Abstract
Sea turtles use coastal and offshore waters along the southeast coast of Florida, and other inlets and bays as feeding grounds, home ranges, and migratory routes. These waters have likely overlap with marine current energy testing and deployment sites, for both tidal and ocean current energies. It is unknown whether sea turtles will avoid or be attracted to structures associated with marine current turbines. Sea turtles do not avoid other in-water structures or moving objects and are often struck by marine vessels. Unfortunately, blunt force trauma to the shell can result in serious injury or death. To assess potential risks, field studies on the effects of blade collision with seal carcasses (a proxy for live marine mammals) show that extensive skeletal damage can result from the tip of the turbine blade. Though turbine blades are slow-moving, turtles may respond defensively, turning the shell toward the blade, if they are viewed as a threat and the result would be blade impact. Since strike risks are currently unknown, it is important to understand how sea turtle shells respond to impact forces to best assess if hazard is likely. The aim of this study is to quantify the biomechanical properties of sea turtle shells throughout ontogeny to assess the shell’s mechanical behavior. The sea turtle species investigated in this study include the loggerhead (Caretta caretta), green (Chelonia mydas), and Kemp’s ridley (Lepidochelys kempii) turtle due to their occurrence in continental shelf waters and their stranding abundance along the US coast. Shells were collected from fresh dead and freshly frozen turtles made available from states, federal sea turtle stranding networks, and sea turtle rehabilitation facilities. Material testing of the shell is used to assess mechanical behavior and includes both quasi-static compression and impact testing. Stiffness (resistance to deformation), toughness (ability to absorb energy), yield strength, and impact toughness of shells were evaluated from the material tests. In compression, our data suggests that marine turtle shells are much less stiff than other turtle and tortoise species’ shells. The greater flexibility of sea turtle shells likely reflects the pressure oscillations routinely encountered while diving and represents functional ecological tradeoff between protection and other essential biological shell functions. Variation also exists among species with green turtles having the strongest and stiffest shells, followed by Kemp’s ridleys, and loggerhead shells were the least stiff (i.e., they are highly compliant). In general, low stiffness and strength values seen across life stages suggest that the shell does not offer sea turtles the same level of mechanical defense that is assumed for turtles. Initial data on impact testing also supports these results. Ultimately, we hope to use these material properties to simulate turbine strikes drawing on finite element analyses for evaluation and risk assessment.