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Spiral ribs are among the most common morphological features in mollusk shells and previous studies have shown them to have functional significance with expected evolutionary consequences. Many previous studies, however, have treated these features as potentially analogous across taxa, without examining whether they may have important constructional dissimilarities. Mollusk shells are made of multiple layers of calcite or aragonite which may exhibit different microstructure or microstructure orientations which may in turn impact their mechanical properties. In this study, five specimens of marine mollusks with spiral ribs, including three turritellid gastropods and two bivalves, were examined under SEM to examine microstructure of ribbed region in comparison of non-ribbed region. SEM imaging revealed differences in the number and thickness of distinct microstructural layers of each shell and allowed comparisons to be made between the ribbed and non-ribbed region of each specimen, providing a greater understanding of how these ribs were constructed during shell deposition. Ribs in all specimens are formed through the thickening of single or multiple crossed-lamellar layers, but differences in rib ultrastructures were found among species and different ribs of same species, showing great diversity and complexity of constructional mechanisms. This diversity in rib construction might indicate heterology in the development of shell sculpture, especially mechanisms for differences in concurrently deposited rib strength. This is especially notable for turritellids where the pattern of onset of spiral ornamentation is phylogenetically informative, suggesting homology of rib identity. Further study will be conducted on turritellid gastropods in different lineages to explore the taxonomic meaning of different rib constructional mechanisms.
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Specimen alignment with limited point-based homology: 3D morphometrics of disparate bivalve shells (Mollusca: Bivalvia)
Background Comparative morphology fundamentally relies on the orientation and alignment of specimens. In the era of geometric morphometrics, point-based homologies are commonly deployed to register specimens and their landmarks in a shared coordinate system. However, the number of point-based homologies commonly diminishes with increasing phylogenetic breadth. These situations invite alternative, often conflicting, approaches to alignment. The bivalve shell (Mollusca: Bivalvia) exemplifies a homologous structure with few universally homologous points—only one can be identified across the Class, the shell ‘beak’. Here, we develop an axis-based framework, grounded in the homology of shell features, to orient shells for landmark-based, comparative morphology. Methods Using 3D scans of species that span the disparity of shell morphology across the Class, multiple modes of scaling, translation, and rotation were applied to test for differences in shell shape. Point-based homologies were used to define body axes, which were then standardized to facilitate specimen alignment via rotation. Resulting alignments were compared using pairwise distances between specimen shapes as defined by surface semilandmarks. Results Analysis of 45 possible alignment schemes finds general conformity among the shape differences of ‘typical’ equilateral shells, but the shape differences among atypical shells can change considerably, particularly those with distinctive modes of growth. Each alignment corresponds to a hypothesis about the ecological, developmental, or evolutionary basis of morphological differences, but we suggest orientation via the hinge line for many analyses of shell shape across the Class, a formalization of the most common approach to morphometrics of shell form. This axis-based approach to aligning specimens facilitates the comparison of approximately continuous differences in shape among phylogenetically broad and morphologically disparate samples, not only within bivalves but across many other clades.
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- Award ID(s):
- 1633535
- PAR ID:
- 10359591
- Date Published:
- Journal Name:
- PeerJ
- Volume:
- 10
- ISSN:
- 2167-8359
- Page Range / eLocation ID:
- e13617
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.7717/peerj.13617
