FUNCTIONAL SHELL COLOR PATTERN POLYMORPHISM IN PUPERITA PUPA

The functionality of color pattern in mollusks has been debated for decades. Previous studies identified both functional color patterns and nonfunctional color patterns, and both types may be influenced by many factors, ranging from salinity to diet and natural selection. This is a difficult question to address using fossils alone, but combined paleontological and ecological approaches can more successfully examine the possible functions of molluscan color pattern.

This study uses repair scars and measurements of abiotic parameters to examine factors influencing color pattern in the Zebra Nerite, Puperita pupa, a small, locally abundant, intertidal gastropod inhabiting rocky tide pools in southeast Florida and the Caribbean. It is a member of the family Neritidae, whose fossils frequently preserve color patterns. The color pattern of P. pupa is polymorphic and ranges from mostly black, to white with thin black stripes.

Twenty-five P. pupa were collected from four tide pools at four different localities on San Salvador Island, Bahamas. Water temperature, salinity, and pool depth were measured at each tide pool. Each snail was photographed and measured, and the presence/absence of scarring was noted. Average greyscale values, a quantitative proxy for shell pattern, were calculated for each snail using ImageJ (v. 1.46r). Variance in average greyscale value was then plotted against scar frequency to assess selection pressure across populations varying in extent of polymorphic color pattern (PCP).

Shell lengths and scar frequencies ranged from 2.8mm to 11.5mm and from 19% to 76%, respectively, among the sixteen tide pools. Greyscale variance was not correlated with any of the abiotic parameters. A significant negative correlation was observed between greyscale variance and scar frequency (r = -0.71, p = 0.0019), with pools having greater scar frequency expressing lower color variance. These results suggest that more highly polymorphic P. pupa populations are attacked less frequently, which is consistent with the hypothesis that PCP confers an adaptive advantage on populations in which it occurs. Thus, selection may control PCP in P. pupa, as opposed to environmental factors, indicating that PCP may be functional as camouflage and suggesting that the polymorphic color pattern of Puperita pupa is adaptive.