THOMPSON: SNAIL GENUS MARSTONIA


TABLE 1.-MEASUREMENTS (IN MM) AND WHORL COUNTS FOR SELECTED SPECIMENS OF Marstonia
 lustrica, REPRESENTING RIVERINE, LACUSTRINE, AND MARSH HABITATS.


 Riverine Lacustrine Marsh

Shell Length 3.3-3.8 3.2-4.3 4.3-4.9
Shell Width 2.2-2.6 2.0-2.4 2.0-2.6
Aperture Height 1.4-1.7 1.3-1.5 1.3-1.6
Aperture Width 1.3-1.5 1.1-1.3 1.1-1.3
Whorls 4.4-4.7 4.9-5.5 5.2-6.0
Shell Width/Length 0.66-0.76 0.53-0.64 0.46-0.56
Aper. H/Length 0.41-0.46 0.35-0.41 0.30-0.36
Aper. H/Aper. W 0.82-0.93 0.80-0.88 0.78-0.93


 VARIATION.-Throughout its range M. lacustrica shows a remarkable de-
gree of ecologically-associated variation, as is indicated by the lengthy syn-
onymy for the species. Specimens from lacustrine populations, for which the
name lustrica Pilsbry was proposed (Figs. 2C; 16A-B), are usually more slender
than are specimens from riverine habitats, for which the name decepta Baker
was proposed (Figs. 2A-B; 16G-J). I consider the lake form to be typical of the
species, because it is the most widespread and common variant and is found in
the most commonly occurring habitat throughout its range. Some specimens
from marshy lakes along the southern edge of the snail's range are extremely
elongate or terete (Figs. 2D; 16M-N); these elongate forms were named oneida
Pilsbry and gelida Baker. Two other named forms, winkleyi Pilsbry and per-
lustrica Baker, are minor variations of the riverine form. All degrees of inter-
mediate variation in the shell occur between the various forms, even within
single ecological stations (Turkey Foot Lake, Summit Co., Ohio and Lake
Maxinkuckee, Marshall County, Indiana). I find no other morphological evi-
dence to support recognition of more than one taxonomic entity. However, I
have examined only a limited amount of anatomical material, representing
riverine and lacustrine populations from Ohio and Michigan.
 M. lustrica cannot be segregated into distinct subspecies of obvious zoo-
geographic significance. Consequently, I think it unwise to recognize its eco-
logically segregated forms as subspecies until additional evidence is forth-
coming. Two genetic explanations can account for the observed ecologically
associated variation: (1) these forms are different genetic entities that segre-
gate ecologically due to selection for different environmental factors, or (2)
they are genotypic responses to different environmental factors, thus pro-
ducing different phenotypes. If variation is due to the former (1), then our
system of trinomial nomenclature is applicable. If variation results from the
latter (2), then trinomial epithets should not be applied. The terms ecotype
and ecophenotype have been used rather haphazardly in malacology to ex-
plain similar patterns of variation observed among other freshwater mollusks.