Tightly shut: flexible valve margins and microstructural asymmetry in pterioid bivalves

Elizabeth M Harper,Antonio G Checa

doi:10.1007/s00227-020-03693-y

Elizabeth M Harper, Antonio G Checa

Open Access

https://doi.org/10.1007/s00227-020-03693-y

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Journal: Marine biology	Publication Date: May 6, 2020
Citations: 15	License type: open-access

Affiliation: University of Cambridge, University of Granada

Abstract

An organic-rich columnar prismatic outer shell layer, which extends far beyond the underlying nacre, has allowed pterioid bivalves (the pearl oysters and their allies) to develop flexible valve margins, allowing a tight hermetic seal when shut. In some taxa, the microstructural arrangement is known to be asymmetrically developed between the two valves. The asymmetry was surveyed across 29 taxa of pterioids (including representatives of known genera) confirming that it is typically the right valve which has a greater expanse of prism-only shell (and less nacre) and showing that this portion of the right valve has more organic content (more than twice the value in some instances) than the equivalent in the left. A more detailed investigation of prismatic material in Pteria penguin comparing the right and left valves revealed that the right valve flange has a higher density of smaller prisms, each with its organic envelope, and not a greater thickness of the organic envelopes themselves. The flange is also thinner on the right valve and shown here to be very flexible when wet. This allows it to bend against the rigid left valve when the shell is closed. Comparison of this structural asymmetry in the pterioids with five outgroup taxa in the Ostreidae and Pinnidae suggests that clades with the asymmetry have been freed from the constraints of a flattened valve morphology and to develop inequivalved forms.

Highlights

Molluscan shell is a biocomposite in which a stiff mineralised phase is embedded in a softer organic matrix that allows energy dissipation, toughness and flexibility (Currey 1999; O’Toole-Howes et al 2019; Strąg et al 2020)
For some taxa, most notably all Pteria species examined, some Pinctada and some Isognomon (I. isognomon and I. legumen), there is a high degree of asymmetry in the shell microstructure between the right and left valves
In only those taxa for which we had more than ten specimens, we investigated whether there was any systematic link between valve size and the degree of asymmetry displayed but no cases were discovered

Summary

Introduction

Molluscan shell is a biocomposite in which a stiff mineralised phase (usually calcite or aragonite) is embedded in a softer organic matrix that allows energy dissipation, toughness and flexibility (Currey 1999; O’Toole-Howes et al 2019; Strąg et al 2020).

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