Comparative morphophysiology and thermal stress resistance in the Hawaiian endemic octocoral Sarcothelia edmondsoni (Verrill 1928)

Abstract

As coral reefs face escalating threats from climate change and local stressors, identifying mechanisms of stress resistance in coral species is critical. This dissertation examines thermal tolerance in Sarcothelia edmondsoni, a dominant and potentially resilient Hawaiian octocoral with two distinct color morphotypes—blue and brown. Chapter 1 explores photophysiological responses under controlled irradiance conditions, revealing that while blue morphs exhibit higher maximum photosynthetic rates, brown morphs maintain significantly greater productivity under thermal stress, indicating enhanced stress-resistance. Their performance under low irradiance suggests that brown morphs are a low-light-acclimated phenotype, consistent with their prevalence in turbid, lagoon-like habitats. Chapter 2 utilizes ITS2 sequencing and SymPortal to analyze symbiont communities, demonstrating that both morphotype and island significantly influence Symbiodiniaceae assemblages. Although both morphs are dominated by Symbiodinium tridacnidorum (Clade A; ITS2 type A3), blue morphs host more diverse profiles, and no single ITS2 profile is shared across all samples. Notably, strong ecological differentiation in symbiont profiles occurs alongside phylogenetic similarity, suggesting intragenomic variation or ecotypic divergence within a conserved lineage—indicating fine-scale adaptation to local environmental conditions. Chapter 3 investigates symbiont dynamics under thermal stress using histology and fluorescence microscopy. Although both morphotypes lose symbionts, blue morphs show faster declines and minimal recovery. In contrast, brown morphs retain more symbionts, demonstrate redistribution to the gastrovascular lumen, and exhibit partial recovery—indicative of a possible migration-based stress-resistance mechanism. Collectively, these findings highlight that morphotype-specific differences in photobiology, symbiont diversity, and stress response may underpin the success of S. edmondsoni in variable reef environments. The brown morph consistently outperforms under stress, suggesting it may play a central role in future reef trajectories as stony corals decline. This study underscores intra-species variability as a key axis of resilience in coral communities under climate change.