The Acropora palmata, commonly known as “elkhorn coral,” is a species of coral that has captured the attention of marine biologists and conservationists due to its crucial role in the structure of Caribbean coral reefs. This coral is not only part of a vibrant and diverse ecosystem but also faces significant challenges due to climate change, pollution, and human intervention.
This article delves into the biology, habitat, reproduction, and conservation efforts of the elkhorn coral (Acropora palmata), as well as its care and cultivation in aquaculture systems, with the aim of providing a comprehensive understanding of this essential species.
Characteristics of Acropora palmata
The elkhorn coral (Acropora palmata) belongs to the group known as stony corals, also called hard corals, due to their ability to secrete a calcium carbonate skeleton (SEMARNAT, 2018).
Taxonomy
- Kingdom: Animal
- Phylum: Cnidaria
- Class: Anthozoa
- Order: Scleractinia
- Family: Acroporidae
- Genus: Acropora
- Species: Acropora palmata (Lamarck, 1816)
- Common name in Spanish: Coral Cuerno de Alce
- Common name in English: Elkhorn Coral
Physical Description
Acropora palmata is easily recognizable by its branching structure, which resembles elk antlers, hence its common name. This coral is one of the fastest-growing species on coral reefs, reaching up to 10-12 cm per year under ideal conditions. A single specimen can grow over 4 meters in diameter and 2 meters in height, while colonies can extend for several meters, creating a critical habitat for various marine species.
Differences and Comparisons with Other Corals
Acropora palmata is often confused with other coral species due to its similar structure. However, it stands out for its rapid growth and its unique ability to form large colonies that are essential to reef structure. Compared to Acropora cervicornis, another Acropora species, A. palmata is more robust and prefers shallower waters, while A. cervicornis is found at greater depths and has thinner branches, making it less resistant to storms.
Habitat and Geographic Distribution of Acropora palmata
Acropora palmata is primarily found in the shallow waters of Caribbean coral reefs, including the Gulf of Mexico, the Bahamas, and the Florida Keys, in a temperature range of 20 to 29°C.
Elkhorn coral prefers shallow waters between 1 and 5 meters deep, where sunlight is abundant, and currents are strong enough to provide a steady flow of nutrients; however, specimens have been reported at depths of 20 meters. This coral is particularly important in the formation of reef crests, where it acts as a natural buffer against waves, protecting coastlines from erosion.
Conservation Status of Elkhorn Coral
Acropora palmata is classified as critically endangered by the IUCN due to its drastic decline over recent decades. Major threats include climate change, which causes coral bleaching, diseases like white band syndrome, and habitat destruction due to coastal development. Several restoration programs are underway, using fragmentation techniques to grow new colonies in underwater nurseries before reintroducing them to natural reefs. Fargo et al., (2023) reported that coral restoration costs vary significantly, ranging from US$10,000 to 50,000,000 per hectare, depending on the restoration methods and locations.
The Future of Acropora palmata and the Importance of Conservation
The future of Acropora palmata largely depends on conservation and restoration efforts. Management strategies include protecting key habitats, regulating fishing and tourism, and ongoing research into the diseases affecting this species. Additionally, conservation aquaculture is a relatively new approach for many reef-building species but shows great potential in promoting species recovery and reinforcing resilience to stressors (Ridlon et al., 2023).
Moreover, public education and participation in restoration programs can play a vital role in preserving this crucial species for marine ecosystems. Along these lines, Banaszak et al., (2023) highlight four priority areas for research and cooperative innovation to enhance the effectiveness and scale of coral breeding in restoration:
- Expanding the number of restoration sites and species,
- Improving the selection of breeders to maximize genetic diversity and adaptive capacity of restored populations,
- Enhancing cultivation conditions to improve the health of juveniles before and after outplanting, and
- Scaling up infrastructure and technologies for large-scale coral breeding and restoration.
Diet and Feeding Mechanisms of Elkhorn Coral
Acropora palmata obtains most of its energy through symbiosis with zooxanthellae, photosynthetic algae that live within its tissues. These algae convert sunlight into energy via photosynthesis, which the coral then uses for growth and reproduction. In addition to this symbiotic relationship, elkhorn coral also captures plankton and other dissolved nutrients from the water through its polyps.
The polyps have tentacles with cnidoblasts that paralyze and capture organisms, then move the immobilized prey to the mouth for digestion in the gastrovascular cavity. Organisms captured by elkhorn coral polyps include small zooplankton.
Reproduction and Life Cycle of Acropora palmata
Acropora palmata can reproduce both sexually and asexually.
Sexual Reproduction of Elkhorn Coral
In sexual reproduction, which occurs once a year during the summer, corals simultaneously release gametes into the water column in an event known as “spawning.” In the genus Acropora, fertilization and larval development are external (SEMARNAT, 2013). This synchronized process increases the chances of fertilization, resulting in planktonic larvae that eventually settle on the substrate to form new colonies.
Chamberland et al., (2015) conducted the first successful transplant and achieved better long-term survival of A. palmata settlers raised from gametes collected in the field, demonstrating that early planting of sexually raised coral settlers may be more cost-effective than the traditional nursery approach for restoration efforts aimed at rehabilitating coral populations. In this regard, Erwin y Szmant (2010) used the neuropeptide Hym-248 to effectively trigger the metamorphosis of elkhorn coral larvae (Acropora palmata).
