Schistoloma alta sibuyanica Bartsch, 1915
(Sinistral Specimen On The Left)
in land prosobranchs (operculate land snails) is a rare and
interesting phenomenon. It is a "normal" occurrence in about six
lineages of the cyclophoroidean family
Diplommatinidae L. Pfeiffer, 1858,
and one species, Palaina taeniolata hyalina Quadras and
Möllendorff, 1894 from Guam has somewhat indifferent chirality. In
the H. G. Lee collection there are 240 dextral and 4617 sinistral
specimens of this tiny oddity (just under 5% dextral mutants). See:
Dextral Palaina taeniolata hyalina
Quadras and Möllendorff, 1894
Records for mutant reversal of coil in the cyclophoroideans, to which Schistoloma Kobelt, 1902 [type: Cyclostoma alta G. B. Sowerby II, 1842] belongs, are reported in the table below (Dautzenberg; 1914, Nisters, 1999; Nunes and Santos, 2007; Gerber, 2009; taxonomy and nomenclature updated by H. G. Lee). As one can see, there are fifteen records from three families, Cyclophoridae Aciculidae, and Diplommatinidae, but Schistoloma Kobelt, 1902 is not in any of these three; it appears to be the first known instance of mutant reversal in the Pupinidae.
Also treated in the table is the other large group of terrestrial prosobranchs, the annulariid-pomatiid stock, which arose independently of the Cyclophoroidea and comprises the terrestrial radiation of the Littorinoidea. The land snails of this superfamily also seem disinclined to reversal of coil. In fact, until Dr. Gary Rosenberg recently found two sinistral specimens of Adamsiella irrorata in Jamaica, only a single instance of reversal in the entire (dextral) family Annulariidae Henderson and Bartsch, 1920, which numbers about 700 species, was known (Jacobson and Boss, 1971; Watters, 2006). In the other littorinoidean landsnail group, the (dextral) Pomatiidae Newton, 1891, there is only one species known to exhibit reverse chirality (Dautzenberg, 1914).
MUTANT REVERSAL OF COIL IN TERRESTRIAL PROSOBRANCHS (sinistrality)
Bartsch, P., 1915 The Philippine land
shells of the genus Schistoloma Proc. U. S. N. M. 49(2104):
195-204 + 1 pl. (51).