Bleaching events are becoming more frequent and intense worldwide. Southwestern Atlantic corals are considered stress-tolerant species that suffer less during bleaching episodes. Therefore, we investigated if the reef-building corals Mussismilia hispida and M. harttii are capable of spawning while fully bleached. We compared spermatozoa concentration and motility, egg diameter and embryo viability between seemingly healthy and bleached colonies for both species. Findings show that both species spawned viable gametes. Concentration and motility of newly-released spermatozoa were similar between healthy and bleached colonies for both species. Unlike M. hispida, a 10% reduction in size was found in egg diameter for M. harttii. Embryo viability for bleached M. hispida colonies did not decrease; for bleached M. harttii there was a reduction from 14.7 to 6.3%, but fertilization and larval development still took place. These findings describe the unprecedented episode of corals spawning viable gametes when entirely bleached, supporting the suggestion of greater resilience for Southwestern Atlantic corals.

Zooxanthellate corals live in symbiosis with phototrophic dinoflagellates of the family Symbiodiniaceae, enabling the host coral to dwell in shallow, nutrient-poor marine waters. The South Atlantic Ocean is characterized by low coral diversity with high levels of endemism. However, little is known about coral-dinoflagellate associations in the region. This study examined the diversity of Symbiodiniaceae associated with the scleractinian coral Favia gravida across its distributional range using the ITS-2 marker. This brooding coral endemic to the South Atlantic can be found across a wide range of latitudes and longitudes, including the Mid-Atlantic islands. Even though it occurs primarily in shallower environments, F. gravida is among the few coral species that live in habitats with extreme environmental conditions (high irradiance, temperature, and turbidity) such as very shallow tide pools. In the present study, we show that F. gravida exhibits some degree of flexibility in its symbiotic association with zooxanthellae across its range. F. gravida associates predominantly with Cladocopium C3 (ITS2 type Symbiodinium C3) but also with Symbiodinium A3, Symbiodinium linucheae (ITS2 type A4), Cladocopium C1, Cladocopium C130, and Fugacium F3. Symbiont diversity varied across biogeographic regions (Symbiodinium A3 and S. linucheae were found in the Tropical Eastern Atlantic, Cladocopium C1 in the Mid-Atlantic, and other subtypes in the Southwestern Atlantic) and was affected by local environmental conditions. In addition, Symbiodiniaceae diversity was highest in a southwestern Atlantic oceanic island (Rocas Atoll). Understanding the relationship between corals and their algal symbionts is critical in determining the factors that control the ecological niches of zooxanthellate corals and their symbionts, and identifying host-symbiont pairs that may be more resistant to environmental changes.