Experimental tests of bivalve shell shape reveal potential tradeoffs between mechanical and behavioral defenses

Abstract

Bivalves protect themselves from predators using both mechanical and behavioral defenses. While their shells serve as mechanical armor, bivalve shells also enable evasive behaviors such as swimming and burrowing. Therefore, bivalve shell shape is a critical determinant of how successfully an organism can defend against attack. Shape is believed to be related to shell strength with bivalve shell shapes converging on a select few morphologies that correlate with life mode and motility. In this study, mathematical modeling and 3D printing were used to analyze the protective function of different shell shapes against vertebrate shell-crushing predators. Considering what life modes different shapes permit and analyzing the strength of these shapes in compression provides insight to evolutionary and ecological tradeoffs with respect to mechanical and behavioral defenses. These empirical tests are the first of their kind to isolate the influence of bivalve shell shape on strength and quantitatively demonstrate that shell strength is derived from multiple shape parameters. The findings of this theoretical study are consistent with examples of shell shapes that allow escape behaviors being mechanically weaker than those which do not. Additionally, shell elongation from the umbo, a metric often overlooked, is shown to have significant effects on shell strength.

Breaking down shell strength: inferences from experimental compression and future directions enabled by3Dprinting

“…Similarly, 3D printing can be used to create a wide range of prey morphologies to study relationships between shell morphology and strength. Johnson (2020) used 3D printing to test potential trade‐offs between bivalve shell shapes that allow burrowing or swimming and shapes associated with less mobility in taxa that rely on the shells as a mechanical defence. Theoretical shell shapes created for this study were generated mathematically, allowing the comparison of theoretical morphologies with those found in nature.…”

“…The results demonstrated potential trade‐offs: morphologies that enable escape may be less resistant to shell crushing by vertebrate predators. In addition, the ability to parameterize carefully the tested morphologies revealed that the direction of shell elongation (away from the umbo) affected shell strength (Johnson, 2020).…”

“…gypsum powder) exhibit mechanical behaviour much like real shells in compression tests (Fig. 2) (Crofts & Summers, 2014; Kolmann et al ., 2015; Johnson, 2020). Despite differences in microstructure, the dominant failure and crack propagation in 3D models made from carefully selected brittle materials will be through layers rather than along them.…”

“…Both real and 3D‐printed shells show brittle behaviour in compression demonstrating a small region of initially plastic behaviour followed by largely linear elastic behaviour and a subsequent second small region of plastic behaviour before peak load is reached at the breakage point. Modified from Johnson (2020).…”

Out of Their Shells

“…in contours, growth lines, and umbo curvature, which might be responses to environmental stress (Morais et al 2013). Alongside functional trade-offs in resource allocation between shell building and soft-tissue growth, shell sculpture has been revealed as a functional trait for defence through experiments with printed models ( Johnson 2020). The 3D models I created are effective in visualising preliminary qualitative results that imply differences in developmental processes and resource allocation for the production of different anti-scouring shell morphology -shell inflation and shell sculpture (see section:…”