USING MICRO-CT SCANNING AND NANO SEM TO ASSESS THE STATE OF PTEROPODS IN THE MODERN AND FUTURE OCEAN

Shelled pteropods are a group of nektonic molluscs that live in the upper ocean. They form their shells from aragonite, the more soluble form of calcium carbonate, and are abundant in polar waters where carbon dioxide is most soluble. When carbon dioxide dissolves in the ocean, it reacts with water, decreasing the concentration of carbonate ions, essential for carbonate shell formation, and decreasing the pH of seawater in a process called ocean acidification. Due to their importance in the marine food chain and their threat from ocean acidification, it is with some urgency that we need to understand how pteropods may react to future changes in ocean chemistry.

Visual assessment of pteropods shells following incubation in aragonite undersaturated waters indicate that reduced concentrations of carbonate ions can have a detrimental effect on shell condition. At present, light and scanning electron microscopy are the standard means of qualitatively assessing pteropod shells. Although these observations are useful for documenting large scale changes in shell preservation, we expect that preservational differences related to changes in ocean chemistry will be more subtle.

This study uses micro-CT scanning to image pteropod shells in three dimensions. This novel technique is applied to samples from five locations globally which represent a range of aragonite saturation values. These data enable shell properties such as thickness and volume to be quantified, allowing for impartial comparisons between sites to be made. We find that there is a significant difference in pteropod shell thickness between the five different ocean basins and discuss the potential reasons for this.

There has been an increased interest in the field focused on how pteropods may react to future changes in ocean chemistry. We present data from incubation experiments where polar pteropods were kept at 2 and 2.5 times modern pCO₂ levels for 14 days. Micro-CT scans on these specimens highlight pteropods kept at 2.5 times modern levels exhibit significant pitting to the shell surface. These pits are investigated further using nano-SEM to analyse the changes that are occurring during shell dissolution. From this, the mechanisms for dissolution are assessed, and the implications of these changes are discussed.