Hard corals vs soft corals

Corals are marine invertebrates that typically live in compact colonies of identical individual polyps. There are over 70,000 species of corals in the world, and the vast majority of them can be found in tropical marine waters.

Corals are largely categorised into two groups: hard and soft.

Hard Corals: the reef architects

Also called stony corals or reef-building corals, hard corals have hard, calcium-based skeletons which form the bulk of a coral reef’s structure.

Birds Nest Coral
Birds Nest Corals are one of the fastest growing hard corals. Image credit: Boon Ping, aquarist at S.E.A. Aquarium

These calcium-based structures are secreted by the polyps, each measuring only a few millimeters wide and several centimeters long. Surrounding the central mouth opening of each polyp is a ring of six smooth tentacles.

Most hard corals are photosynthetic, with symbiotic algae living within them. Called Zooxanthellae, these colourful marine algae provide the corals with pigmentation and food through photosynthesis. Hence, hard corals are usually found at shallower depths where they can gain access to plenty of sunlight – the algae’s main food source.

Brain Coral
Close-up of the brain coral, a common type of hard coral. Image credit: Boon Ping

When hard corals eventually die off, their skeletons become anchors for other corals, both hard and soft ones.

Soft Corals: the unsuspecting predators

The chief difference between soft corals and hard corals lie in their structural makeup. Instead of a calcium-based skeleton, soft corals contain minute, spiny skeletal elements called sclerites that provide some degree of support and give their flesh a spiky, grainy texture that deters predators.

Additionally, most soft corals are non photosynthetic. Instead of Zooxanthellae, they have eight fuzzy tentacles for capturing food (plankton) themselves.

Extreme close-up of the Rumphella Coral shows eight fuzzy tentacles on each polyp. Image credit: Boon Ping

While it is usually easy to differentiate between hard and soft corals based on their physical structure, it can be tricky at times. Take for example this Goniopora Coral:

Goniopora Coral

Its soft, swaying body can easily convince anyone that it is a soft coral. But look what happens when it goes into ‘protective mode’:

Goniopora Coral
Its tentacles retract upon contact with ‘potential predator’ before transforming into a rock-like structure – the Goniopora Coral is a hard coral.

Caring for corals

Hard corals are among the hardest organisms to keep under human care. In order to thrive, they require long hours of powerful lighting, precise water parameters as well as supplemental minerals.

“We check water parameters at least three times a week and maintain adequate levels of minerals in the water using equipment like calcium reactors,” explained Curator Jason Lim who oversees coral husbandry at S.E.A. Aquarium.

Hard Coral Habitat
Jason checking on corals in the back-of-house-area of the Hard Coral Habitat. From coral selection to design aesthetics, he was the key person who developed this habitat from scratch.

Most importantly, our team constantly monitors the corals to ensure their well-being. In fact our corals are reproducing very well in the various habitats. A sure sign that they are healthy.

Our Hard Coral Habitat is so well designed and maintained, it was listed as one of the six most spectacular reef tanks in the world by reefbuilders.com. It was even described as a masterpiece and “firmly leads the pack, especially in the public aquarium space”.

Hard Coral Habitat
Hard Coral Habitat at S.E.A. Aquarium. Image credit: Boon Ping

The next time when you’re at S.E.A. Aquarium, do take some time to take in the spectacular beauty of our coral habitats.

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