Asian Diver – March 2018

ALL ABOUT CORALS

Hard and Soft Corals

What Do Corals Need to Survive?

There are two main types of coral:

hard coral and soft coral. As the name

implies, hard corals build a hard

calcium carbonate skeleton which

grows to become the coral reef. Soft

corals are flexible and are held

together by jelly-like mesoglea and

rigid, spiny structures called sclerites.

The main differences between these two

Sunlight: Corals need to grow in
shallow water where sunlight can
reach them. They depend on the
zooxanthellae for oxygen, which
need sunlight to survive.

Clear Water: Clear water is needed
to let sunlight through. Sediment and
plankton can cloud water, decreasing
the amount of sunlight that reaches
the zooxanthellae.

coral types are the number of tentacles

on each polyp, and the skeletal structure

as they grow.

Soft corals come from the subclass

Octocorallia, whose species have

eight tentacles, and it is fairly easy to

count the number of tentacles on most

species. Hard corals come from the

subclass Hexacorallia, whose species

generally have six-fold symmetry. When

you look at the skeleton of hard corals

below the polyp in the corallite, the

skeleton is divided into six sections.

However, hard corals don’t have six

tentacles, but rather, their tentacles

grow in multiples of six such as 12 or 24

tentacles, a number that is much harder

to count than for soft corals.

Hard Base

Stomach

Throat

Mouth

Stomach

Throat

12 or 24 Mouth

Tentacles

REFERENCE: Coral Reef Alliance

8 Tentacles

Hard Coral Soft Coral

Warm Water: Corals require warm

water to survive. Different corals living

in different regions can withstand

various temperature fluctuations.

However, corals generally live in water

temperatures of 20–32°C.

Clean Water: Corals are sensitive to

pollution and sediments. Sediment can

create cloudy water and be deposited

on corals, blocking out the sun and

harming the polyps. Wastewater

discharged into the ocean near the reef

can contain too many nutrients that

cause seaweeds to overgrow the reef.

Saltwater: Corals need salt water

to survive and require a certain

balance in the ratio of salt to water.

This is why corals don’t live in estuaries,

areas where rivers drain fresh water

into the ocean.

20–32°C