THE ADAPTIVE SIGNIFICANCE OF SHELL MORPHOLOGY AND COLOR IN CERION (MOLLUSCA, GASTROPODA, PULMONATA)

JOHN FREDERICK QUENSEN, Purdue University

Abstract

Cerion is a morphologically diverse genus of land snails inhabiting Cuba, the Cayman Islands, Bahama Islands, Hispanola, Virgin Islands, Dutch Antilles, and the Florida Keys. Due to extensive interpopulational variation over 600 species have been described, but nearly all hybridize freely, and only one case of sympatry is known. Morphological variation within populations is much more moderate. The distribution of morphological types has been explained as the result of hurricanes casting ashore lone propagules which founded new populations. But more recent work indicates there is a systematic pattern to the distribution of morphological types. Literature on the adaptive morphology of snail shells is reviewed and applied to Cerion. Specifically considered are shell size, color, strength of ribbing, and resistance to crushing. Experiments demonstrating the possible adaptive value of variation in each of these traits were performed and correlations between morphology and habitat were noted. The force necessary to crush shells of seven species and ten populations of Cerion was determined using a mechanical crab claw. The populations sampled represent a variety of degrees of shell thickness, ribbiness, shape, and overall size. Models predicting shell strength from measures of shell height, breadth, thickness, and rib height were developed. Predictions of shell strength are consistent with determinations of snails' susceptibility to being crushed by the land crab Gecarcinus lateralis, a predator of Cerion. The color of different Cerion ranges from white to almost solid dark brown. It has been demonstrated for other snail species that more darkly pigmented shells absorb more radiant energy, and correlations between color and climate have been noted. Differences in the temperatures of Cerion shells of different degrees of mottling were estimated by measuring the temperatures of shells exposed to direct sunlight. Comparisons were also made between shells of different surface textures and ribbiness. The maximum difference between hourly mean temperatures was 3.1(DEGREES)C, between white and heavily mottled shells. The maximum shell temperatures of pigmented shells (48(DEGREES)C) was below the lethal temperture determined in the laboratory when snails were exposed to elevated temperatures for 5 hours (52.5(DEGREES)C). But snails exposed to 42.5(DEGREES)C for one week died of dehydration. It is concluded that white shells should be favored in habitats where there is little or no shade. Nine morphospecies of Cerion were collected from Abaco Island and Long Island in the Bahamas and from the Florida Keys. Habitat data for each collection site were recorded and included location, elevation, substrate type, shade, predominant vegetation, presence of other snail species, and evidence of predators. Notes were also made of the estivation position at each site, i.e., whether the snails were in leaf litter or above ground on plants or rocks. Snails with stronger shell types, as determined by experimental means, were more commonly found where there were signs of potential predators, especially Gecarcinus lateralis. Pigmented snails were more often found in shaded habitats, and white snails in habitats where they were exposed to direct sunlight. Snails at the most exposed sites had a lower height to breadth ratio, and small adult snails and juveniles behaviorally compensate for their greater susceptibility to desiccation by estivating in leaf litter rather than above ground. Thus, the distribution of different Cerion shell morphologies is consistent with functional adaptations to local habitat.

Degree

Ph.D.

Subject Area

Ecology

COinS