Examining the effects of ocean acidification on the shells of thecosomatous pteropods with micro-computed tomography
To understand the organismal and ecosystem impacts of ocean acidification, some recent research has focused on the shell building capability and other metrics of wellbeing for a taxon of pelagic gastropod called thecosomatous pteropods (thecosomes). The shape and mass of thecosomes’ aragonite shells have important implications for the ecology of thecosomes and their role in the carbon cycle. The results of several laboratory experiments that test the quality of shell under projected increases of CO2 concentrations in the surface ocean have shown that shells appear less healthy under light and electron microscopes. The use of a micro-computed tomography (micro-CT) scanner on pteropod shells is a logical, and potentially exciting, extension of existing approaches for studying shell morphology in the context of ocean acidification. With a micro-CT system, an X-ray source is used to image a sample in 3D, and this would allow measurements of thickness and density of pteropod shells to be made over the entirety of the organism without compromising the integrity of the sample. This technique has not yet been used for studying thecosome morphology, although CT scans have been successfully used to examine the effects of ocean acidification on coral fragments1.
I have recently successfully defended my thesis proposal, part of which focuses on examining spatial differences in the shell morphology of thecosomes and the consequence of shell form and quality on the sinking velocities of these organisms. Within the taxon of thecosomes there is great diversity in shell size and shape which I plan to investigate as adaptations to living as holoplankton. I propose to explore the utility of micro-CT for the quantification of pteropod shell quality and structure. Specifically, my objectives for the COI funds are to investigate spatial differences in shell morphology among Limacina helicina and obtain parameters for the modeling of thecosome sinking dynamics for 3 species. If the micro-CT system proves to be a useful tool, then more individuals of different species will be analyzed in the future.
The shells of L. helicina have shown evidence of dissolution in undersaturated conditions, where dissolution of aragonite is thermodynamically favored, in laboratory experiments2 and in situ3. The use of micro-CT may be a more appropriate tool for studying shell properties than microscopes that only allow researchers to look at dissolution marks on the shell surface and thickness just at the shell opening.
The morphology of thecosome shells may be important for buoyancy regulation, behaviors such as diel vertical migrations and predator avoidance, and the export of carbonate to depth. Scans of animals with a micro-CT system would provide measurements of density, volume, and surface area, necessary for the calculation of the drag, gravitational force, and buoyant force on shells in the water column. Variability in shell structure between co-located individuals, conspecifics in different regions, and between different species also has not been investigated with micro-CT before, but could provide important insight into my thesis questions concerning the effects of ocean acidification on pteropods. I recently had the opportunity to obtain a test scan of Limacina retroversa on a dried animal sent to a micro-CT manufacturing company and it shows promise for measuring shell thickness (Figure 1).
Micro-CT scans of L. helicina will be analyzed on individuals caught over a gradient of saturation states in the Northern Pacific. Live individuals residing in undersaturated water will have their overall shell thickness and density compared to individuals living in more hospitable waters with respect to shell growth and maintenance. Individuals of L. helicina were preserved in 70% ethanol during a 2012 cruise that spanned 34°N to 50°N in the open Pacific Ocean.
Analysis of micro-CT images for 3 species of thecosomes (Limacina retroversa, Cavolinia uncinata, Clio pyramidata) will be used to compare measured sinking rates and modeled sinking rates to better understand how thecosomes regulate their sinking as live individuals and how post-mortem animals sink through the water column. Testing the use of micro-CT on these species is important since they vary considerably in size and shell structure. Thecosome individuals are available from Pacific and Atlantic Ocean cruises, as well as from more recent sampling efforts in the Gulf of Maine.
I propose to examine scans of:
20 Limacina helicina caught in nets at four different stations in the Pacific
20 Limacina retroversa from the coastal (Gulf of Maine) and the offshore Atlantic environment
10 Cavolinia uncinata from the offshore Pacific environment
10 Clio pyramidata from both the Atlantic and Pacific OceansGreater numbers L. helicina might need to be scanned to fully address spatial differences in shell morphology, but this preliminary investigation will allow me to identify this need. A larger, more morphologically diverse set of species could be examined in the future to learn more about how shell forms affect sinking velocities and the effect of ocean acidification on these organisms.