How do Corals reproduce in Easter Island (Rapa Nui)?


Introducing guest contributor Itziar Burgués

Hi, I am Itziar! I am a marine scientist interested in marine resources sustainability and in understanding the contribution of healthy oceans to human well-being, topic in which I will be starting PhD very soon. Nevertheless, before decided to go for a PhD I was involved in other marine conservation projects in different locations and I would like to share my stories with you. I am a multidisciplinary and multifaceted scientist, I worked as a research assistant in Chile, I also take jobs as a scientific observer for fisheries, I also love scuba diving and I have participate in several underwater survey campaigns. I am always curious about the oceans!

Known by its local name of Rapa Nui, Easter Island is one of the most isolated habited islands in the world. Polynesians settled in Easter Island around 980 AD and have always lived in close connection to the sea. Located at about 29 degrees south, over 4000 kilometres from the coast of Chile and 7000 from New Zealand, it is like an oasis in the middle of the vast Pacific Ocean.  It is at the southern limit of the range in which corals can thrive, and its extreme isolation has led to a depauperate marine community with extremely high endemism.

Here, the crystalline and nutrient-poor waters are relatively colder and more variable in temperature than in other tropical areas where corals form reefs. Nevertheless, the corals cover the bottom, dominating the underwater landscape and filling the sea with life in the coastal area. In this environment, the coral species that dominate the seabed of Rapa Nui are Pocillopora verucosa and Porites lobata. Whereas coral reefs around the world are currently in decline, reefs at Easter Island appear to have been increasing in coral cover for the last several decades, moving away from an algal-dominated state.

Coral species  Porites lobate . Credit: Itziar Burgués

Coral species Porites lobate. Credit: Itziar Burgués

Coral species  Pocillopora verrucosa . Credit: Itziar Burgués

Coral species Pocillopora verrucosa. Credit: Itziar Burgués

Corals are animals that belong to the Cnidarians order, related to jellyfish, gorgonians and anemones among others. Millions of small bodies called polyps, which generate a hard skeleton from calcite carbonate, create and gives shape to the coral colonies. The colonies develop by means of the individual growth of the polyps and their duplication by two types of asexual reproduction - budding or fragmentation. The tiny polyps grow little by little, and so does the colony, at about a rate of 25cm per year. With enough time the polyps form colonies up to 5 meters in diameter, such as the Porites lobata colony near the Hanga Roa bay.

These animals can also reproduce sexually by developing male and female gametes (oocytes and sperm, respectively). When an oocyte and sperm join, they form an embryo that grows into a larva (named larva planula), which will give rise to a new individual. This larva will spend a period of time in the water column, being moved by currents, until finding a favourable place to settle, metamorphosise into a polyp, multiply, and develop a new colony. By this mechanism, corals can settle in other places different from their place of origin, mix with other populations, and recolonize areas of previously degraded reef.

Sexual reproduction is of extreme importance for the healthy maintenance of coral populations in isolated places such as Rapa Nui. Large numbers of larvae from other coral populations rarely arrive from other coral communities around the Pacific so sexual reproduction allows coral populations to recover from natural or human disasters, without relying on far away populations. In fact, this happened in the year 2000 in Rapa Nui when after a sea heat wave, more than 80% of some corals suffered bleaching and died, but recovered in just five years - surprisingly fast considering that Rapa Nui is an isolated place and disconnected from other places that can act as a source of individuals.

Little was known about the process of sexual reproduction of the two dominant species - Pocillopora verrucosa and Porites lobata - at Rapa Nui, but recently I was involved in a study on the patterns of sexual reproduction of corals from Rapa Nui with scientist from the Pontificia Univesidad Católica de Chile. Since the moon cycle is one of the main factors that affects sexual reproduction of many reef organisms, samples, we decided to collect samples of Pocillopora verrucosa and Porites lobata every month for two years during the full moon. Back at the lab, the samples were processed and analysed under the microscope to see if the corals had developed oocytes and sperm.

We concluded that, in Rapa Nui, the two dominant coral species reproduce only in the austral summer - between the months of December and April. We also found that each species has its own strategy. While Pocillopora verrucosa is hermaphrodite, which means that the same polyp develops oocytes and sperm at the same time, Porites lobata has colonies that develop female gametes and other colonies that develop male gametes.

Although both species expel the gametes to the water column for external fertilisation, it does not occur at the same time for both species, with Porites lobata acting a month later than Pocillopora verrucosa. Environmental factors around the island, such as water temperature, lunar cycle, wind and tides, can have an effect on the development and timing of the release of gametes into the water, but the precise moment where this happens is still not accurately known. With this work we completed the first step to better understand coral populations in Rapa Nui, and contributes to our understanding of their sexual reproduction.

Sea bottom form Easter Island (Rapa Nui). Credit: Alejandro Pérez-Matus - Subelab (with permission)

Sea bottom form Easter Island (Rapa Nui). Credit: Alejandro Pérez-Matus - Subelab (with permission)

The protection of marine ecosystems begins with knowing about ecosystem functioning and the species that are part of the system. Corals in Rapa Nui are the base of the flow of matter and energy in the coastal ecosystem. They fixate carbon and nitrogen, other animals feed on the corals and use them as refugees and nurseries. In addition, they offer a multitude of ecosystem services to the human inhabitants and visitors of the island, such as providing habitat for fish of commercial interest, protection against storms, and recreational and tourist services.

Thanks to this study, a little more is known about the reproductive strategies of the dominant coral species in Rapa Nui, which affects the maintenance, adaptation and the formation of reefs. Based on this new information, we suggested  that specific protection measures are created and applied, especially in the summer months during the period of coral reproduction. The study also helps us to begin to understand how climate change can affect these populations in the future. Each piece of knowledge brings new light to understanding the functioning of this unique marine ecosystem. However, there are still many more puzzles to solve on the mysterious island of Rapa Nui.

For further information, you can have a look at the open access article: Buck-Wiese H, Burgués I, Medrano A, Navarrete-Fernandez T, Garcia M, Wieters EA Patterns in sexual reproduction of the dominant scleractinian corals at Rapa Nui (Easter Island): Pocillopora verrucosa and Porites lobate Aquatic Biology 27:1-11

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