Comparisons of Population Structure and Morphology of a Saltmarsh Keystone Species (Malaclemys terrapin) Across Coastal Louisiana

Abstract

Diamondback terrapins (Malaclemys terrapin) are an ecologically important species in salt marsh habitats, and they occur along the Gulf of Mexico and the Atlantic coastlines of the USA. Terrapins are subjected to a myriad of threats including loss of coastal marsh/nesting habitat and differential adult mortality in fisheries bycatch. In Louisiana, knowledge of terrapin populations is lacking even though they likely face these same threats. To address this knowledge gap, we sampled terrapins across coastal Louisiana from 2011 to 2015 using unbaited fyke nets at six study sites. We sexed all individuals and measured morphometric characters (including plastron length, PL and carapace height, CH). We captured 1111 individuals at all sites, and each site was represented by ≥ 96 individuals. Skewed sex ratios were found at five of the six sites, while three of the six populations were lacking individuals in the 13–15 cm PL size classes. Male and female PL varied, with Chenier Plain males being larger than Deltaic Plain males, while Deltaic Plain females were larger than Chenier Plain females. Similarly, CH also varied, with Deltaic Plain males and females being relatively deeper than individuals from the Chenier Plain. A number of biotic, abiotic, and anthropogenic factors likely interplay to influence population structure, shell morphology, and demographics. However, the latter, including bycatch in crab traps, is possibly influencing terrapin population structure at some study sites via selective mortality of males and juvenile females. We suggest that future studies research the effects of anthropogenic factors on terrapin populations.