Although corals closely represent characteristics of plants and are often mistaken as colorful rocks, they are, in fact, classified as animals.

Corals are classified under the phylum Cnidaria and the class Anthozoa, which makes them relatives of jellyfish and anemones.

Corals are separated into two main categories – hard and soft corals.

Let’s take a deeper look at how hard and soft corals compare.

Visual comparison of hard coral (left) and soft coral (right)


Main Differences Between Hard Coral and Soft Coral

  • Appearance: Hard corals have tentacles in multiples of 6, while soft coral tentacles form in multiples of 8. Hard corals closely represent rocks, while soft corals look like underwater plants.
  • Growth characteristics: Hard corals deposit calcium carbonate skeletons that remain behind after they die, forming the base of coral reefs. Soft corals have internal structural support known as spiracles.
  • Defense Mechanisms: Polyps of hard coral can retreat into their skeletons for protection, while soft corals rely more on chemical defense.

We’ll explore these differences and exciting facts about these two corals below.

What Is Hard Coral?

Hard corals, also known as stony coral or scleractinian coral, produce a skeleton-like structure made up of calcium carbonate.

The coral polyps live within this hard, rock-like skeleton where they can retreat for protection. Each skeleton is made of numerous individual polyps living in a colony that closely represents tiny sea anemones.

Hard corals are known as the building blocks of coral reefs, with their dead skeletons becoming anchoring points for new corals, both hard and soft.

Common characteristics of hard coral include:

  • Hard corals deposit a hard calcium carbonate skeleton (limestone) which is left behind when they die.
  • A symbiotic relationship with zooxanthellae provides the coral energy through the algae’s photosynthesis.
  • The coral has a rigid structure with each polyp containing six tentacles or multiples of six.


Types of Hard Coral

Hard coral is separated into two groups, which include:

  • Large Polyp Stony Coral (LPS)
  • Small Polyp Stony Coral (SPS) 

Although hard corals are separated into these two groups, the main difference between the two is simply the size of the Polyps.

LPS corals, as the name suggests, have larger, more prominent polyps when compared to SPS corals. 

Appearance

Hard corals are made up of a hard ridged calcium carbonate (limestone) skeleton and closely represent the appearance of a colorful rock.

Polyps of hard corals are built of a 6-fold symmetry pattern. In other words, when you look closely, you will notice that hard coral polyps have six testicles or in multiples of 6.

Hard corals can be found in a variety of shapes, including but not limited to branching corals, finger-shaped clumps, table corals, and boulder corals.

Growth Characteristics

Hard coral polyps deposit limestone skeletons in which they live. The polyp periodically lifts and deposits a new layer of limestone, thus lifting it higher and expanding the size of the colony.

When the polyp dies, the internal structure remains, providing an anchoring point for new polyps and expanding the coral colony.

Hard internal limestone skeleton of dead hard coral lying on the beach
Hard internal limestone skeleton of dead hard coral

Stony corals have small polyps, usually only 1 – 3 mm in diameter. This means that they grow at incredibly small rates, often no more than 0.3 – 2 cm a year.

That said, some species of branching coral, such as staghorn coral, can grow up to 10cm a year. This is approximately the same rate of human hair growth.

Defense Mechanisms

Hard corals polyps can retract themselves almost entirely into their hard skeletal structure. This aids as protection against harsh sea conditions and predators.

Additionally, as stony corals are the building blocks of reefs, they create homes for animals such as crabs who defend them against predatory fish, snails, and bacteria.

Additionally, some hard coral uses their tentacles to sting predators such as starfish, which has been recorded to slow down starfish production and ultimately preserve the coral colony.

What is Soft Coral?

Soft corals (Alcyonacea) are different from hard corals in that they don’t produce a hard limestone skeleton, but instead have soft bodies that move with the ocean currents.

Soft corals closely represent plants, but are, just like stony coral, a cluster of tiny individual animals known as polyps.

Common characteristics of soft coral include:

  • Contains small calcareous sclerites in their body instead of a hard external skeleton.
  • It has a soft structure that can move and sway, closely resembling a plant moving with the tide.
  • Soft corals do not always have symbiotic zooxanthellae (particularly deep sea soft coral).
  • Each polp has eight tentacles or multiples of eight.


Appearance

As soft corals don’t deposit hard skeletons, they often appear to be flexible and leathery. Soft corals have 8 tentacles or testicles that appear in multiples of 8.

These testicles appear feathery, which further gives them a “soft” appearance.

Soft corals can be found in a variety of bright colors and shapes, including Fans, whips, moving fingers, and grasses.

Growth Characteristics

Although soft corals do not deposit hard skeletal structures, they are anchored to rocks or reef beds.

Various soft corals with colors blue, orange, and pink, are anchored to a reef
Various soft corals anchored to a reef

Softy corals don’t form reefs. However, they are often found among them.

Although soft corals don’t produce calcium-based skeletons, they do deposit small amounts of limestone within their structure, which are known as spiracles. These spiracles act as a spine or as the stem of a plant.

Soft corals grow slightly faster on average when compared to stony coral and expand by approximately 2 – 4 cm per year (in well-balanced conditions).

Defense Mechanisms

As soft corals don’t have the privilege of retreating into their hard exoskeletons, they have developed a chemical defense to aid off predictors and encroaching stony corals.

Soft corals are territorial and use chemicals not only to defend themselves against predators but to prevent the growth of other species in the immediate vicinity.

Soft corals also have spicules that can deter some predators.

A variety of colorful hard and soft corals
A variety of colorful hard and soft corals


FAQs

Do all hard and soft corals photosynthesize?

Most, but not all, corals have a symbiotic relationship with algae known as zooxanthellae. This plankton photosynthesizes and feeds the coral.
Although most reef-building coral has zooxanthellae, deep water hard and soft coral without light do not.

Where does coral live?

Coral can be found scattered across the globe, but reef-building coral prefers shallow clear water that allows for easy light penetration.
Furthermore, the photosynthesizing algae thrive in temperatures between 70-85°F (22-29°C).

What are the natural predators of hard and soft coral?

Both hard and soft corals share most of the same predators. 
These include, but are not limited to sea snails, parrot fish, sea worms, barnacles, crabs, and sea stars.
All coral reefs are also susceptible to climate change as their energy-producing algae are extremely sensitive.

Do both hard and soft coral bleach?

Coral bleaching has become a common problem and threatens reefs (both hard and soft) around the globe, but it is not the coral that bleaches
Coral is, for the most part, colorless, and it is the zooxanthellae that form their color. As the zooxanthellae are extremely sensitive, when water temperatures change, the algae dies and leaves the white coral exposed.