The difference of morphological diversity between land and marine gastropod shells

Background/Question/Methods

The morphological diversity of gastropod shells is a classic question that has been subjected to various physical, geometrical and ecological explanations. Vermeij (1971) pointed out that the apertures of high-spired shells are generally not highly “inclined”. Inclination is defined as the angle between the aperture plane and the coiling axis, and it is positive when the aperture tilts downward. Okajima and Chiba (2013) discovered a correlation between spire height and aperture inclination in land snails, and attributed this correlation to adaptation to gravity. However, many questions remain: does the correlation between spire height and aperture inclination differ among habitats? If there is a difference, what is its cause? Here I study these questions using both biometric and biomechanical analyses. To quantify morphological diversity I measured spire height and aperture inclination in 511 specimens, which were classified into three categories based on their habitats: land (n = 116), freshwater (n = 127) and marine (n = 268). In addition, I calculated two properties: shell balance (average moment of force) and area of orthogonal projection, using Raup’s classic model of shell development. 

Results/Conclusions

I found that the correlation between height and aperture inclination was different in land snails than in freshwater and marine species. I also found the aperture inclination for marine species was significantly lower than for land snails. Few species in marine gastropods had planispiral. Morphological patterns observed by Vermeij (1971) and by Okajima and Chiba (2013) were also confirmed by this biometric analysis. Through biomechanical analysis, I found that optimal aperture inclination is reduced as spire height becomes higher, which corresponds to the empirical pattern observed in land snails. Moreover, I found marine species have significantly smaller areas of orthogonal projection. Taken together, these results suggest that land snails are primarily adapted to the effects of gravity, which are stronger on land than in water; whereas marine species are primarily adapted to decrease drag, which is stronger in viscous water compared to air. These results, in a classical study system, illustrate how functional constraints imposed by the habitat shape the distribution of species in morphospace.