Volume of the whale shark and their mechanism of vertical feeding

Abstract

The present study provides a noninvasive method to estimate the body volume of sharks (Elasmobranchii, Selachii) using a computational geometric model. This method allows the volume of sharks to be estimated from lateral and ventral photographs assuming an elliptical body cross-sectional geometry. A comparison of the estimated and actual body volumes of several shark species showed that the estimation error was < 0.5%. The accuracy of the model decreased if photographs that were inclined to the orthogonal plane were used, although this error was on average < 2.3% if the inclination angle was 10° or less.Applying this model to captive whale sharks (Rhincodon typus) that were 8.0 and 8.8 m in total length revealed that their body volumes were 3.5 and 4.5 m3, respectively. These estimates allowed for the quantitative evaluation of our hypothesis, that the whale shark uses suctioned air for buoyancy control during vertical feeding—a behavior unique to this species among elasmobranchs. The volume estimates of the captive whale sharks, together with the density estimates from their liver proportions, revealed that the air occupying a part of oro-pharyngeal and branchial cavities can help the whale sharks to keep their body floating. This hypothesis may explain how the whale shark sometimes stays at the water surface without fin motion during vertical feeding, even though their body density is greater than that of seawater.