The inner layer of many bivalve and gastropod molluscs consists of iridescent nacre, a material that is structured like a brick wall with bricks consisting of crystalline aragonite and mortar of organic molecules. Myostracal layers formed during shell growth at the point of muscle attachment to the shell can be found interspersed within the nacre structure. Little has been done to examine the effect the myostracal layer has on subsequent nacre structure. Here we present data on the structure of the myostracal and nacre layers from a bivalve mollusc, Pinctada fucata. Scanning electron microscope imaging shows the myostracal layer consists of regular crystalline blocks. The nacre before the layer consists of tablets approximately 400 nm thick, while after the myostracal layer the tablets are approximately 500 nm thick. A new technique, imaging polarimetry, indicates that the aragonite crystals within the nacre following the myostracal layer have greater orientation uniformity than before the myostracal layer. The results presented here suggest a possible interaction between the myostracal layer and subsequent shell growth.
Video S1 - polarimetry of shell as a function of horizontal position
Polarimetry imaging of the Pinctada fucata shell as the shell was moved horizontally through the beam, thereby illuminating nacre, myostracal layer, nacre, and prismatic layer, in order, shows distinct light transmission patterns for each region. The sample was moved horizontally through the beam with the movie beginning with imaging the light before it is transmitted through the shell, followed by imaging the light as it is transmitted first through the nacre, then the myostracal layer, the nacre between the myostracal layer and the prismatic layer, and finally the prismatic layer. The regions of low intensity within the images are indicative of regions of the sample where the transmitted light did not match the selected polarization filter, which was held constant throughout the movie. As can be seen within the movie, there are repeating fringes of high and low intensity with varying periodicity in both nacre regions.
Video S1.mov
Video S2 - polarimetry of shell as a function of vertical position
Polarimetry imaging of the Pinctada fucata shell as a nacre portion of the shell was moved vertically through the beam, shows the repetitive nature of the polarization fringes within the sample. The sample was moved vertically through the beam while the horizontal position was held constant such that the beam was transmitted through nacre. The regions of low intensity within the images are indicative of regions of the sample where the transmitted light did not match the selected polarization filter, which was held constant throughout the movie. As can be seen within the movie, there are repeating fringes of high and low intensity throughout the shell.
Video S2.mov
Backscatter scanning electron microscope image of the post-myostracal layer nacre.
Backscatter SEM image showing the nacre layers in the post-myostracal layer.
6500x BEC 20kV cross-section postgrowth nacrescale.tif
Backscatter scanning electron microscope image of the pre-myostracal layer nacre.
Backscatter SEM image of the pre-myostracal layer nacre highlighting the nacre layers.
6500x BEC 20kV cross-section pregrowth nacrescale.tif