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.2013 Apr 9;110(15):6015-20.
doi: 10.1073/pnas.1220443110. Epub 2013 Mar 25.

Mechanical basis of morphogenesis and convergent evolution of spiny seashells

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Mechanical basis of morphogenesis and convergent evolution of spiny seashells

Régis Chirat et al. Proc Natl Acad Sci U S A..

Abstract

Convergent evolution is a phenomenon whereby similar traits evolved independently in not closely related species, and is often interpreted in functional terms. Spines in mollusk seashells are classically interpreted as having repeatedly evolved as a defense in response to shell-crushing predators. Here we consider the morphogenetic process that shapes these structures and underlies their repeated emergence. We develop a mathematical model for spine morphogenesis based on the mechanical interaction between the secreting mantle edge and the calcified shell edge to which the mantle adheres during shell growth. It is demonstrated that a large diversity of spine structures can be accounted for through small variations in control parameters of this natural mechanical process. This physical mechanism suggests that convergent evolution of spines can be understood through a generic morphogenetic process, and provides unique perspectives in understanding the phenotypic evolution of this second largest phylum in the animal kingdom.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Spiny shells in mollusks. (A) Bivalve (Spondylus imperialis, recent, Philippines), (B) gastropod (Bolinus cornutus, recent, Senegal), (C) ammonoid (Collignoniceras praecox, Middle Turonian, United States), and (D) nautiloid (Paracenoceras spinatum, Middle Oxfordian, France). (Scale bars, 10 mm.)
Fig. 2.
Fig. 2.
Setup for modeling spine formation. The elastic mantle edge, showing an excess of length compared with the previous and fixed shell edge, deforms, then secretes new material in the deformed shape.
Fig. 3.
Fig. 3.
Simulation of spine formation with uniform growth and thickness, for growth ratesformula image (left to right). Other parameters are provided inSI Text.
Fig. 4.
Fig. 4.
Form of bending stiffness. A decreased stiffness allows for higher curvature at the tip of the spine.
Fig. 5.
Fig. 5.
Phase diagram showing the effect of the growth rate and the degree of stiffness variation on the evolving spine. Variations in spine morphology among species of the same family (Muricidae) are shown for comparison: (A)Hexaplex erythrostomus, (B)Bolinus brandaris, (C)Pterynotus phyllopterus, and (D)Ceratostoma burnetti. Other parameters are provided inSI Text. (Scale bar, 10 mm.)
Fig. 6.
Fig. 6.
(A) Time evolution of the shell edge via our model. (B) Proposed evolution when including self-contact of the mantle edge (dashed lines). Self-contact prohibits the middle section from pinching.
See this image and copyright information in PMC

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