Abstract
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 macrophotography 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 with others obtained with micro‐CT scanning to verify their accuracy.The models can be obtained quickly and cheaply compared with micro‐CT scanning and have sufficient precision for quantitative interspecific morphological analyses. Our work shows that underwater photogrammetry is a fast, noninvasive, 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 nonportable. 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.
Highlights
Quantifying the complexity and the variability of biological objects has been a long-standing challenge, yet it is critical to understand the evolution of phenotypic diversity (Haleem & Javaid, 2019; Houle et al, 2010; Pears et al, 2012; Sansoni et al, 2009; Vázquez-Arellano et al, 2016)
Underwater, photogrammetry protocol in order to add these types of biological structures to the panel of photogrammetric possibilities
Our work shows that underwater photogrammetry is a fast, noninvasive, efficient, and accurate way to reconstruct 3D models of dry-sensitive objects while conserving their shape
Summary
Quantifying the complexity and the variability of biological objects has been a long-standing challenge, yet it is critical to understand the evolution of phenotypic diversity (Haleem & Javaid, 2019; Houle et al, 2010; Pears et al, 2012; Sansoni et al, 2009; Vázquez-Arellano et al, 2016). For the imaging of soft tissues that consist largely of water, magnetic resonance imaging (MRI) is an alternative (Digital Fish Library, 2009; Zanette et al, 2013) and is widely used in medicine, physiology, and neurobiology (Rasmussen et al, 2010) This method does not allow the accurate imaging of “dry” objects such as bones and it typically has a longer acquisition time and lower spatial resolution than micro-CT scanning (Ziegler et al, 2010). Cephalopods beaks combine many of the difficulties associated with the imaging of objects sensitive to drying and 3D modeling Their shape is complex, the walls of the structure thin, the texture is smooth and often homogeneous, and these objects can show a gradient from transparent (untanned) to dark brown (heavily tanned) (Miserez et al, 2008). We present a close- range underwater photogrammetry protocol adapted to dry-sensitive objects, validated by a quantitative comparison between these photogrammetric models with ones obtained through micro-CT scanning
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