On the other hand, Schutter et al., (2023) optimized conditions for raising A. palmata recruits in aquaculture facilities. The researchers tested the effects of different nutrient levels and cultivation durations on survival rates. These are the main findings:
- Feeding is essential: Providing the larvae with Artemia nauplii significantly improved survival and growth rates both during and after reef transplantation.
- Increased TA does not benefit survival: While high total alkalinity may be beneficial in some scenarios, it did not improve survival rates in this study.
- Optimal cultivation duration: A 9-week aquaculture period yielded the best results, with recruits showing double survival and size compared to those raised in environmental conditions.
- Surpassing natural recruitment: Even in environmental conditions, aquaculture-raised recruits significantly outperformed natural recruitment, demonstrating the effectiveness of this approach.
Asexual Reproduction of Acropora palmata
Asexual reproduction, on the other hand, occurs through fragmentation, where parts of the colony break off and establish new colonies, contributing to the rapid expansion of this coral.
Coral fragmentation has become the primary method for restoring populations. In this regard, Papke et al., (2021) evaluated the effects of coral genotype (genetic lineage) and substrate type on the growth and survival of elkhorn coral (Acropora palmata) fragments. These were the main conclusions:
- High survival rates: All fragmented corals survived the experimental period, demonstrating the effectiveness of asexual propagation.
- Rapid growth: Corals doubled their initial size in 45 days, with an average growth rate of 545% during the study.
- Genetic importance: The coral’s genetic lineage had a significant impact on growth rates. Some genotypes grew significantly faster than others.
- Substrate influence: While the type of substrate (ceramic vs. cement) also affected growth, its influence was less pronounced than that of genetics.
- Cost-effective solution: Cement substrates, being more economical and readily available, offer a practical and cost-effective alternative to ceramic substrates.
Aquaculture of Acropora palmata
Maintaining Acropora palmata in aquariums is a challenge that should only be undertaken by experienced aquarists. This coral requires specific conditions to thrive, including high-intensity lighting, strong and constant water flow, and well-balanced nutrient levels. It is crucial to replicate its natural environment as closely as possible, which includes maintaining a stable temperature of 25-28°C and adequate calcium levels for the growth of its calcareous skeleton.
Ng et al., (2024) developed a cultivation system that promises to revolutionize the way corals are raised in large tanks, significantly increasing breeding capacity while reducing costs and environmental impact. This novel system addresses these challenges by intelligently utilizing vertical space within large tanks. Here’s how it works:
- Modular coral units: Coral fragments are securely attached to specially designed “coral units,” which are essentially custom-designed building blocks for coral growth.
- Aluminum column system: These units are strategically placed on vertical “columns” made of aluminum rods, creating tiered structures within the tank.
- Centralized rotation: The columns are suspended from horizontal “spokes” radiating from a central rotating platform. This ingenious design allows the entire tank to rotate, providing easy access to corals at all levels for monitoring and maintenance.
- Customizable lighting regimes: Fixtures mounted under the spokes provide targeted lighting, mimicking natural light conditions and promoting optimal coral growth for different species.
Ng et al., (2024) modularized vertical coral cultivation system offers numerous advantages:
- Increased capacity: Compared to traditional methods, this system allows for a staggering 3.7-fold increase in coral fragment yield within a single tank. A 5000-liter tank can house an impressive 1,310 coral units, resulting in a significant boost in coral production.
- Adaptable design: The modular nature of the system allows for easy expansion and customization. Different coral units can be used to accommodate various coral morphologies.
- Energy efficiency: Strategically positioning the corals according to light needs and rotating the tank ensures optimal light distribution, resulting in a 73% reduction in energy consumption compared to conventional methods.
- Cost-benefit ratio: The system prioritizes readily available materials, making it cost-effective to implement in existing aquarium facilities.
- Scalability: The modular design adapts to tanks of various shapes and sizes, allowing large-scale coral production in diverse aquarium setups.
In addition, Lippmann et al., (2023) designed a mathematical programming model to determine the optimal location and size of coral growth facilities, impacted by the survival rate of the resource based on its growth time.
Diseases Affecting Acropora palmata
Diseases affecting corals have become the main threat to restoration efforts. Susmaa et al., (2024) provides a detailed description of coral diseases, including black band disease, white band disease, aspergillosis, dark spot disease, stony coral tissue loss disease, pink line syndrome, yellow band disease, and skeleton-eroding diseases.
In this regard, Young et al. (2023) cite two well-documented diseases of A. palmata: white band type I (WBTi) and acroporid serratiosis, the latter caused by Serratia marcescens, one of the few coral diseases with a known causative pathogen.
Conclusion
Acropora palmata is much more than just a component of the marine landscape; it is a keystone species that sustains biodiversity and the health of Caribbean coral reefs. Protecting this coral is protecting an entire ecosystem that provides countless ecological, economic, and cultural benefits. As environmental challenges intensify, conservation efforts must be doubled to ensure that Acropora palmata and the reefs it supports continue to thrive in the future.
References
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Chamberland, V. F., Vermeij, M. J., Brittsan, M., Carl, M., Schick, M., Snowden, S., Schrier, A., & Petersen, D. (2015). Restoration of critically endangered elkhorn coral (Acropora palmata) populations using larvae reared from wild-caught gametes. Global Ecology and Conservation, 4, 526-537. https://doi.org/10.1016/j.gecco.2015.10.005
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