Incredible internal strains within a biogenic single crystal viewed by X-ray diffraction tomography

The dorsal arm plates (DAPs) of the Ophiocoma Wendtii brittle star are highly functional single crystalline biominerals whose optimized structure and nanostructure enable them to fullfill mechanical and optical functions in the organism. Here, a large DAP bulk piece is characterized by means of synchrotron X-ray Diffraction Tomography (XRDT). This non-destructive crystallographic characterization revealed an astounding feature: the presence of very high compressive strains which relax when the mineral is cracked or grinded into a powder. Thus, previous destructive characterization techniques did not allow their detection. We attribute the compressive strains to the previously identified high-Mg calcite particles, which are coherently included and thereby compress the low-Mg calcite matrix. The measured slice contained both the bulk DAP sample as well as DAP powder. The data generated by the bulk piece could be separated from those by the powder, and the latter was used to calibrate and interprete the former. This study reveals yet another awe-inspiring feature of a biogenic structure, highlights the importance of non-destructive crystallographic characterization for biominerals, and exemplifies the potential of XRDT use in studying a single crystalline material, as well as the advantage of complementary measurement of bulk and powder for data calibration and interpretation.

Magnesium-rich nanoprecipitates in calcite: atomistic mechanisms responsible for toughening in Ophiocoma wendtii

Atomistic simulations provide insight into an example of the superiority of biogenic crystals, where Mg-rich nanoprecipitates in calcite inhibit crack propagation.

From spinodal decomposition to alternating layered structure within single crystals of biogenic magnesium calcite

As organisms can form crystals only under ambient conditions, they demonstrate fascinating strategies to overcome this limitation. Recently, we reported a previously unknown biostrategy for toughening brittle calcite crystals, using coherently incorporated Mg-rich nanoprecipitates arranged in a layered manner in the lenses of a brittle star, Ophiocoma wendtii. Here we propose the mechanisms of formation of this functional hierarchical structure under conditions of ambient temperature and limited solid diffusion. We propose that formation proceeds via a spinodal decomposition of a liquid or gel-like magnesium amorphous calcium carbonate (Mg-ACC) precursor into Mg-rich nanoparticles and a Mg-depleted amorphous matrix. In a second step, crystallization of the decomposed amorphous precursor leads to the formation of high-Mg particle-rich layers. The model is supported by our experimental results in synthetic systems. These insights have significant implications for fundamental understanding of the role of Mg-ACC material transformation during crystallization and its subsequent stability.

Whole-body photoreceptor networks are independent of ‘lenses’ in brittle stars

Photoreception and vision are fundamental aspects of animal sensory biology and ecology, but important gaps remain in our understanding of these processes in many species. The colour-changing brittle star Ophiocoma wendtii is iconic in vision research, speculatively possessing a unique whole-body visual system that incorporates information from nerve bundles underlying thousands of crystalline ‘microlenses’. The hypothesis that these might form a sophisticated compound eye-like system regulated by chromatophores has been extensively reiterated, with investigations into biomimetic optics and similar supposedly ‘visual’ structures in living and fossil taxa. However, no photoreceptors or visual behaviours have ever been identified. We present the first evidence of photoreceptor networks in three Ophiocoma species, both with and without microlenses and colour-changing behaviour. High-resolution microscopy, immunohistochemistry and synchrotron tomography demonstrate that putative photoreceptors cover the animals' oral, lateral and aboral surfaces, but are absent at the hypothesized focal points of the microlenses. The structural optics of these crystal ‘lenses’ are an exaptation and do not fulfil any apparent visual role. This contradicts previous studies, yet the photoreceptor network in Ophiocoma appears even more widespread than previously anticipated, both taxonomically and anatomically.

Interactions between ophiuroids and beaugregory damselfish

The interactions between beaugregory damselfish and ophiuroids, potential damselfish egg predators, were investigated using several field studies. A survey of back reef areas showed that the two most common ophiuroid species, Ophiocoma echinata and Ophioderma appressum, were significantly associated with the territories of beaugregory males within the coral rubble habitat where both ophiuroids and beaugregory's can be found. Feeding experiments showed that three ophiuroid species (Ophiocoma echinata, Ophioderma appressum and Ophiocoma wendtii) and Echinometra viridis consumed late development stage beaugregory eggs, whereas earlier stages were only consumed by Ophioderma appressum and Echinometra viridis. Manipulation of ophiuroid densities had no significant effect on the survival of beaugregory eggs and had no measurable impact on female mate choice. The aggressive response of male beaugregory damselfish towards three ophiuroid and one echinoid species showed significant differences among species but were lower than those shown to bluehead wrasse, Thallasoma bifasciatum; the principal daylight predator of eggs. The intensity of attacks by male beaugregory damselfish towards Thallasoma bifasciatum increased significantly if males were guarding eggs, but did not appear to change for the four echinoderm species. Male habitat selection was shown to be unaffected by the presence of ophiuroids.