The effect of enhanced carbon dioxide on the sinking and swimming of the shelled pteropod Limacina retroversa

Shelled pteropods (thecosomes) are planktonic mollusks that are expected to be negatively impacted by ocean acidification. The shells of live pteropods exposed to enhanced CO₂ are known to exhibit degradation in condition, but the impacts on the fitness of the animals are unclear. Limacina retroversa from the Gulf of Maine were used to investigate the impact of enhanced CO₂ on shell condition as well as swimming and sinking behaviors. L. retroversa were caught in the summer, fall, and spring and maintained in seawater at either ambient or two levels of enhanced CO₂, and then filmed in a mirrored tank to measure the 3D velocities and other characteristics of the animals’ movements while sinking or swimming. Shell condition was also examined by a suite of imaging techniques including light microscopy, SEM, and micro-computed tomography. After exposures to enhanced CO₂ of as little as 3 days the pteropod shells became darker and more opaque. The pteropds had slower sinking velocities when kept under medium and high CO₂ (800 and 1200 ppm) in comparison to the ambient (~400 ppm) control group for exposure periods between one and four weeks. The swimming velocities of animals ascending in the tank were similarly decreased for animals maintained under the enhanced CO₂ conditions for one to three weeks. The wing beat frequency and the path of motion were analyzed to further characterize swimming ability. Pteropods use both sinking and swimming as anti-predation techniques and hence the observed decrease in sinking and swimming speeds observed for animals exposed to increased CO₂ could have a direct impact on their fitness by increasing their mortality risk to predators.