Retyped and reformatted (but otherwise verbatim) from:
Giusti, F. & Manganelli, G. (eds.) 1992. Abstracts of the 11th International Malacological Congress, Siena, Italy, 30th August 5th September 1992. Pp. 434436.© 1992, University of Siena.
Developmental regulation of snail shell shape; multiple roles of the preceding whorl
J. M. C. Hutchinson
School of Mathematics, University of Bristol, University Walk, Bristol, BS8 1TW, England.
Snail shells invite measurement. Many dimensions seem required to properly describe their shape but multivariate analyses reveal much covariance (e.g. Gould, 1987). Some correlations follow from isometric size variation, or because, for instance, spire height is a component of overall height. When no such inevitable geometrical constraint is responsible, a correlation is evidence either of characters independently adapting to the same selection pressures, or of developmental constraints. I view the latter as geometrical constraints that follow once particular rules of developmental regulation are specified.
Here I list 5 ways in which the shell one revolution behind the aperture acts as a cue to guide (constrain) growth. One may guess the adaptive purposes of these developmental rules, but proximate mechanisms are my primary concern. Thus a definitive test of the hypotheses would be to manipulate shape or position of the preceding whorl. My tests are merely confirmations that shell measurements on single populations do intercorrelate as predicted.
The 1st process of regulation is the Roadholding Model (Hutchinson, 1989a). This concerns where the next whorl attaches. I have proposed that the shellsecreting mantle follows contours on the preceding whorl. This is clearest when a sharp keel is tracked so as to generate a flush suture (Fig. 1a). An analogy is the way a cable neatly coiling on a drum slips in alongside the preceding turn; but I do not deny that the mantle may also be aligned by the shell just behind the aperture, rather like a carpet rolling up askew.
The 2nd process of regulation concerns expansion of the aperture, bearing also on its shape (Hutchinson, 1989b). There are two parts to the mantle: around the inner margin of the aperture shell material is merely accreted onto the preceding whorl; around the outer margin shell grows out into space (Fig. 1b). Consider how these margins expand (either per revolution, or per distance traversed along a spiral trajectory). Expansion rates of both margins increase then decrease through ontogeny, but the peak is one revolution later for the outer margin (Fig. 3.). The reason, I propose, is that the inner margin is constrained to expand in concert with the outer margin of the preceding whorl, to which it attaches. This would follow from the Roadholding Model if sharp contours around the umbilicus and periphery acted as railway tracks setting a gauge (Fig. 1a). Surprisingly my data are from Trichia hispida (L.), which lacks a sharp keel (Fig. 1b); we might have expected that expansion of the body would have stretched the inner mantle as much as the outer.
The 3rd process of regulation concerns the inclination of the aperture relative to the coiling axis. Most apertures are roughly planar, so two angles will quantify inclination. Elsewhere (Hutchinson, 1989b) I consider several potential determinants of these angles; each is used to predict relationships with other dimensions measured on T. hispida. The successful hypothesis is that the apertural plane is maintained at a tangent to the preceding whorl; this correctly predicts how the two inclination angles vary with size, and (once size is factored out, where necessary using polynomial regression) with umbilicus width and the angle of the underside of the preceding whorl. In fact the aperture is not fully tangential (Fig. 2a,b), but evidently developmental processes do regulate the alignment. An adaptive basis is that a tangential aperture makes a close seal against a flat surface (Linsley, 1977). This benefit also explains the downturn in growth of mature snails, and why immature stages are often keeled rather than rounded in profile (Fig. 2c,d).
Elder and Sibatini (1991) proposed a 4th process of regulation, concerning the periodic deposition of projecting spines and ribs. In species such as Epitonium scalare varices on adjacent whorls are aligned, and these authors argue that each varix entrains varix formation one revolution later. A 5th process concerns not just the preceding whorl but also earlier whorls. Okamoto (1988) showed that changes in growth direction of heteromorph ammonoids could be caused by the changing balance of the shell. It is conceivable, but untested, that in land snails the torque on an unbalanced shell affects the position of the shellsecreting mantle.
Conchologists should be aware of the above developmental processes: (1) they may avoid measuring two characters that developmental processes make nonindependent; (2) patterns of covariation may simplify if characters are chosen to directly reflect the parameters of the developmental rules; (3) as the null hypothesis about normal growth changes, deviations from this pattern may be recognised, and adaptive reasons for these sought.
The morphological integration with the preceding whorl also makes a concrete example to inspire embryologists. The closest parallel is the way spiders space out the capture spiral of their web (Vollrath, 1987). In models of plant growth, the shade or chemical sinks of old shoots may redirect or inhibit new shoots (Bell, 1986). In Drosophila, compartments subdivide the compartments last set up. Thus the preceding homologue is often a significant component of the new structures environment, and a robust developmental strategy is to accommodate to it.
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