Amphidromus Kruehni

''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 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 Fulton 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) on the Philippine species, Bernhard Rensch (1932) 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. 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) 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.

File:Amphidromus floresianus shell 2.png, In '' A. floresianus'', subgenus ''Syndromus'', shell coiling is normally sinistral. Scale bar 10 mm.
File:Amphidromus perversus natunensis shell 2.png, Shells in the amphidromine species ''A. perversus'' can be dextral, as shown here.
File:Amphidromus perversus natunensis shell 3.png, But shell coiling in ''A. perversus'' can also be sinistral, as shown here.
File:Amphidromus perversus shell.jpg, Abapertural view of a sinistral shell (left), and apertural view of a dextral shell (right) of ''A. perversus''

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.

Generally the whorls of the shell increase rather regularly in size, however, species which are probably closely related, such as ''A. sinistralis'' (Reeve) and '' A. heccarii'' Tapparone-Canefri, can have quite different degrees of whorl increment. No attempt has been made to express these differences meristically, since most of the available material was inadequate for statistical treatment. Actual dimensions of the shell vary greatly both within and between species. The minimum adult size is about 21 mm high, the observed maximum about 75 mm. There is not much variation in adult size within species: only a few species, notably '' A. maculiferus'', '' A. sinensis'' and '' A. entobaptus'', have a variation in adult size that is greater than seven or eight millimeters in total.

Shell coloration

The single most major aspect of shell variation within the genus is the color patterning. In general, many arboreal snails are brightly colored, obvious examples being the bulimulid genera ''Drymaeus'' and ''Liguus'', the cepolid '' Polymita'', and the camaenid '' Papuina''. However, '' Polymita'', ''Liguus'' and ''Amphidromus'' are particularly noted for their color variations. The basic ground color of ''Amphidromus'' appears to be yellow, and this color is usually (except for '' Amphidromus entobaptus'') confined to the surface layers of the shell, since worn specimens appear to be nearly devoid of color. In some species the background color is whitish, and a few have dark background colors. The apical whorls are pale, purple, brown, or black, and this sometimes varies within a population (as in '' A. quadrasi''). A few species, for example '' A. schomburgki'', have a deciduous green periostracum.

Continuous zonal patterns can take the form of whitish sub sutural bands ('' A. similis''), heavy subperipheral pigmentation (''A. perversus'' var. ''infraviridis''), subsutural color lines (''A. columellaris''), broad spiral color bands ('' A. metabletus'', '' A. webbi''), or narrow spiral bands ('' A. laevus''). Interrupted zonation can consist of the interruption of bands into spots in ('' A. maculatus''); highly irregular splitting of zones (''A. perversus'' vars. ''sultanus'' and ''interruptus''); formation of oblique radial streaks which run parallel to (in '' A. inversus'') or cross (in '' A. latestrigatus'') the incremental growth lines; or almost every conceivable combination and variation of these factors. Often the pattern will change radically from the apex to the body whorl (in '' A. quadrasi'' vars.). The aperture, parietal callus, columella, lip, and umbilical region are variously marked with pink, brown, purple, white, or black. Haniel (1921) includes several color plates which clearly demonstrate the extent of color variation within two species of the ''Syndromus'' type. ''A. perversus'' and '' A. maculiferus'' of the subgenus ''Amphidromus'' are equally variable, whereas species such as ''A. inversus'' and '' A. similis'' are almost uniform in coloration.

In shells of most of the species in the subgenus ''Amphidromus'', resting stages are marked by the deposition of a brown or black radial band called a varix. This appears to be rare in the subgenus ''Syndromus'', although the shell of ''A. laevus'' does show evidence of interruption of the spiral banding after a resting phase.

Species recognition

Species recognition is based on combinations of minor structural variations in the shape, aperture, whorl contour, umbilical region, and color pattern. It appears to be the case that many species have a stable color pattern, while other species seem to vary tremendously. Adequate unselected field samples will enable a better understanding of the relative stability or variability of particular species in single localities.

Anatomy

Information concerning the soft anatomy of ''Amphidromus'' is widely scattered and fragmentary. The most complete account is that of Arnold Jacobi (1895) on specimens from Great Natuna (Natuna Islands) and Djemadja ( Anamba Islands). Unfortunately, although it is clear that anatomical differences exist in the two species Jacobi dissected, unfortunately we do not know which forms he worked on, because he had incorrectly identified his material. In his paper he referred to the two species as ''Amphidromus chloris'' and the ''interruptus'' phase of ''A. perversus''. However, that is not possible, because in reality ''Amphidromus chloris'' is a species found only in the Philippine Islands, and the ''interruptus'' phase of ''A. perversus'' is not present in the Natuna Islands.

Carl Arend Friedrich Wiegmann (1893, 1898) discussed portions of the anatomy of '' A. adamsii'', '' A. porcellanus'', '' A. contrarius'', and ''A. sinistralis''. Walter Edward Collinge (1901, 1902) briefly noted features of '' A. palaceus'' and '' A. parakensis'' (reported as ''A. perversus''). Haniel (1921) dissected ''A. contrarius'' and '' A. reflexilabris'', and Bernhard Rensch published a few scattered notes in his various faunistic surveys. A few earlier notes are mentioned in Pilsbry (1900).

Characters such as the long, narrow kidney with reflexed ureter and closed secondary ureter, the penial complex with distinct penis, which is continuous with the epiphallus, epiphallic caecum (a flagellum and an appendix), unbranched gametolytic duct, lack of vaginal accessory organs, and the basic condition of the nervous and retractor muscle systems support the inclusion of ''Amphidromus'' in the family Camaenidae. This group of snails occur in a wide variety of habitats in the tropics of Eastern Asia and Australasia, and is one of the most diverse families in the clade Stylommatophora. Though Laidlaw & Solem (1961) provided no more additional details on the anatomy of ''Amphidromus'', subsequent studies by distinct authors, e.g., Bishop (1977) and Solem (1983), have demonstrated that the reproductive system can provide valuable data for species recognition.
dissection