Colony-Level 3D Photogrammetry Reveals That Total Linear Extension and Initial Growth Do Not Scale With Complex Morphological Growth in the Branching Coral, Acropora cervicornis

Abstract

The ability to quantify changes in the structural complexity of reefs and individual coral colonies that build them is vital to understanding, managing, and restoring the function of these ecosystems. However, traditional methods for quantifying coral growth in situ fail to accurately quantify the diversity of morphologies observed both among and within species that contribute to topographical complexity. Three-dimensional (3D) photogrammetry has emerged as a powerful tool for the quantification of reefscape complexity but has yet to be broadly adopted for quantifying the growth and morphology of individual coral colonies. Here we debut a high-throughput method for colony-level 3D photogrammetry and apply this technique to explore the relationship between linear extension and other growth metrics in Acropora cervicornis. We fate-tracked 156 individual coral transplants to test whether initial growth can be used to predict subsequent patterns of growth. We generated photographic series of fragments in a restoration nursery immediately before transplanting to natural reef sites and re-photographed coral at 6 months and 1 year post-transplantation. Photosets were used to build 3D models with Agisoft Metashape, which was automated to run on a high-performance computing system using a custom script to serially process models without the need for additional user input. Coral models were phenotyped in MeshLab to obtain measures of total linear extension (TLE), surface area, volume, and volume of interstitial space (i.e., the space between branches). 3D-model based measures of TLE were highly similar to by-hand measurements made in the field (r = 0.98), demonstrating that this method is compatible with established techniques without additional in water effort. However, we identified an allometric relationship between the change in TLE and the volume of interstitial space, indicating that growth in higher order traits is not necessarily a linear function of growth in branch length. Additionally, relationships among growth measures weakened when comparisons were made across time points, implying that the use of early growth to predict future performance is limited. Taken together, results show that 3D photogrammetry is an information rich method for quantifying colony-level growth and its application can help address contemporary questions in coral biology.

A fast, precise, in‐vivo method for micron‐level 3D models of corals using dental scanners

“…For volume averages, these ranged from 0.65% to 11.25% (overall mean 5.5%, mm range), with small and large CVs at 3.55% and 8.77%. These CVs are on-par or lower compared to SfM methods, which are deemed within suitable ranges of a few mm to cm (SA: 1.5%, V: 2% Ferrari et al, 2017; 14%-15% Lange & Perry, 2020; SA: 3.7% and V: 5.7% Million et al, 2021).…”

Evidence for adaptive morphological plasticity in the Caribbean coral,Acropora cervicornis

“…Specifications for model building and all scripts can be found at https://github.com/wyattmillion/Coral3DPhotogram. 3D models were imported into Meshlab v2020.6 (103) to measure four growth-related traits following protocols described in Million et al (102) and detailed in the Supplementary Methods: TLE, SA, V, and V inter . We assessed the final shape of colonies that survived to T12 with no detectable fragmentation by calculating SA-to-V and TLE-to-V ratios, in addition to packing, convexity, and sphericity (59).…”

“…Ten coral genets maintained long-term (5+ years) at Mote Marine Laboratory's in situ coral nursery (Table S10) were outplanted in a multi-site transplant study under FKMNS permits 2015-163-A1 and 2018-035. In April 2018, 270 coral (mean TLE of 8.4 cm) ramets representing 10 genets (27 ramets per genet) affixed to concrete pucks were photographed following Million et al (102) and manually measured for TLE immediately before transplantation to nine active restoration sites (Table S1, Fig. 1).…”

“…Photographs taken in situ were used to generate individual 3D models of each coral ramet in Metashape 1.5.4 (Agisoft LLC, St. Petersburg, Russia) using a high-throughput pipeline (102). Specifications for model building and all scripts can be found at https://github.com/wyattmillion/Coral3DPhotogram.…”

Structure-from-motion photogrammetry demonstrates variability in coral growth within colonies and across habitats

“…Emerging approaches, using underwater photogrammetry and structure-from-motion (SfM) software to create three-dimensional (3D) models of coral colonies [ 12 , 13 ], are now helping to address many of the above analytical constraints. Recent studies explored colony-scale growth patterns by either measuring extension, surface area or volume repeatedly for the same colony [ 14 , 15 ] or by overlying models from subsequent years and directly quantifying the change in dimensions [ 16 ]. Besides being non-invasive and thus allowing repeated measurements of the same coral colony over many years, photogrammetry-based methods are providing novel ways to quantify a whole range of different colony-scale growth metrics and to analyse intra-colony variability in growth.…”