Ephemeral masks in the ellipsoidal foraminifera Alveolinella and Borelis (Alveolinoidea): resilient solutions to stabilization in coral-reef settings

In modern Indo-Pacific coral-reef areas, the porcelaneous larger foraminifera Alveolinella quoyi and Borelis schlumbergeri, the only extant prolate ellipsoidal alveolinoids, live on sandy and coral-rubble substrates. Apertural features are functional elements of shell morphology; therefore, specimens were investigated to assess these characters. The primary apertures of these alveolinoid forms can be obstructed by two different masks (mineralized infraskeletal shell elements): sinusoidal reverse masks (pending from the upper border of the aperture) and upright (rising from the lower border). Alveolinella quoyi exhibits sinusoidal reverse masks, whereas B. schlumbergeri has upright masks. These masks consist of discrete groups of needle-shaped crystals. The reverse masks are part of the apertural rim, whereas the upright masks form from the basal layer. These ephemeral characters occur at the terminal stage of the formation of the apertural face and are resorbed after the calcification of the new chamber. These features, with no or very low fossilization potential, appear to reflect distinct functions during chamber growth. The extruding protoplasm from the reverse-mask and upright mask-bearing apertures enhances shell stabilization. In the poles, where the reverse masks and upright masks do not occur, the protoplasm likely enables motility.