Supplementary 1: Underwater photogrammetry for close-range 3D imaging of dry-sensitive objects: The case study of cephalopod beaks

Technical advances in 3D imaging have contributed to quantifying and understanding biological variability and complexity. However, small, dry-sensitive objects are not easy to reconstruct using common and easily available techniques such as photogrammetry, surface scanning, or micro-CT scanning. Here we use cephalopod beaks as an example as their size, thickness, transparency, and dry-sensitive nature make them particularly challenging. We developed a new, underwater, photogrammetry protocol in order to add these types of biological structures to the panel of photogrammetric possibilities.
We used a camera with a macro-photography mode in a waterproof housing fixed in a tank with clear water. The beak was painted and fixed on a colored rotating support. Three angles of view, two acquisitions, and around 300 pictures per specimen were taken in order to reconstruct a full 3D-model. These models were compared to others obtained with micro-CT scanning to verify their accuracy.
The models can be obtained quickly and cheaply compared to micro-CT scanning, and have sufficient precision for quantitative inter-specific morphological analyses. Our work shows that underwater photogrammetry is a fast, non-invasive, efficient, and accurate way to reconstruct 3D models of dry-sensitive objects while conserving their shape. While the reconstruction of the shape is accurate, some internal parts cannot be reconstructed with photogrammetry as they are not visible. In contrast, these structures are visible using reconstructions based on micro-CT scanning. The mean difference between both methods is very small (10-5 to 10-4 mm) and is significantly lower than differences between meshes of different individuals.
This photogrammetry protocol is portable, easy-to-use, fast, and reproducible. Micro-CT scanning, in contrast, is time-consuming, expensive and non-portable. This protocol can be applied to reconstruct the 3D shape of many other dry-sensitive objects such as shells of shellfish, cartilage, plants and other chitinous materials.

三维成像技术的进步推动了生物变异与复杂性的量化研究与认知。然而，小型、易受干燥影响的物体难以通过摄影测量法（Photogrammetry）、表面扫描或显微CT扫描（micro-CT扫描）等通用且易获取的技术完成三维重建。本研究以头足类喙（cephalopod beak）为例，这类物体的尺寸、厚度、透明度以及易受干燥影响的特性使其重建难度尤高。为此我们开发了一种全新的水下摄影测量法方案，旨在将此类生物结构纳入摄影测量法可覆盖的重建对象范围。

我们采用搭载微距摄影模式（macro-photography mode）的相机，将其置于固定于清水水槽中的防水外壳（waterproof housing）内。对头足类喙进行喷涂处理后，将其固定于带颜色的旋转支架上。每个标本需从3个视角完成拍摄、开展2次采集，单视角拍摄约300张照片，最终重建完整的三维模型。将这些模型与显微CT扫描得到的模型进行对比，以验证其重建精度。

相较于显微CT扫描，该方法可快速且低成本地获取三维模型，且具备足够的精度以支持种间形态学定量分析。本研究结果表明，水下摄影测量法是一种快速、非侵入、高效且精准的重建方式，可在保留物体形态的前提下完成易受干燥影响物体的三维建模。尽管外形重建精度达标，但部分内部结构因不可见而无法通过摄影测量法完成重建；而基于显微CT扫描的重建则可呈现此类结构。两种方法的平均差异极小（10^-5至10^-4毫米），且远低于不同个体三维网格模型间的差异。

该摄影测量法方案具备便携性强、易于操作、快速高效且可重复的优势。相较之下，显微CT扫描耗时久、成本高昂且无法便携部署。本方案可推广应用于诸多其他易受干燥影响物体的三维形态重建，例如贝类贝壳、软骨组织、植物及其他几丁质材料（chitinous materials）。