Acoustic and Vibrational Properties of the American Horseshoe Crab Shell

Among the most ancient living species on earth the Limulus polyphemus has evolved extraordinarily effective survival strategies. Covered by a shell of chitin exhibiting physical properties that set it apart from other shell bearing animals: including a low density of 0.15 [g/cm3], a complex gradient of thickness, and a closed porous structure. It is reasonable to assume that the role of the shell is to shield the soft tissue of the crab from predators; however, one would expect that its mechanical strength would be high and yet this shell is comparatively weak. At the same time, it displays a structural complexity yielding a high acoustic impedance mismatch with water. These observations raise the prospect that the shell may be contributing in other ways to the crab’s survival. Harnessing ultiple measurement approaches we study physiological sources of sound including the magnitude of heart and gill movement as expressed close to the source of excitation itself as well as its vibrational impact on the motion of the shell surface. Gill motion and heartbeat, which generate large macroscopic displacements below the shell result in sub microns vibration of the shell surface. Given the large impedance mismatch with water, we show through sound transmission measurements that that the shell has very high acoustic reflectivity below 1000 Hz corroborated by a finite element analysis. These observations raise the prospect that the Limulus polyphemus shell may serve as an acoustic and vibration attenuator, lowering the vibration and acoustic signature of the horseshoe crab and conferring a potential survival advantage.