A marine biologist investigated the physiological stress levels of green sea turtles (Chelonia mydas) in different habitats. A matched-pairs design was used, comparing 14 turtles captured in coastal lagoons affected by urban runoff with 14 turtles captured in pristine offshore reef environments. The biologist measured the concentration of corticosterone (a stress hormone, in nanograms per millilitre) in blood samples taken from each turtle. It was found that turtles from urban coastal lagoons had significantly higher corticosterone concentrations than those from offshore reefs.