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 leucoxanthus
has a shell form that is typical for the genus Amphidromus'': it is moderately convex and smooth. 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 '' shell is brightly colored, a feature which is typical of arboreal snails in general. '' shell marks a resting stage. 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 ==