Written by Jasmine Haskell
Coral reefs are supported, created, and maintained by tiny animals called corals. For most of their life, they are sedentary, forming large colonies of hundreds of polyps that work together to create the incredible architecture of reef environments. However, in the very beginning of their life they are expelled from their parent colony to live as free-swimming larvae for a time. These larvae can survive for a few days to hundreds of days all in search of a new place to call home (Randall et al., 2024).
The process of coral spawning from sexual fertilization to the free-living larval stage and finally settlement is known as larval dispersal (Fig. 1). It is a critical process responsible for the proliferation of coral populations and is linked to coral reef recovery from disturbance events. Connectivity is how researchers explain how many coral larvae are introduced from one reef to another. Quantifying connectivity of reef systems can help describe the resilience of the entire system (Fig. 2).
As coral population declines are becoming more acute, the ability to predict recovery through larval dispersal and connectivity is a focal point for conservation. Researchers have created high level models that attempt to justify complex hydrodynamic processes to support management decisions and conservation practices. These models are used in the creation of Marine Protected Areas, coral restoration site selection, and to quantify the resilience of entire reef systems (Hock et al., 2017; Balbar & Metaxas, 2019; Faryuni et al., 2024). However, recent research has called into question the accuracy of these models.
Antoine Saint-Amand, from the Earth and Life Institute in Belgium, has conducted an analysis on larval dispersal models by comparing the size of the meshes, or scales, that are commonly used within larval dispersal models (Fig. 3). He calculated the Mean Absolute Error (MAE) and bias of different variables (e.g., current speed and direction) against ground-truthed data to see how realistic the simulated variables were.
The study found that almost all mesh sizes used are too coarse to accurately represent water movement in reef environments. This caused an overestimation or underestimation of current speeds dependent on the location. Why does this matter? Well, most models designate coral larvae as passive particles, meaning they go where the water flows, if current speeds are being over or underestimated, then where these little larvae end up will most likely be vastly different from the real location.
Despite this discrepancy, it is important that natural processes for coral reefs are incorporated into conservation practices. This has been standard for restoration planning of terrestrial environments, such as rainforests. In the marine environment fish spawning aggregations of key economic species are routinely included in the decision making of MPA’s. Less precise information is better than no information and provides a foundation for current management strategies. The study continues to highlight the importance of working at a scale that is relevant to the focus species and something future coral larvae dispersal models should address.
References
Balbar AC, and Metaxas A (2019) The current application of ecological connectivity in the design of marine protected areas. Global ecology and conservation 17:e00569
Faryuni ID, Saint-Amand A, Dobbelaere T, Umar W, Jompa J, Moore AM, and Hanert E (2024) Assessing coral reef conservation planning in Wakatobi National Park (Indonesia) from larval connectivity networks. Coral Reefs 43:19-33
Hock K, Wolff NH, Ortiz JC, Condie SA, Anthony KR, Blackwell PG, and Mumby PJ (2017) Connectivity and systemic resilience of the Great Barrier Reef. PLoS biology 15:e2003355
Randall CJ, Giuliano C, Stephenson B, Whitman TN, Page CA, Treml EA, Logan M, and Negri AP (2024) Larval precompetency and settlement behaviour in 25 Indo-Pacific coral species. Communications Biology 7:142
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