What are Coral Reefs?

Coral reefs are well known for their spectacular beauty. These tropical reefs result from massive deposits of calcium carbonate that were formed by billions of tiny individual animals called polyps. Each polyp has inside it a special symbiotic association with a certain type of single-celled algae called Zooxanthellae. These algae can photosynthesize, like all plants, and the coral polyps utilize the algal photosynthetic by-products as an energy source.  This relationship is beneficial to both organisms since the coral polyps provide the algae with a protective environment, and the coral harnesses energy from the algae. Except when under stressful environmental conditions, then the algae can be expelled from the polyps and since much of the color of corals is due to the pigmentation of the algae, the result of this expulsion is referred to as coral “bleaching” because bleached corals appear bright white.

Figura 1. Coral Algae symbiosis, Zooxantellae. Far-Reaching Ecological Systems https://www.behance.net/gallery/4040147/Far-Reacing-Ecological-Systems.

Corals are animals and also carnivores, capturing zooplanktons, suspended particles, and bacteria, and absorbing dissolved organic matter to feed. They can reproduce in two ways: asexually, forming colonies with the same genotype by fragmentation or fission; or sexually by planktonic larvae that disperse, settle and establish new colonies with different genes from their parent colonies.

Imagen 1. Polyps of  Orbicella anularis, the color of corals is due to the pigmentation of the zooxantellae.  Photo: Ximena Lizaola

These marine invertebrates usually grow upward, toward the light, as each polyp deposits new carbonate layers under itself, making the thin layer of tissue on the skeleton the only part alive. Branching corals, in particular, create a variety of microhabitats, contributing to an extraordinarily rich faunal diversity. Organisms like mollusks, fishes, sponges, crustaceans, worm tubes and other types of non-reef-building corals, such as fire corals and soft corals are often associated here. Since the coral reef provides food and shelter for many other organisms, they are called keystone or foundation species, because they are the most dominant element in this ecosystem, similar to the trees in the forest.

In general, corals are regarded as slow-growing, with measured growth rates usually varying from one to ten cm per year, although there are some exceptions. Despite the slow growth of coral reefs, they are unique formations among the oldest of marine communities, with a geological history of more than 500 million years.  And that is not at all! Coral reefs play an important role as natural wave breakers, which minimize impacts from tropical storms and surges. They, in turn, provide environmental services to local communities, like fish production, tourism, and recreation, becoming the central source of livelihood for thousands of people.

Imagen 2. Coral Reefs offers environmental Services and is the support of the  local economy

Unfortunately, these systems are under threat by many natural and anthropogenic factors such as climate change, elevated temperatures, pollution, and unsustainable fishing practices. As global and local threats to coral reefs increase in frequency and severity, it is imperative that government agencies, private sectors, institutions, non-governmental organizations, and the general public work collectively to strategically address common challenges to increase resiliency and reduce threats.

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