Larval Enhancement: An Approach to Replenish Coral Reefs

Written by Charlotte Jensen    

Edited by Sandra Schleier Hernández

Figure 1. A. loripes at 8 months post-settlement a) on natural substrate and b) on tile. 35-month-old A. loripes c) on natural substrate and d) on tile. doi:10.1371/journal.pone.0242847

Coral Decline: The Cause and Why it Matters

            Over the past half-century, scientists have observed a decline in coral populations that could reach 60% by 2030 (Hughes et al. 2003). This decline is due to multiple reasons, including global warming, pollution, and overfishing. Due to the importance of coral reefs, it is critical to look for new management strategies to reduce their loss. A new study examines the viability of introducing lab grown larvae to areas where reefs are degraded and isolated to boost their population.

Management Strategy

            Due to the diversity of corals, scientists need to perform trials for specific coral species to determine if introduced larva is an applicable solution. Luckily, scientists have already begun to perform these trials on different species of coral. Dela Cruz and Harrison (2020), brought lab grown larvae to the Bolinao Anda reef complex in the northwest Philippines to try and replenish the coral species: Acropora loripes or Bluetip coral.

Experiment Methods

            From two nearby reefs, Cruz and Harrison took 25 gravid (pregnant) colonies of Bluetip coral to the Bolinao Marine Laboratory to spawn (reproduce). After the colonies spawned in the lab, the larvae were collected and then transported to the destination reef. They designated one section with three plots to place the lab larvae, and another section where they didn’t add any of their larva to serve as a control. Each plot had ten settlement tiles made of dead Bluetip coral so they could visually monitor the settlement better. They released the larvae at the designated section and a fine mesh enclosed the area for five days so the larva could settle. After the initial five days the mesh was removed and there was periodic monitoring for the next 35 months.

Experiment Results

            Before the experiment, both the designated plots and control plots were in poor condition. Live coral cover, which is the proportion of the reef that is covered with coral, was very low at 12%. Sixty-seven percent was dead coral and algae. After the five-day period, there was an average of 278 settlements of larvae on each of the ten tiles in the designation plots, and zero settlements on the control plots. After the 35-month period, 47 adult colonies of Bluetip coralsurvived on the settlement tiles, along with colonies found on the natural parts of the reef.  

Figure 2. a) Sample dead coral tile deployed in larval-enhanced and control plots used to determine initial larval settlement. b) The mesh matting placed onto the larval enhancement plot. doi:10.1371/journal.pone.0242847
Figure 3. Mean volume of juvenile A. loripes on the plot tiles and natural reef substrata from 8 months post settlement to 35 months. * no data collected doi:10.1371/journal.pone.0242847

Larval Enhancement: Effectiveness and Future Use

            Formerly, projects have been conducted by transplanting juvenile colonies or broken branches of corals onto reefs in order to immediately increase coral cover. However, releasing coral larvae onto reefs increases genetic diversity, as well as reduces the stress of transplantation. In this experiment, ~900,000 larvae were released onto the reef, and 47 colonies were produced. Looking at the number of larvae that settled on the tiles, this illustrates that far less than 50% of the larvae were able to successfully settle during the five-day period. Based on these numbers and previous research, it is likely that this experiment was not cost effective because so few colonies were produced. Despite this, valuable information was still obtained from this experiment because the data on the growth rates and survivability of Bluetip coral is the first collected on this species. Though the experiment did not result in a large increase of Bluetip coral, colonies were still added to this reef site through larval enhancement, as no larvae from the natural population of Bluetip coral were found on the control plots. This experiment also shows that post-settlement mortality is an important issue that needs to be further addressed in order to ensure larva enhancement as a viable and effective strategy. Possible reasons for post settlement mortality include predation and other interactions with reef organisms, but pinpointing which interaction is most detrimental could help the success of these enhancement projects.

Conclusion

Though this experiment was not cost effective, it provided valuable information that future researchers can utilize to make these experiments more efficient and effective. This study also brings forth a new method to coral restoration that could greatly improve the current restoration movement that mostly consists of transplantation. It is also significant to note that due to the diversity of corals the success of this method will vary from species to species, so it is important to have continued research to understand these differences. Coral reefs are incredibly important ecosystems because they provide habitat for a diverse variety of marine animals, along with a large biomass of marine animals, which are critical sources of protein for small island nations and are utilized globally. Coral reefs also produce a large amount of revenue from tourism. As human actions play a significant role in their decline, and as we greatly benefit from them, it is only right that we learn and work to rebuild coral populations.

References

Dela Cruz, D. W., & Harrison, P. L. (2020). Enhancing coral recruitment through assisted mass settlement of cultured coral larvae. PLOS ONE, 15(11). doi:10.1371/journal.pone.0242847

Hughes, T. P. (2003). Climate Change, Human Impacts, and the Resilience of Coral Reefs. Science, 301(5635), 929–933. https://doi.org/10.1126/science.1085046

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