Watercress Alga (Halimeda opuntia): All Interesting Facts

Watercress Alga Featured Image

Watercress Alga, also known as Halimeda opuntia, is a type of green seaweed that is found in oceans all around the world.

It belongs to the order Bryopsidales and is native to reefs in the Indo-Pacific region, the Atlantic Ocean, and the Mediterranean Sea.

This seaweed is unique because it’s made up of calcified, leaf-like segments that form thick clumps.

What Does Watercress Alga Look Like?

Shape and Size

Watercress Alga has a distinct appearance with its flat, kidney- or fan-shaped segments that can grow up to 0.3 in (8 mm) high and 0.4 in (10 mm) broad.

The segments have a central rib and a smooth, sinuous, or lobe-shaped upper margin.

Watercress Alga can grow up to 10 in (25 cm) high and form dense mats of herbage.

Growth and Attachment

Rhizoids, which are root-like structures, grow where the segments of the Watercress Alga touch the substrate, or the surface it grows on.

This helps these plants to attach themselves to rocks and coral in their environment.

The individual plants are often crammed closely together, making it difficult to tell them apart.

Where Does Watercress Alga Live?

Watercress Alga is found in various parts of the world, including the Indo-Pacific Ocean, the Atlantic Ocean, the Mediterranean Sea, the Caribbean Sea, and around the coasts of Florida and the Bahamas.

It prefers to grow in grooves, depressions, and cracks in rocks and between coral heads.

This seaweed is found in moderately protected parts of the reef, at depths down to about 180 ft (55 m).

How Does Watercress Alga Affect Its Surroundings?

Impact on Coral Reefs

When coral reefs become degraded and have an abundance of macroalgae, research has shown that coral larvae will settle on the seaweed just as much as they will on the rubble substrate.

This means that the coral larvae are attracted to the Watercress Alga and will settle on it, even though it may not be the best environment for them to grow and survive long-term.

In a study, researchers found that the larvae of Favia fragum, a type of coral, readily settled on Watercress Alga.

However, since this plant is ephemeral and doesn’t last long, it’s not suitable for the long-term survival of the coral.

This can have significant consequences for the recruitment of corals on degraded reefs, as the coral larvae may not find a suitable environment to grow and thrive.

Secondary Metabolites

Watercress Alga produces secondary metabolites, which are substances that can have various effects on its surroundings.

Some of these metabolites include growth regulators like auxins, gibberellins, and cytokinins. These substances can help this plant grow and develop.

Other metabolites have antibacterial and antifungal properties, which can help protect this plant from harmful microorganisms.

However, these substances are not currently being harvested for commercial use.

What Are the Physical Characteristics of Watercress Alga?

Thallus Structure

The thallus, or body of the Watercress Alga, forms loose clumps or extensive colonies that can measure 15.7 to 19.7 in (40 to 50 cm) in diameter and up to 10 in (25 cm) high.

The color of this plant can range from dark green to whitish yellow-green. The branching of the plant is irregular, occurring in all planes or at right angles to each other.

Segment Details

The segments of Watercress Alga are heavily calcified, flat to contorted, ribbed, and transversely oval, ear-shaped, or trilobate.

They measure 0.2 to 0.7 in (6 to 17 mm) wide, 0.2 to 0.4 in (5 to 10 mm) long, and 0.01-0.02 in (0.3-0.5 mm) thick. The joints between the segments are flexible.


Utricles are small, bladder-like structures found in the Watercress Alga. The outermost utricles are polygonal in surface view and can be angular or slightly rounded.

In the longitudinal section of the segment, the cortex is composed of 3 to 4 layers of utricles.

Attachment and Growing Conditions

Watercress Alga attaches itself to surfaces using a fibrous holdfast, which can measure up to 1.2 in (3 cm) in diameter, and secondary rhizoids that develop from any segment touching the substrate.

This seaweed is found on dead corals covered with sand, as well as in shallow lagoons and intertidal to subtidal areas.

You can check out what this plant looks like over here.

Halimeda opuntia Characteristics

Care Level:Easy
Growth Rate:Moderate
Nutrient Uptake:Fair
Required Lighting:Moderate High
Required Water Flow:Moderate
Is Palatable?No
Maximum Size:10 in (25.4 cm)

Scientific Classification

Scientific Name:Halimeda opuntia
Also Known As:Watercress Alga, Prickly Pear
Conservation Status:Unknown

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