Fig. 3

Hydrostatic balance of the coelacanth at different depths. a,b Modeled overall center of gravity (COG) (red reticle), overall center of buoyancy (COB) (blue reticle), bone mineral COG (gray reticle), lipid COG (yellow reticle), and muscle COG (purple reticle) overlaid on bone mineral volume reconstruction (a) and photogrammetry reconstructed surface model (b) of the coelacanth. c Physical measurement of COG using the plumb line method at surface pressure and room temperature of 70% v/v EtOH preserved coelacanth. The intersection of the extrapolated vertical lines from two different anchor points (left photo: ventral side, caudal to the pelvic fins; right photo: dorsal side, caudal to the 2nd dorsal fin) reveal the COG (under the assumption of a negligible buoyancy force provided by the displaced atmospheric air). d Balance point of coelacanth immersed in its 70% EtOH preservation fluid. A line that was adjustable in the long axis of the specimen was tied around the circumference of the specimen and the specimen was hung from two vertical lines with adjustable connections to the circumferential line in the dorsoventral (short) axis. Long and short axis position of the anchor points were adjusted until the specimen was in balance indicating the balance point for the specimen in the preservation fluid between the COG and COB. e Distances between COG and COB relative to total length (TL) and total height (TH) in the long and short axis at different depths with varying temperatures, pressures, and salinities. f Absolute and relative torque (relative to torque at surface) at different depths with varying temperatures, pressures, and salinities. Distance between the COG and COB in both long and short axis increases slightly with depth in the modeled depth range resulting in increased torque and the need to produce extra hydrodynamic force to counteract the hydrostatic force acting to push COB directly above COG. If brought rapidly to the surface, the GOG to GOB distance will gradually decrease as the coelacanth heats up to surface temperatures resulting in a decreased torque (exemplified from 190, 400, and 1000 m of depth to surface with increasingly dark blue circles and dashed lines)