Caught between dimensions: 2D versus 3D geometric morphometrics in biodiversity assessment

Area of Science:

• Comparative morphology
• Geometric morphometrics
• Ichthyology

Background:

• Geometric morphometrics quantifies shape variation, but the accuracy of 2D data for estimating 3D morphology is debated.
• Dimensionality reduction from 3D to 2D may obscure subtle yet biologically significant shape differences.

Purpose of the Study:

• To evaluate the impact of dimensionality reduction on shape analysis by comparing 2D and 3D landmark data.
• To assess the concordance of shape variance patterns between 2D and 3D datasets across different taxonomic levels.

Main Methods:

• Utilized photogrammetric 3D models of museum fish specimens from the Lower Mississippi River Basin.
• Compared bilateral 3D, left-side-only 3D, and 2D projected landmark configurations using Principal Component Analyses and Mantel tests.
• Analyzed whole-body shape variation across the fish assemblage and within the Catostomidae family.

Main Results:

• 2D and 3D datasets showed strong concordance in capturing broad-scale shape variance and primary axes of variation (PC1-PC2).
• Correspondence decreased for lower-variance PCs, with 2D analyses underestimating disparity in taxa with pronounced 3D features like body width.
• The effect of dimensionality reduction was more pronounced within the morphologically constrained Catostomidae family than in the diverse assemblage.

Conclusions:

• 2D geometric morphometrics reliably capture dominant body shape gradients but fail to represent fine-scale 3D structural variation.
• 3D approaches are crucial for studies focusing on subtle shape disparity, functional morphology, and traits like body width.
• The choice between 2D and 3D methods should align with research objectives, with 2D suitable for broad surveys and 3D for detailed analyses.