Quantified Ventral Color and Dorsal Pigment Invasion for Nerodia Erythrogaster

The subspecies concept is a useful tool when describing individual populations that possess unique characteristics, but are members of a larger ranging species system. Phenotypic variation within populations, however, can often make subspecies distinctions difficult. This is particularly true in the plainbelly water snake (Nerodia erythrogaster). In this taxonomic system, the variation in the diagnostic characters has no clear evolutionary significance, which contributes to the high levels of phenotypic variation seen throughout its range, particularly in the lower Ohio and middle Mississippi River drainages. To better understand the taxonomy and geographic distribution of this species in this region, variation of the diagnostic phenotypic characters of N. erythrogaster in extreme southern Illinois, western Kentucky, and western Tennessee were studied. Prior to this study, the existing data suggested that the southeastern tip of southern Illinois was occupied by the copperbelly water snake (N. e. neglecta) and the remainder of the study area was occupied by the yellowbelly water snake (N. e. flavigaster). From the spring of 2002 to the summer of 2004, adult Nerodia erythrogaster were captured and scored for sex, size, ventral color, and dorsal pigment invasion onto the ventral scales. Specimens indistinguishable from all four United States N. erythrogaster subspecies were found within this region. N. e. neglecta has the greatest influence on regional phenotypes to the north and to the east along the Ohio and Cumberland Rivers, giving way to N. e. flavigaster to the south and west throughout the Mississippi River valley. This represents an extension of the current range of N. e. neglecta through the remainder of the lower Ohio River drainage to its confluence with the Mississippi River. This includes portions of the lower Cumberland River. The remainder of this region, from south of the Ohio and Mississippi River confluence, south to the Hatchie River in southwest Tennessee is an area of intergradation between these two subspecies. Multiple regression analysis of the diagnostic characters for N. erythrogaster revealed that ventral color and invasion of dorsal pigment onto the ventral scales were dependent characters. They were also dependent on size (snout to vent length). This suggests that there are sustained ontogenetic changes in the diagnostic phenotypic characters of Nerodia erythrogaster, which further complicates subspecies designation in this system. The findings of this study, in conjunction with anecdotal reports from other regions of N. erythrogasters range, bring into question the validity of subspecies distinctions in this system.