Variability in Fitness Trade-Offs Amongst Coral Juveniles With Mixed Genetic Backgrounds Held in the Wild

Abstract

Novel restoration methods are currently under consideration worldwide to help coral reefs recover or become more resilient to higher temperature stress. Critical field-based information concerning the paradigm of “local is best” is lacking for many methods; information which is essential to determine the risk and feasibility associated with restoration. One method involves breeding corals from different reef regions with expected variation in heat tolerance and moving those offspring to new locations to enhance offspring survival; thereby augmenting local stock to enhance survival for anticipated warming. In this study, surviving colonies from the 2016 to 2017 mass bleaching events on the Great Barrier Reef (GBR) were reproductively crossed and they included colonies sourced from northern (three) and central (two) reefs. The gravid colonies of Acropora tenuis were collected across 6° of latitude, and they were spawned to produce a total of 17 purebred and hybrid crosses. Juvenile corals (3,748 individual colonies settled on 1,474 terracotta tiles) were deployed to Davies reef in the central GBR after 4 months of aquarium rearing. Survival, growth, and coral colour (as a proxy for bleaching) were assessed after 0, 91, and 217 days of field deployment. Overall, a high percentage of juveniles (17% ± 2.5 SE) survived relative to expected survival at the final census. Survival was significantly higher for central purebred crosses, hybrid crosses had intermediate survival while northern purebreds had the lowest survival. Colour and growth rates (0.001−0.006 mm2 day–1) were not significantly different amongst central, northern, or hybrid crosses but were of a reverse pattern compared to survival. On average, northern purebred crosses grew the fastest, followed by hybrid crosses, and then central purebred crosses. Modelled growth trajectories suggest that northern purebreds would take 8 years to grow to reproductive size, hybrids would take nine, and central purebreds would require 12. All deployed juvenile corals paled over time in the field although the colour of A. tenuis juveniles did not differ significantly amongst central, northern, or hybrid crosses. Growth and survival trade-off analysis showed that although most crosses did not outperform the native central juveniles, two of the eight hybrid crosses (SBxLS, DRxCU) demonstrated faster time to reproductive age and increased survival. Overall, reduced time to reach reproductive size and minimal trade-offs in at least two of the eight hybrids suggest that these crosses may accelerate and supplement recovery through natural re-seeding of genes sourced from northern reefs.

Machine Learning for the Fast and Accurate Assessment of Fitness in Coral Early Life History

“…While a global effort to reduce carbon dioxide emissions should be central, large investments are being made to design coral restoration interventions that can be used to mitigate degradation [67]. Some of these interventions include the seeding of enhanced, heat tolerant reef-building coral species using assisted gene-flow methods onto reefs to prepare them for future warming [19,68]. This may include the seeding of hundreds of thousands of individuals, necessitating tools that can rapidly count and assess corals on large scales.…”

“…We present a pipeline for coral juvenile analysis targeted at three key coral fitness traits relevant for ecological and restoration purposes that performs up to 36× faster with minimal differences in accuracy, especially in survival and size. While employing current methods of manual measurements using tools such as ImageJ, analysis of coral juveniles can take many months [19]. This pipeline, therefore, has the potential to accelerate the turnover of results between experiments and scale-up experimental size using a user-friendly, free, and reproducible interface.…”

“…Specifically, there were 1200 tile images with three different time points from the field, and 900 slides with images taken at 12 time points in the laboratory. Terracotta tile images were obtained from a year-long field study conducted at Davies reef on the Great Barrier Reef (site description and map can be found in Quigley et al [19]. Images were taken at three time points using a Nikon D810 camera body and Nikon AF-S 60 mm micro lens (image resolution: 7360 × 4912 pixels) and an Olympus Tough TG-5 camera with Ikelite DS160 strobes (image resolution 4000 × 3000 pixels).…”

“…For example, although this analysis used 2D images, this pipeline is also capable of processing 3D images [34]. This will deliver estimates of the size and color of 3D scanned corals, further improving biological information, throughput, time efficiency, accuracy, and ease of use in the field of current 3D methods [34,[73][74][75] and apply them to early life stages, which are especially challenging for small coral juveniles (see Quigley et al [19]). Understanding coral ontogeny over all life stages is essential, in which multiple methods are needed to understand growth form and structural complexity of corals.…”

Extending the natural adaptive capacity of coral holobionts

“…Unwanted domestication effects such as a growth advantage for corals that do better under aquaria conditions, however, could make it challenging to maintain coral genotypes that 'thrive' under environmental extremes in the wild 34 . Thus, the use of pre-settlement larvae screened for increased thermotolerance for deployment in local reefs and subsequent environmental selection of suitable genotypes might be the most promising approach 103 .…”