Many species of chiton are known to deposit magnetite (Fe3O4) within the cusps of their heavily mineralized and ultrahard radular teeth. Recently, much attention has been paid to the ultrastructural design and superior mechanical properties of these radular teeth, providing a promising model for the development of novel abrasion resistant materials. Here, we constructed de novo assembled transcripts from the radular tissue of C. stelleri that were used for transcriptome and proteome analysis. Transcriptomic analysis revealed that the top 20 most highly expressed transcripts in the non-mineralized teeth region include the transcripts encoding ferritin, while those in the mineralized teeth region contain a high proportion of mitochondrial respiratory chain proteins. Proteomic analysis identified 22 proteins that were specifically expressed in the mineralized cusp. These specific proteins include a novel protein that we term radular teeth matrix protein1 (RTMP1), globins, peroxidasins, antioxidant enzymes and a ferroxidase protein. This study reports the first de novo transcriptome assembly from C. stelleri, providing a broad overview of radular teeth mineralization. This new transcriptomic resource and the proteomic profiles of mineralized cusp are valuable for further investigation of the molecular mechanisms of radular teeth mineralization in chitons.

Abstract: The quest for making a triplet positronium (Ps) Bose–Einstein condensate confined in a micron-sized cavity in a material such as porous silica faces at least three interrelated problems: (1) About $$10^7$$ 10 7 spin polarized Ps atoms must be injected into a small cavity within a porous solid material without vaporizing it. (2) It is known that Ps atoms confined in 30–100 nm diameter cavities in porous silica do not remain in the gas phase, but become stuck to the cavity walls at room temperature (Cooper et al., Phys. Rev. B 97:205302, 2018). (3) Cooling a gas of Ps atoms to cryogenic temperatures by energy exchange with the walls would be a very slow process (Saito and Hyodo, in: Surko, Gianturco (eds) New Directions in Antimatter Chemistry and Physics, Springer Dordrecht, Netherlands, 2001) because of the relatively low collision rate with the walls and the large mismatch between the masses of the Ps and the wall atoms. A possible solution of these difficulties is presented, based on cooling the implanted positrons in an isotopically pure diamond single crystal target, subsequent saturating of the wall Ps coverage so that a substantial portion of the Ps will be in the gaseous state, and thermalizing the gas-phase Ps via collisions with the low effective mass wall Ps. A design process for the target material is outlined as well, including preliminary results in porous cavity fabrication using focused ion beam milling. Graphical abstract:

Chitons are herbivorous invertebrates that use rows of ultrahard magnetite-based teeth connected to a flexible belt (radula) to rasp away algal deposits growing on and within rocky outcrops along coastlines around the world. Each tooth is attached to the radula by an organic structure (stylus) that provides mechanical support during feeding. However, the underlying structures within the stylus, and their subsequent function within the chiton have yet to be investigated. Here, we investigate the macrostructural architecture, the regional material and elemental distribution and subsequent nano-mechanical properties of the stylus from the Northern Pacific dwelling Cryptochiton stelleri. Using a combination of μ-CT imaging, optical and electron microscopy, as well as elemental analysis, we reveal that the stylus is a highly contoured tube, mainly composed of alpha-chitin fibers, with a complex density distribution. Nanoindentation reveals regiospecific and graded mechanical properties that can be correlated with both the elemental composition and material distribution. Finite element modeling shows that the unique macroscale architecture, material distribution and elemental gradients have been optimized to preserve the structural stability of this flexible, yet robust functionally-graded fiber-reinforced composite tube, providing effective function during rasping. Understanding these complex fiber-based structures offers promising blueprints for lightweight, multifunctional and integrated materials.

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Woodpeckers avoid brain injury while they peck at trees up to 20Hz with speeds up to 7m/s, undergoing decelerations up to 1200g. Along with the head, beak and neck, the hyoid apparatus (tongue bone and associated soft tissues) is subjected to these high impact forces. The shape of the hyoid apparatus is unusual in woodpeckers and its structure and mechanical properties have not been reported in detail. High-resolution X-ray micro-computed tomography and scanning electron microscopy with energy dispersive X-ray spectroscopy were performed and correlated with nanoindentation mapping. The hyoid apparatus has four distinct bone sections, with three joints between these sections. Nanoindentation results on cross-sectional regions of each bone reveal a previously unreported structure consisting of a stiff core and outer, more compliant shell with moduli of up to 27.4GPa and 8.5GPa, respectively. The bending resistance is low at the posterior section of the hyoid bones, indicating that this region has a high degree of flexibility to absorb impact. These new structural findings can be applied to further studies on the energy dissipation of the woodpecker during its drumming behavior, and may have implications for the design of engineered impact-absorbing structures.

Statement of significance

Woodpeckers avoid brain injury while they peck at trees, which results in extreme impact conditions. One common adaptation in woodpeckers is the unusual shape of the elongated tongue, also called the hyoid apparatus. The relationship between the structure and mechanical properties of the bony part of the hyoid apparatus has not been previously reported. A three dimensional model of the bony tongue was developed, and the hardness and stiffness were evaluated. A new type of bone structure, which is opposite of typical skeletal bone structure was found. The combined microstructural and mechanical property analysis indicate possible energy absorption routes for the hyoid apparatus and are applicable to the design of engineered structures.