The life span of the Greenland shark (Somniosus microcephalus) is at least 272 years and may be as long as 500 years (1). This extreme longevity is particularly interesting with respect to the heart, because ageing in humans is associated with cardiovascular dysfunction including fibrosis, atherosclerosis and heart failure. Accumulating evidence has suggested a causative link between mitochondrial dysfunction and major phenotypes associated with ageing (2). In this study we have performed quantitative analysis on mitochondrial morphology and subcellular organisation on mitochondria for the compact layer of the Greenland shark. We have used scanning electron micrographs of ultrathin serial sections to segment individual mitochondria (n=208) and performed three-dimensional computational reconstructions of these within the cardiomyocyte. We report an average mitochondrial volume of 1.33±1.21 μm3 and a mitochondrial fraction/volume density of 0.251. Furthermore, we report cases of severe mitomegaly and a large number of caveolae. This is of interest because, caveolins, proteins associated with caveolae (small cave-like invaginations of the sarcolemmal membrane), have been implicated as signalling mediators coordinating cellular protection responses during oxidative stress and ageing (3). To our knowledge, this is the first study to present a deep qualitative and quantitative analysis of an elasmobranch cardiomyocyte and its organelles. Further studies will lead to better understanding of mitochondrial morphology and subcellular associations in a model of extreme longevity.