Which Way Does A Sundial Turn?

By Harry G. Lee

Figure 1.

    Recently the author received two anomalous specimens of a common and widespread member of the Sundial Shell family (aptly dubbed Solariidae, now Architectonicidae) from two adroit dealers, Phillip Clover and Guido Poppe. Each specimen was said to be "sinistral," yet there were a few unusual features that came to light after closer examination.

    Figure 1 shows each anomalous specimen of Heliacus variegatus (Gmelin, 1791) on either side of a single normal specimen. The two shells on the left were taken off the island of Negros; the one on the right, Siasi Sulu, all from the Philippines.  From left to right they measure 13, 15, and 14 mm.  The apical whorls of the left and right shells are missing, and their growth is in a disjunct conical pattern in contrast to the normal low turbinate form of the normal shell in the center.  Certainly, the conical specimens appear to coil in a direction opposite to the normal one.

    Now, Heliacus, like all architectonicids begins life with a left-coiling larval shell, which is an unusual condition among gastropods.  We know that the architectonicid animal has a right-handed, or dextral, organization, and, once the larval shell has settled and the animal begins its the benthic lifestyle, the shell reverses coil and remains dextral for the remainder of the snail's life (and beyond). To help visualize this topology, when one observes the apex of a well-preserved of a larger architectonicid like a Sundial Shell, a large Architectonica species for example, it appears that the apex is turned inward.  When the shell is inverted, the larval shell (protoconch) can actually be seen through the open umbilicus with its apex pointing toward the observer (see image). We may think of this left-coiling protoconch as having grown "up-side down" as contrasted the conventional growth of its surrounding adult shell and the majority of more familiar gastropod shells, like the middle specimen. We'll return to this notion a bit later.

    Are these two oddities in figure 1, like most anomalous left-coiled snail shells, mirror images (albeit somewhat contorted) of the normal one, or could they be the result of "upside-down" shell growth of the snail much like the relationship between animal and the larval architectonicid shell?  The shell on the right is posed to demonstrate the "upside-down" orientation; note that, thus displayed, the aperture is on the same side of the axis of growth as is the normal shell in the center.

    Are either or both of these anomalous shells the result of true reversal of coiling (a true mirror-image sinistral, growing in a normal or orthostrophic manner) or did they grow "upside-down" ("hyperstrophic pseudosinistral")? How does one determine the difference?

    Lets take a closer look at the shells' structure and sculpture using the left and middle specimens:



In the images of paired shells (figures 2-6), the normal specimen is on the left.
Figure 2. Adapical (umbilical) aspect of the body whorl is juxtaposed with that of the abnormal shell from Negros.
Figure 3. Same aspect of the normal shell next to the apical aspect of the abnormal.
Figure 4. Apical aspect of both shells.
Figure 5. Apical aspect of the normal with adapical of the abnormal.

    Accounting for some aberration of the contour of the abnormal shell, it is apparent that the sculpture of the adapical (basal) aspect of the normal shell, with more, smaller, and more finely-beaded cords, is far more similar to that of the apical aspect of the abnormal one (figure 2,3).  Likewise, the less numerous, larger, more coarsely-beaded sculpture basal (adapical) cords of the apical half of the normal shell's body whorl more closely resemble that of the basal aspect of the abnormal (figure 4,5).

Figure 6.    Now let's look more closely at the aperture (figure 6), which in this species has two distinct grooves on the columellar aspect as visible on the normal specimen. Note one groove (as well as the ornate cord runs back from it around the umbilical rim) is at the anterior (adapical) aspect of the columella.  In the abnormal shell that groove is at the apical ("opposite") aspect of the columella, where the aperture joins the body whorl (and the ornate cord is hidden beneath the latter).

