In this minute of visual meditation, we invite you to absorb the details of hard coral polyps (Hexacorallia). Identifying coral species is rather difficult as corals constantly grow and can take on quite different shapes as they adapt to environmental conditions such as water movement.

Take a Minute to Relax: Hard corals (Hexacorallia)


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We have to admit we don‘t know which coral Yoeri filmed here in Wakatobi. The Indonesian region in the South-East of Sulawesi is almost at the centre of the so-called coral triangle, the area of the world where the diversity and abundance of corals are the highest in the world. We wrote about „The reefs of Wakatobi“ in episode XI and „Coral Reef Protection“ in episode XXV of our series „Take a Minute to Relax“.

Soft and hard corals: Differences and similarities

Diving along those reefs was such a joy, in particular, because of the thriving corals. Total bliss in forms and colours. During our three years as Dive Experience Managers at Wakatobi Dive Resort, we learned an easy way to distinguish between hard and soft corals. While soft corals have eight feathery tentacles (these little arm-like extensions), hard coral polyps have six smooth-looking tentacles or a multiple of six tentacles. Hence, soft corals form the class Octocorallia, whereas hard corals are Hexacorallia.

Both classes of corals use stinging cells, nematocysts, located in their tentacles and outer tissues to capture their food. It is the same kind of stinging cells jellyfish possess.

Nematocysts are capable of delivering powerful, often lethal, toxins, and are essential in capturing prey. A coral’s prey ranges in size from nearly microscopic animals called zooplankton to small fish, depending on the size of the coral polyps. In addition to capturing zooplankton and larger animals with their tentacles, many corals also collect fine organic particles in mucous film and strands, which they then draw into their mouths.

National Ocean Service: What are corals?

Under the cover of darkness or with strong currents, coral polyps extend their arms to feed. The prey is pulled into the polyps‘ mouth in the centre from where it goes to the stomach for digestion.

Hard corals (Hexacorallia)

Stony corals have skeletons of calcium carbonate (CaCO3) which is secreted by the lower portion of the polyp. This way every polyp produces a cup, or calyx, to sit in. The floor of this cup is called the basal plate and the walls around it are called the theca. From time to time, each polyp lifts off its base and secretes a new basal plate above the old one. This way the colony keeps growing outwards.

Even though for us it looks like corals consist of many single polyps, a coral is in fact one big colonial organism. All these polyps are genetically identical. Apart from secreting CaCO3, the colony grows by budding. When a polyp reaches a certain size, it divides. You can observe all the different phases of budding on the reefs.

Living together

Most corals have a unique partnership with zooxanthellae, tiny algae. These algae live within the coral polyps. Out of carbon dioxide, a waste product for the coral, and sunlight the algae create sugar for energy (photosynthesis). This energy goes to the polyp, providing most of its daily food. In return, coral polyps provide a protected home for zooxanthellae.

The brilliant colours of certain types of corals stem from the algae. Multiple environmental factors such as water temperature, sedimentation or nutrients can throw off the balance of this fruitful symbiosis, the algae turn toxic. Eventually, the coral is forced to expel it. As a result, the corals turn white, a phenomenon that is known as coral bleaching.

In addition to being among the most captivating and biologically diverse habitats in the ocean, barrier reefs and atolls are some of the oldest marine habitats of our planet. Coral reefs make up less than 1% of the ocean’s floor, yet are home to more than 25% of all marine life. Even a lot of pelagic species start their life out on coral reefs. The oceans depend on healthy coral reefs for their inhabitants and so do we! Coral reefs provide a lot of different services.

Ecosystem services

To begin with, coral reefs are considered key to finding new medicines for the 21st century. Drugs developed from coral reef animals and plants are tested as cures for cancer, arthritis, human bacterial infections, viruses, and other diseases.

Sustainably managed coral reefs support commercial and subsistence fisheries and provide jobs and income beyond this sector. Tourism and recreation with all the backward and forward linkages to local economies depend on healthy reefs and oceans.

Intact coral reef structures protect the shores against 97% of the energy from waves, storms, and floods. This way corals help prevent loss of life, property damage, and erosion. So, by protecting the reefs, we are in fact protecting ourselves!

From macro shots to gentle giants: Take a Minute features it all

For more visual meditation and marine information, watch the whole playlist on our YouTube channel or browse through the different clips on our designated page „Take a Minute“ on this website.

Screenshot of Take a Minute to Relax XLVII: Hard coral polyps (Hexacorallia). Super macro shot of hard coral polyps filmed in Wakatobi, Sulawesi, Indonesia.
Take a Minute to Relax XLVII: Hard coral polyps (Hexacorallia)

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