helicoid shell of 5 to 8 whorls. The shell may be thin and fragile, or very heavy and solid, with no known correlation of shell structure with distribution or habitats.
Shell coiling
In some species within this genus, the shell coils invariably to the right, and in many others just as invariably to the left. However, a significant number of species in this genus are "amphidromine"; this term means that both left- and right-handed shell coiling are found within the same population. One could say they are "polymorphic" for the direction of shell coiling, but because there are only two possible types of shell coiling, they are described as "dimorphic" in coiling. The two types of shell coiling occur in some species in approximately equal numbers, other species have a distinct predominance of one phase. There is as yet no information on the heredity of this character in Amphidromus.
Because almost all other species of amphidromine gastropods, such as ones within the genera Partula and Achatinella, have already become extinct, the genus Amphidromus, containing over 110 species, is uniquely useful for the study of the evolution of asymmetry in animals, and this is why the conservation of this genus is of essential importance to biologists.
Shell shape and sculpture
The whorls of the shell of species of Amphidromus are moderately convex and, with only a few exceptions, are smooth or have a faint sculpture of growth lines. However, a sculpture of moderately heavy oblique radial ribs has appeared at least four separate times in the genus, and can be seen in the following species: Amphidromus costifer Smith from Binh Dinh Province in Vietnam; A. begini Morlet from Cambodia; A. heccarii Tapparone-Canefri from Celebes; and the A. palaceus-A. winteri complex from Java and Sumatra. Correlated with the ribbing is a light, monochrome coloration, and a thin shell with a large aperture and a flaring lip. Many solid shells in other species do show a slight roughening of the surface, but this is very different from the ribbed sculpture mentioned above.
The aperture is generally large, varying from about two-fifths to one-third the height of the shell, often within the same population. Usually the lip is at least somewhat expanded, and in forms such as A. reflexilabris Schepman and A. winteri (Pfeiffer) var. inauris Fulton, the lip can only be called flaring. In A. perversus (Linnaeus) and most other thick-shelled species, the lip is internally thickened, forming a "roll" in its expansion, and has a very heavy parietal callus. In thin-shelled species, the lip is usually a simple reflected edge. The umbilical area can be partially open, nearly closed, or sealed. This feature sometimes provides a useful criterion for specific identification. The angle of the parietal wall varies, but no precise information on this has been compiled.