    It may help to think of this anomalous shell by an imaginary process of transformation. Think of inserting the finger of a tiny rubber glove through the umbilicus at the aperture and pushing it all the way to the other end along this central axis.  Now imagine the shell becoming the same consistency as, and attached to, the glove finger. Now slowly pull the fingertip through the finger so as to turn it inside out. The spire will gradually shorten and then emerge through the umbilicus.  After a little more eversion, the transmuted shell becomes apparently right-coiling and, furthermore, the apertural grooves and outer shell sculpture are just (well almost) like that of the normal shell.  The latter features would not be in normal position if the anomalous shell were a truly (orthostrophic) sinistral specimen and put through this same contortion; they'd be opposite. Ergo: the anomalous shell in figures 2 through 6 is "hyperstrophic pseudosinistral" - also dubbed "ultradextral," very much like the larval shell discussed above.  Without doing the rubber glove trick, we can correct the anomalous position of the apertural groove by turning the aberrant shell "upside down" (as the shell on the right of figure 1); figure 7 shows the "upside down" aperture of the anomalous shell (bottom image) to have the groove on the same (lower; in the photo) end of the columella as the normal specimen. Corollary to this analysis: Hyperstrophic and orthostrophic shells of a single species are not mirror images of one another - even if they exhibit the same dimensions whereas true (orthostrophic) sinistral and dextral forms of a single species will match up in a mirror; on them the rubber glove technique fails.

    In case you're wondering, all the shell features discussed were also exhibited by the other anomalous specimen (on the right of figure 1), making it also hyperstrophic pseudosinistral rather than truly (orthostrophic) sinistral.  Furthermore, three congeners, Heliacus implexus (Michel, 1845)  [Ekawa, 1991 as H. dorsuosus (Hinds, 1844, a nude name teste Bieler, 1993], H. bicanaliculatus (Valenciennes, 1832) [Lagoda, 1868 as H. variegatus, a misidentification which was perpetuated for over a century teste Robertson and Merrill, 1963], and H. cylindricus (Gmelin, 1791) [Robertson and Merrill, 1963] have been reported to exhibit the same pseudosinistral condition. Like our shells, each of these is unnaturally elongate.  Robertson and Merrill (1963) demonstrated compelling evidence for hyperstrophy including the operculum morphology with that of the shell. Robertson (1974) went on record stating that "(true) sinistrality (is) unknown in the Architectonicidae," confirming this later (Robertson, 1993).

    So it appears that, throughout life, thus far all Sundials, like the dials on the non-digital chronometer descendants of their namesake, turn clockwise. This condition is has come to be the equivalent of right-handedness, which various cultures have deemed more virtuous and functional (linguistics reminds us: "dexterous" and "adroit" like Clover and Poppe above) than left-handedness ("sinister").  Perhaps aptly, the dextral condition also predominates in the gastropod world, and, at present, Sundials cannot claim exemption from majority rule.

    A shell of the South African Turbo sarmaticus Linnaeus, 1758 with a the same kind of anomaly is featured at http://www.jaxshells.org/tursar.htm

Bieler, R., 1993.  Architectonicidae of the Indo-Pacific (Mollusca: Gastropoda). G. Fisher, Stuttgart, pp. 1-377 incl. 286 figs., 3 pls.
Ekawa, K., 1991. A "sinistral" abnormality of Heliacus dorsuosus (Hinds). The Chiribotan 21(4): 87-88 April 3.
Lagoda, A. de, 1868 Note sur une variété abnormale de Torinia variegata, Lamarck. Journal de Conchyliologie 16: 264-265; pl. 9, fig.    7.
Robertson, R., 1974.  Sinistrality unknown in the Architectonicidae. La Conchiglia 6(60): 14 Feb.
Robertson, R., 1993. Snail handedness. The coiling direction of gastropods. National Geographic Research and Exploration 9(1):  104-119.
Robertson, R. and A. S. Merrill, 1963. Abnormal dextral hyperstrophy in post-larval Heliacus (Gastropoda: Architectonicidae). The Veliger 6: 76-79 Oct. 1.