Orbicella annularis is an important reef-building coral and an endangered species. Previous studies suggest it is also one of the most intensely grazed corals by parrotfishes in the Caribbean. Yet, the healing capacity of Caribbean corals from parrotfish predation scars remains poorly understood. In a recent study published in Coral Reefs, researchers from California Polytechnic State University monitored the healing of over 400 recent parrotfish predation scars on O. annularis colonies on the islands of St. Croix and Bonaire for up to two months in 2018 and 2019, respectively.
Parrotfishes: herbivores & corallivores
Parrotfishes are abundant herbivores that primarily graze upon algae, thereby mitigating coral-algae competition and indirectly benefiting the recruitment and growth of coral colonies. At a local scale, management efforts to increase populations of parrotfishes are believed to be critically important to maintaining
resilient, coral-dominated reefs. Yet, some parrotfish species also occasionally graze coral – a behavior known as corallivory. Corallivory can cause the localized mortality of coral tissue and may have long-term impacts such as reduced coral growth and fecundity and increased susceptibility to disease. While evidence suggests that parrotfishes likely have an overall net positive impact on coral communities, some researchers have expressed concern that they may have detrimental impacts on heavily predated coral species, such as O. annularis.
To better understand the consequences of corallivory for O. annularis, they evaluated (1) coral healing rates, (2) total coral healing, and (3) developed a model to predict tissue loss from a standing stock of parrotfish predation scars. This is the first study to monitor coral tissue healing rates and recovery thresholds from naturally occurring parrotfish bite scars in the Caribbean.
Coral healing rates
The team evaluated how coral healing rates were influenced by factors such as the initial scar size, number of scars per coral colony, colony size, colony water depth, and island. They found temporal thresholds in O. annularis healing rates, where the majority of scar healing occurs within the first few weeks and scars have minimal healing after approximately 45 days. Additionally, they found that smaller scars have higher initial healing rates than larger scars. However, the number of scars per coral colony, colony size, colony water depth, and island had no observable effect on coral healing rates.
Total coral healingThey compared how these same factors influenced the total amount of coral healing over two months, which their research suggests encompasses the full timespan during which healing occurs. They found that the initial scar size alone was an important predictor of total coral healing and that there were distinct size based thresholds in coral tissue regeneration. They observed that small scars (≤1.25 cm2) often fully heal, while larger scars (≥8.2 cm2) – presumably resulting from repetitive, localized predation by parrotfishes – had minimal tissue regeneration. They used these findings to develop a predictive model of total O. annularis tissue loss based on the initial size of parrotfish predation scars.
Predicted coral tissue loss
They surveyed the size and abundance of recent parrotfish predation scars present on O. annularis colonies at a point in time and used their model to predict coral tissue loss from these scars. They observed that 87% of scars were small (≤1.25 cm2) and their model predicted that these small scars would fully heal or result in minimal tissue loss. In contrast, while only 6% of observed scars were large (≥8.2 cm2), their model predicted that these large scars had minimal healing and would result in 96% of the total tissue loss for predated colonies.
Conclusions & future directions
This study suggests that the immediate negative consequences of parrotfish corallivory for O. annularis appear to be driven primarily by a few exceptionally large bite scars. Interestingly, there was no observable difference in healing rates or total coral healing between islands despite stark contrasts in parrotfish and coral community composition on St. Croix and Bonaire. This suggests that these findings and this predictive model may be broadly applicable to other regions of the Caribbean. This study presents an important advance in understanding the recovery of a heavily predated, ecologically important, and endangered Caribbean coral from parrotfish corallivory. The group’s ongoing work seeks to address how the intensity of parrotfish corallivory is influenced by the community composition of corals and parrotfishes across multiple spatial scales and regions of the Greater Caribbean. Areas for future research include quantifying the indirect consequences of corallivory for coral growth and fecundity and the factors driving selective parrotfish corallivory.
More info in the Dutch Caribbean Biodiversity Database