Functional morphological MR Imaging: a case study on the Antarctic silverfish Pleuragramma antarctica

Modern non-destructive imaging techniques offer distinct advantages for particular anatomical and morphological questions in zoology. The possibility to capture multi- slice images and create 3D representations of complex and fragile morphological structures that can be digitized and animated in combination with image processing techniques are just some of the reasons why these techniques are increasingly being used. Using the Antarctic silverfish Pleuragramma antarctica as an example, we show in this presentation how high-field magnetic resonance imaging (MRI) can be used as a tool to address ecological questions in preserved animal samples. P. antarctica is a pelagic key species in the food web of high-Antarctic waters and is considered highly vulnerable to changes in its environment. P. antarctica is extremely delicate and capturing and keeping individuals of this species alive for in vivo imaging studies is hardly possible. Due to the lack of a swim bladder, neutral buoyancy is mainly attained by large amounts of lipids which are stored in lipid sacks. However, the functional role of lipids in P. antarctica is not yet fully understood, i.e. it is not clear whether the function of lipids is limited to buoyancy or they serve as energy deposits as well. If the lipids are used as energy storage, differences in the nutritional state should be reflected in the amount of body lipid content. Several examples of imaging applications for addressing ecological questions in P. antarctica will be presented. This will include high-resolution morphological 2D- and 3D-MR images from P. antarctica to determine body composition and the overall fat and muscle distribution of individual preserved fishes. We calculated the individual lipid and water content from these images, which allowed us to estimate the percentage of overall lipid content. The percentage of lipid content correlated well with literature data obtained from standard, destructive lipid measurement techniques. 3D image reconstructions were used for non-destructive analysis of stomach volume and comparing lipid deposits in different individuals. We show that functional morphological MRI is a useful tool in marine ecological studies, even in preserved samples and provides a suitable alternative to some classical, destructive methods, in particular for studies of fragile and delicate structure