Fig. 5

Fatty organ and vestigial lung composition and buoyancy contribution. a Sagittal section in the fatty organ imaged for lipid fraction with magnetic resonance imaging (MRI) using the Dixon method. Lipid fraction is visibly elevated in the caudal portion of the fatty organ. b Distribution of net buoyancy provided by the fatty organ (mass of tissue subtracted from mass of displaced seawater) along its long axis at four different depths in Comoran seawater. The caudal portion of the fatty organ provides more buoyancy than the cranial portion. c Magnetic resonance spectra acquired at the cranial (red graph and red line in a) and caudal (blue graph and blue line in a) portion of the fatty organ with the same scanning parameters (i.e., signal intensity reflects proton density). The spectrum at the caudal portion has a more pronounced methylene peak, whereas the cranial portion has a slightly more pronounced water peak, both supporting differences in lipid fraction along the craniocaudal axis of the fatty organ. d Tissue composition of coelacanth vestigial lung completely imbedded in the fatty organ. Low-resolution lipid (top left and top middle) and water fraction (top right) images of the cranial portion of the fatty organ (yellow isosceles triangles) containing the vestigial lung (yellow arrows). The vestigial lung contains very little lipid (see Table 1) and is instead water rich. A high-resolution T2-weighted MR image (bottom left) allows for better visualization of the vestigial lung within the fatty organ. X-ray computed tomography (CT) can be used to visualize the bony plates (yellow arrow heads) surrounding the vestigial lung (bottom middle) allowing for three-dimensional surface rendering of bony plates (bottom right). e,f Modeled absolute and relative buoyancy (e) and torque (f) (relative to surface torque with normal fatty organ) experienced by the coelacanth at surface, 190, 400, and 1000 m of depth in Comoran seawater if the fatty organ consisted of either seawater, muscle (or other water-rich lean tissues), pure lipid (oleyl oleate with no water content), or atmospheric air from inhalation. At the surface, an air-filled structure of similar size as the fatty organ would provide a marked increase in buoyancy and torque. However, as this air volume would compress at increasing depths under the assumption of a non-rigid wall, the lift provided by an air-filled bladder at depths would be less than that of a lipid-filled fatty organ. Magnifications in e and f contain the normal depth range of Comoran coelacanths