Heat-Tolerant Coral Genotype Selection & Outplant Survival

Abstract

Coral restoration that ignores thermal tolerance plants reefs that the next summer kills. We screened 64 wild Acropora cervicornis genotypes from a surviving relict thicket using a standardized acute heat-stress assay (CBASS) and ranked them by the thermal threshold at which photochemical efficiency collapses (ED50 of F_v/F_m). The 18 top-ranked genotypes were propagated in the AERIEL midwater nursery and outplanted alongside an unselected reference set onto a degraded fore-reef. Across the following austral summer the site experienced 6.8 Degree-Heating-Weeks — a Category-2 bleaching event. At twelve months, heat-selected outplants showed 81% survival versus 43% for the unselected reference, a partitioning driven almost entirely by the selected corals' higher bleaching threshold rather than by growth rate. We conclude that ED50 pre-screening is a cheap, high-leverage filter for restoration stock, and we have promoted the assay to standard practice for the program.

Background & Question

The relict thicket designated as the parent source (registry AE-SPX-0004 · Staghorn Coral) is one of the last self-recruiting A. cervicornis stands in its basin: it persisted through two bleaching events that erased the surrounding reef. That persistence implied standing genetic variation in thermal tolerance worth harvesting. The operational question was blunt: does selecting outplant stock on a lab thermal threshold actually buy field survival through a real heatwave, and by how much? If the effect is large, restoration must screen before it plants; if it is marginal, the assay cost is not justified. We pre-registered a survival difference of ≥20 percentage points as the threshold for adopting screening as policy.

Methods

• Source: 64 genotypes sampled from the relict thicket, ≥5 m apart to avoid clonemates; clonality confirmed by 2b-RAD SNP genotyping (no duplicate multi-locus genotypes retained).
• Thermal assay: CBASS acute stress — four parallel tanks at +3, +6, +9 °C above local maximum monthly mean, 18 h hold, dark-recovery, then pulse-amplitude-modulated fluorometry. ED50 = temperature at which F_v/F_m falls to half its control value, fitted per genotype (log-logistic, n=5 fragments/genotype/dose).
• Propagation: Top 18 genotypes (highest ED50) fragmented and grown 6 months on midwater tree structures per AE-SOP-0410 · Rewilding Release Readiness; reference set drawn from 18 unselected genotypes matched for initial size.
• Outplant: 540 fragments (270 selected / 270 reference) epoxied to a degraded fore-reef in a randomized block design, 30 blocks, GPS-tagged photostations.
• Monitoring: AquaLink in-situ thermistor string logged temperature at 10-min cadence; Degree-Heating-Weeks computed against AE-MET-012 · Sea-Surface Temperature Anomaly and AE-MET-003 · Coral Bleaching Index. Quarterly ROV photomosaic; survival scored from imagery by two blinded readers (Cohen's κ = 0.91).
• Statistics: Cox proportional-hazards on time-to-mortality with treatment as fixed effect and block as frailty term; growth from photomosaic planimetry.

Findings

ED50 ranged from 34.6 °C to 37.1 °C across the 64 genotypes — a 2.5 °C spread within a single thicket, confirming exploitable standing variation. Field outcomes tracked the lab ranking. After the 6.8-DHW summer, heat-selected outplants survived at more than 1.8× the rate of the reference, and the survival hazard was cut by 64% (Cox HR 0.36, 95% CI 0.27–0.48, p<0.001). Notably, the selected and reference corals grew at statistically indistinguishable rates — the benefit was survival under heat, not vigor, which lays to rest the common worry that thermal tolerance trades off against growth in this species.

Implications

This is the cheapest large-effect intervention in the restoration toolkit: a one-week assay roughly doubles the odds that an outplant survives the climate it is planted into. We have promoted CBASS pre-screening to a mandatory gate in nursery intake, and the 18 high-ED50 genotypes are now archived as a heat-tolerant broodstock line in the Marine Genebank with cryo-preserved sperm backup. For the mission, resilient reef is not only ecology — a recovering fore-reef restores fish biomass that anchors the legal-fishery argument the League uses to contest industrial encroachment. Restoration that survives is restoration that becomes evidence.

References & Linked Records

• AE-SPX-0004 — Staghorn Coral (Acropora cervicornis), parent relict thicket
• AE-MET-003 — Coral Bleaching Index
• AE-MET-002 — Live Reef Cover
• AE-MET-012 — Sea-Surface Temperature Anomaly
• AE-SOP-0410 — Rewilding Release Readiness
• Marine Compendium — restoration genetics primer
• Related log: RL-2091-022 — Captive-Breeding & Rewilding Survivorship