Interdigitating biocalcite dendrites form a 3-D jigsaw structure in brachiopod shells.

We report a newly discovered dense microstructure of dendrite-like biocalcite that is formed by marine organisms. High spatial resolution electron backscatter diffraction (EBSD) was carried out under specific analytical conditions (15 and 10 kV) on the primary layer of the modern brachiopod Gryphus vitreus. The primary layer of modern brachiopods, previously termed nanocrystalline, is formed by an array of concave/convex calcite grains with interdigitated recesses and protrusions of abutting crystals without any cavities in or between the dendrites. The interface topology of this structure ranges from a few tens of nanometres to tens of micrometres, giving a nanoscale structure to the material fabric. The dendritic grains show a spread of crystallographic orientation of several degrees and can thus be referred to as mesocrystals. Individual dendritic mesocrystals reach sizes in one dimension larger than 20 µm. The preferred crystallographic orientation is similar in the primary and adjacent fibrous shell layers, even though these two layers show completely different crystal morphologies and grain boundary topologies. This observation indicates that two separate control mechanisms are active when the primary and the fibrous shell layers are formed. We propose a growth model for the interdigitated dendritic calcite grain structure based on a precursor of vesicles filled with amorphous calcium carbonate (ACC).

The analysis of the mechanosensory origin of the infrared sensilla in Melanophila acuminata (Coeloptera; Buprestidae) adduces new insight into the transduction mechanism.

The thoracic infrared (IR) sensilla of the pyrophilous jewel beetle Melanophila acuminata most likely have evolved from hair mechanoreceptors (sensilla trichodea). To further elucidate the sensory transduction mechanism, the morphology of IR sensilla and of neighbouring hair mechanoreceptors was investigated by using conventional electron microscopical techniques (SEM, TEM) in combination with focused ion beam milling (FIB). It was assumed that any deviation from the bauplan of a sensillum trichodeum is of particular concern for the transduction of IR radiation into a mechanical stimulus. Thus, the structures supposed to be relevant for stimulus uptake and transduction were homologized. Compared to a hair mechanoreceptor, an IR sensillum shows the following special features: (i) the formation of a complex cuticular sphere instead of the bristle; the sphere consists of an outer exocuticular shell as well as of an inner porous mesocuticular part. (ii) The enclosure of the dendritic tip of the mechanosensitive neuron inside the sphere in a fluid-filled inner pressure chamber which is connected with a system of microcavities and nanocanals in the mesocuticular part. Hence we propose that an IR sensillum most probably acts as a microfluidic converter of infrared radiation into an increase in internal pressure inside the sphere which is measured by the mechanosensitive neuron.