Multifunctionality of chiton biomineralized armor with an integrated visual system.

Nature provides a multitude of examples of multifunctional structural materials in which trade-offs are imposed by conflicting functional requirements. One such example is the biomineralized armor of the chiton Acanthopleura granulata, which incorporates an integrated sensory system that includes hundreds of eyes with aragonite-based lenses. We use optical experiments to demonstrate that these microscopic lenses are able to form images. Light scattering by the polycrystalline lenses is minimized by the use of relatively large, crystallographically aligned grains. Multiscale mechanical testing reveals that as the size, complexity, and functionality of the integrated sensory elements increase, the local mechanical performance of the armor decreases. However, A. granulata has evolved several strategies to compensate for its mechanical vulnerabilities to form a multipurpose system with co-optimized optical and structural functions.

Three-dimensional structure of the shell plate assembly of the chiton Tonicella marmorea and its biomechanical consequences.

This study investigates the three-dimensional structure of the eight plate exoskeletal (shell) assembly of the chiton Tonicella marmorea. X-ray micro-computed tomography and 3D printing elucidate the mechanism of conformational change from a passive (slightly curved, attached to surface) to a defensive (rolled, detached from surface) state of the plate assembly. The passive and defensive conformations exhibited differences in longitudinal curvature index (0.43 vs. 0.70), average plate-to-plate overlap (∼62% vs. ∼48%), cross-sectional overlap heterogeneity (60-82.5% vs. 0-90%, fourth plate), and plate-to-plate separation distance (100% increase in normalized separation distance between plates 4 and 5), respectively. The plate-to-plate interconnections consist of two rigid plates joined by a compliant, actuating muscle, analogous to a geometrically structured shear lap joint. This work provides an understanding of how T. marmorea achieves the balance between mobility and protection. In the passive state, the morphometry of the plates and plate-to-plate interconnections results in an approximately continuous curvature and constant armor thickness, resulting in limited mobility but maximum protection. In the defensive state, the underlying soft tissues gain protection and the chiton gains mobility through tidal flow, but regions of vulnerability open dorsally, due to the increase in plate-to-plate separation and decrease in plate-to-plate overlap. Lastly, experiments using optical and scanning electron microscopy, mercury porosimetry, and Fourier-transform infrared spectroscopy explore the microstructure and spatial distribution of the six layers within the intermediate plates, the role of multilayering in resisting predatory attacks, and the detection of chitin as a major component of the intra-plate organic matrix and girdle.

Quantifying a dynamic risk landscape: heterogeneous predator activity and implications for prey persistence.

Spatial heterogeneity in predation risk can ameliorate impacts on prey populations, particularly for prey of generalists. Spatially heterogeneous risk implies the existence of refugia, and the spatial scale of those refugia and their persistence over time affect whether prey can avoid predation by aggregating therein. Our objective was to quantify the magnitude, spatial scale, and temporal persistence of heterogeneity in risk of predation by white-footed mice (Peromyscus leucopus), an abundant generalist predator of gypsy moths (Lymantria dispar) and songbirds. We used track plates to measure white-footed mouse activity at > 170 trees in each of three forest plots in upstate New York during summers of 2003-2005. We quantified the mean and coefficient of variation of track activity among trees by fitting the beta-binomial distribution to data from each plot and study period. We measured temporal persistence by disattenuated autocorrelation, and spatial scale by fitting exponential variograms. Mice were much less abundant in 2005 than the other two years, leading to lower overall track activity but higher coefficient of variation among trees. Mouse track activity at individual trees was positively autocorrelated between monthly study periods in 2003 and 2004, and even between the two years, whereas temporal autocorrelation in 2005 was much weaker. Track activity showed positive spatial autocorrelation over lag distances from approximately 30 to > 1000 m. These findings indicate that mouse activity, and hence risk to their prey, varies substantially in space at spatial and temporal scales that appear responsive to mouse population dynamics. The spatial scale and temporal persistence of that variation imply that prey may benefit from returning to, or failing to disperse from, refugia.