Low flow enhances pigmentation and lipid reserves but reduces survival in newly settled Acropora digitifera corals

Technical Summary: Flow-Dependent Trade-offs in Acropora digitifera Spat Performance

Problem Statement
Water flow is a critical driver of coral community structure and adult performance, yet its specific effects on newly settled corals (spat) remain poorly understood. The post-settlement period represents a major demographic bottleneck characterized by high mortality, driven by biotic pressures and abiotic stressors. While flow is known to influence nutrient uptake, calcification, and symbiont acquisition in adults, spat differ significantly in morphology, size, and boundary-layer dynamics. Consequently, flow interactions in early life stages cannot be inferred from adult data. There is a critical gap in understanding how flow regimes simultaneously influence survival, symbiont acquisition (pigmentation), and energy reserves (lipids) during the energetically demanding transition from settlement to juvenile stages.

Methodology
The study utilized Acropora digitifera spat reared in a controlled laboratory environment at the Australian Institute of Marine Science (AIMS) National Sea Simulator.

• Experimental Design: Spat were reared under three distinct flow regimes: low (0.01 m s⁻¹), moderate (0.05 m s⁻¹), and high (0.15 m s⁻¹). These velocities were selected to span ecologically relevant conditions found in recruitment habitats and reef-lagoon systems.
• Duration: The experiment ran for 83 days following settlement.
• Symbiont Provisioning: To standardize symbiont acquisition, a continuous supply of Cladocopium goreaui cells was provided for the first six days post-settlement at a target concentration of ~150 cells ml⁻¹ across all treatments.
• Feeding: Spat received daily heterotrophic feeding (mixed algae and rotifers) via a dosing system calibrated to maintain comparable prey concentrations despite varying water throughput in different flow regimes.
• Response Variables:
  • Survival: Assessed via weekly high-resolution imaging.
  • Pigmentation: Scored on a four-point ordinal scale as a proxy for symbiont acquisition and density.
  • Lipids: Total lipid content and lipid-class composition (storage, structural, and mobilization lipids) were measured at six time points (days 0, 8, 19, 28, 56, and 83) using destructive sampling and thin-layer chromatography-flame ionization detection (TLC-FID).
• Statistical Analysis: Survival was analyzed using Kaplan–Meier curves and discrete-time hazard models. Pigmentation was analyzed using cumulative link mixed models (CLMM). Lipid data were analyzed using linear models and PERMANOVA.

Key Results
The study revealed that flow effects on spat performance are response-specific and time-dependent, often showing trade-offs between different physiological metrics.

1. Survival: Survival trajectories differed significantly among flow regimes. Early mortality (days 0–64) was highest under low flow, with mortality risks 6.1-fold higher than moderate flow and 21.7-fold higher than high flow. By the final census (day 83), cumulative survival was lowest under low flow (0.47), intermediate under high flow (0.56), and highest under moderate flow (0.65). Notably, the mortality risk pattern reversed in the late stage (days 65–83), where low flow exhibited the lowest mortality risk, while high flow showed the highest.
2. Pigmentation (Symbiont Acquisition): Contrary to survival trends, spat under low flow developed the highest pigmentation scores, indicating faster or more extensive symbiont acquisition. Pigmentation scores were consistently highest in low flow, intermediate in moderate flow, and lowest in high flow throughout the experiment.
3. Lipid Reserves: Total lipid content per spat was significantly higher in the low-flow regime compared to moderate and high flow regimes. However, lipid content did not vary significantly over time within treatments. Lipid-class composition shifted over time (specifically a decline in storage lipids like TAG and WE/SE), but these compositional shifts were not significantly influenced by flow regime.

Key Contributions

• Decoupling of Performance Metrics: The study demonstrates that hydrodynamic conditions favoring one aspect of spat performance (e.g., symbiont acquisition and lipid accumulation) do not necessarily maximize others (e.g., early survival).
• Time-Dependent Mortality: The research highlights that the impact of flow on mortality is not static; low flow imposes severe early mortality risks despite producing survivors with higher energy reserves and pigmentation.
• Limitations of Single Metrics: The findings suggest that relying on a single performance metric (such as survival or lipid content alone) provides an incomplete picture of spat health and recruitment potential.

Significance and Claims
The authors claim that their findings challenge the assumption that a single "optimal" flow regime exists for all aspects of early coral development. Instead, flow shapes spat performance through multiple, partly decoupled responses.

• Mechanistic Insight: The results suggest a potential trade-off where low flow may facilitate symbiont uptake and lipid retention (possibly due to increased residence time of particles) but creates unfavorable boundary-layer conditions (e.g., restricted gas exchange, waste accumulation, or microbial fouling) that drive early mortality.
• Implications for Restoration: For coral aquaculture and reef restoration, the study argues that managing flow solely to improve settlement or growth may not maximize recruitment success if it compromises early survival. The authors emphasize the need to assess multiple performance metrics across longer timeframes and broader experimental contexts to understand the complex trade-offs in early post-settlement bottlenecks. They caution against generalizing these results across all coral species without further validation, noting that the observed patterns are specific to A. digitifera and the experimental conditions used.