Flexible embryonic shell allies large offspring size and anti-predatory protection in viviparous snails.

The evolutionary conflicts between viviparous reproductive mode and skeleton shape may occur whenever the space available for embryo development or delivery is limited by hard inflexible structures of a parent (bones, shell, etc.). In tetrapods, offspring size is at odds with female locomotion efficiency, which results in obstetric selection. We suggest a similar relationship for viviparous gastropods, where spacious canal needed for embryo delivery may interfere with anti-predatory role of narrow and toothed shell aperture. We explored this hypothesis in the group of viviparous land snails (Clausiliidae, subfamily Phaedusinae), known for complex apertural barriers protecting the shell interior. Most of the shell structure modifications we recorded facilitate the delivery of embryos but simultaneously reduce the safeguard of a narrow shell opening. However, we also observed highly flexible embryonic shells that may withstand squeezing between apertural barriers during birth. We investigated the microstructure of these flexible embryonic shells, compared to the typical hard shells of clausiliid embryos, which are rigid and unpliable already in the genital tract of the parent. Our results suggest that the unusual flexibility, which is related to a low number of organomineral layers in the shell, evolved in two phylogenetically distant lineages of Phaedusinae. This adaptation reduces mechanical constraints for birth of the neonates but allows to maintain the protective function of the apertural barriers.

Simultaneous extension of both basic microstructural components in scleractinian coral skeleton during night and daytime, visualized by in situ 86Sr pulse labeling.

Using in situ (12 h) pulse-labeling of scleractinian coral aragonitic skeleton with stable 86Sr isotope, the diel pattern of skeletal extension was investigated in the massive Porites lobata species, grown at 5 m depth in the Gulf of Eilat. Several microstructural aspects of coral biomineralization were elucidated, among which the most significant is simultaneous extension of the two basic microstructural components Rapid Accretion Deposits (RAD; also called Centers of Calcification) and Thickening Deposits (TD; also called fibers), both at night and during daytime. Increased thickness of the 86Sr-labeled growth-front in the RADs compared to the adjacent TDs revealed that in this species RADs extend on average twice as fast as TDs. At the level of the individual corallite, skeletal extension is spatially highly heterogeneous, with sporadic slowing or cessation depending on growth directions and skeletal structure morphology. Daytime photosynthesis by symbiotic dinoflagellates is widely acknowledged to substantially increase calcification rates at the colony and the corallite level in reef-building corals. However, in our study, the average night-time extension rate (visualized in three successive 12 h pulses) was similar to the average daytime extension (visualized in the initial 12 h pulse), in all growth directions and skeletal structures. This research provides a platform for further investigations into the temporal calibration of coral skeletal extension via cyclic growth increment deposition, which is a hallmark of coral biomineralization.