Modeling underwater visual ability and varied color expression in the diamondback terrapin (Malaclemys terrapin) in relation to potential mate preference by females
Authors:   , Dominy Abigail Elizabeth , Avery Harold W. , Spotila James Robert 
Published: July 2021

Color vision is utilized by animals to navigate the environment, distinguish between objects (e.g. prey and conspecifics), perceive differences in fitness between potential mates, as well as a number of other visual tasks. Color vision varies greatly among species and as a consequence a visual signal varies greatly in how it is perceived. The conditions of the environment additionally impact the ability of an animal to perceive the intrinsic value of a visual signal. The field of visual ecology has only recently received a great deal of attention due to technological advancements that allow objective measures of animal coloration. However, ease of organismal sampling has created a bias in the literature, with avian and mammalian taxa garnering greater attention than reptilian taxa. The estuarine diamondback terrapin (Malaclemys terrapin), hereafter called terrapin, is an aquatic turtle facing human-induced population declines. Population models and estimates of viability only hold under the assumption of random mating, which is often not the case. In this study I tested the hypothesis that female terrapins exhibit preferences for mates with color or color patterns that differ from the general population. I captured gravid females and males in Barnegat Bay, New Jersey as part of a long term population study conducted by my lab. To determine if non-random mating is occurring in the Barnegat Bay terrapin population, I conducted a paternity analysis on hatchlings that were obtained from captured gravid females. I genotyped blood samples for six highly polymorphic microsatellite loci from all adult males and gravid females, and tissue samples from all hatchlings. To determine whether genotype-matched fathers exhibited color phenotypes different from the population, I implemented the three concepts of visual ecology: (1) measure physiological visual ability, (2) measure the visual target being perceived, and (3) measure the light environment in which the visual target is perceived. I found four distinct classes of photoreceptor cones in the terrapin and modeled their sensitivity in the UV, blue, green and red portions of the electromagnetic spectrum (~355-640 nm), conferring tetrachromatic visual ability. Terrapins exhibited unusual absorbance of light in the ocular media, resulting in unusual spectral tuning of photoreceptor sensitivities. I measured the light environment in the water column at the surface and at half-meter intervals below the surface. The downwelling irradiant light environment at the surface of the water column spectrally matched the visual sensitivity modeled here in the terrapin, resulting in optimal photon capture. I measured spectral reflectance of ten distinct color patches on the terrapin which also spectrally matched modeled sensitivities and surface downwelling light, resulting in optimal perception of conspecific coloration at the surface of the water column. Five color patches were measured on the skin and five on the shell. Skin color patches exhibited a clear UV and longwave components, while the shell only exhibited a longwave component. Hatchling paternity results suggested that females potentially preferred males with higher color contrast between the shell and the skin, as well as greater hue saturation of the shell. Although the number of genotype-matched fathers was low (N = 14), this study suggests that non-random mating may be occurring in the Barnegat Bay, NJ terrapin population.