Tensegrity Architecture of an Agglutinated Foraminiferan Shell

Abstract

Shells of agglutinated foraminiferan protists are composed of mineral grains bound by secreted adhesives. As such, they are useful models for examining the evolution of “primitive” exoskeletons. Previous studies revealed the ultrastructure of shells in the giant Antarctic foraminiferan Astrammina rara and demonstrated that shucked specimens would reconstruct shells using glass beads. Here we further investigate shell architecture in this model species.

For micromechanical testing, an intact A. rara shell was placed between a fixed plate and a facing plate in series with a calibrated load cell. Displacement was effected by a high-precision drive, and 2-3 loading cycles were used to determine shell material properties. to assay tensile properties of the adhesive matrix, a network of pseudopodia and extracellular matrix fibers (i.e., the shell adhesive component) was obtained by incubating shucked cell bodies on 200-mesh gold grids. Pseudopodia were subsequently removed by detergent washes. Fibers in the resultant isolated matrix were severed with a Nd: YAG laser using an inverted DIC light microscope equipped with a 60× objective lens. Preliminary loading experiments using glass needles showed that Sepharose 2B beads were suitable strain gauges to assess compression within reconstructed shells (Fig. 1). in this assay, shucked cell bodies were incubated with a mixture of glass and Sepharose beads, and the reconstructed shells were examined by SEM.

Repeated loading and unloading demonstrated the elastic behavior of intact shells (Fig. 2). Adhesive matrix fibers snapped towards their attachment sites within 2 sec after cutting with a laser (Fig. 3), demonstrating that they are deployed under tension. SEM images of shells reconstructed with Sepharose show compressed particle profiles (Fig. 4).