Caulerpa taxifolia, a sea alga that gets its name from its leaf-looking fronds, has been given the unfortunate label of “Killer algae” due to its invasive tendencies.

Spreading and taking over masses of coral reefs, this plant has become a concern for marine biologists around the world, except for those in the Carrabiean sea and the Indian Ocean, where the algae grow well in a balanced ecological manner.

Let’s take a look at exactly what killer algae is, what it looks like, where to find it, and how it makes its way around the world

Featured image credit: Richard Ling – Flickr

Caulerpa Taxifolia Categorization

Kingdom: Plantae
Division: Chlorophyta
Class: Ulvophyceae
Order: Bryopsidales
Family: Caulerpaceae
Genus: Caulerpa
Species: Caulerpa taxifoli


Killer algae closely represent a land plant and its leaves. Regarding comparing the two, killer algae is most like a fern.

This ocean fern is anchored by rhizoids and grows as a runner with 3 – 10cm high branches stemming from its center.

The branches of the Caulerpa taxifolia form flattened, feather-like, light green leaves.

Bright Killer algae spreading along sediment on the ocean floor
Killer algae growing in sediment on the ocean floor

Credit: Richard Ling – Flickr

When found in its invasive form (more on this later), the algae can grow much larger and spread incredibly quickly.

In shallow water, the fronds average 3 – 15 cm high, and 40 – 60cm in deeper waters. However, Killer algae have been recorded to grow up to 2.8m high in some situations.

Habitat and Distribution

Populating most of the world’s oceans, which has become a threat to the ecological balance in many cases, this algae can be found in two strains.

A non-invasive species found in tropical and subtropical waters of the following:

  • Caribbean
  • Gulf of Guinea
  • Red Sea
  • East African coast
  • Maldives, Seychelles
  • Northern Indian Ocean
  • Southern China Sea
  • Japan
  • Hawaii
  • Fiji
  • New Caledonia 
  • Tropical/sub-tropical Australia

A larger, faster-growing, and more resilient species is native to the Mediterranean and favors colder water, where it thrives due to a lack of predators and key chemical defenses.


Killer algae are monoecious, meaning that there are both male and female sexual organs available in an individual. In other words, killer algae are hermaphrodites.

There remains to be an ongoing discussion as to whether Caulerpa taxifolia is 100% asexual or if they are also sexual in nature and require another individual of the opposite species.

It is clear that the invasive species of Killer algae can reproduce asexually and through fragmentation. However, an in-depth study of the sexual nature of Australian native Caulerpa taxifolia has shown signs of sexual reproduction

Killer algae rooting from broken-off fragments
Killer algae rooting from broken-off fragments

Credit Tony Rodd – Flickr

Threats and Predators

Killer algae are well maintained within their natural environment and are fed on by a variety of mollusks, fish, and sea snails.

That said, the Mediterranean strain is vastly different and has no natural predators, which further adds to the impact of this plant’s invasion.

Some of Killer algae’s natural predators include fish from the Acanthuridae family ( surgeonfish, tangs, and unicornfish), specifically the tang species.

These fish are a primary population control of the algae and can survive nearly entirely on the plant, thanks to special adaptations that allow them to metabolize the toxins.

Another natural predator of Killer algae is the Elysia subornata which is a sea slug that feeds only on various species of Caulerpa.

In the Mediterranean, Killer algae grow significantly faster and produce more toxins than their native cousin.

This leaves very few predators to keep the invasive underwater ferns under control.

In cooler waters such as the Mediterranean, two mollusks have been found on killer algae, feeding on them at extremely slow rates.

 These include:

  • Oxynoe olivacea
  • Lobiger serradifalci

Although these sea snails have been documented feeding on the algae, due to the plant’s high toxicity and explosive expansion, the slugs do little in the means of population control.

Defenses and Adaptations

Caulerpa taxifolia, whether it’s the native species or the invasive Mediterranean strain, produces a chemical that is extremely toxic to many potential predators, which makes the algae unappealing to most herbivores.

Although this is true for both plant strains, the invasive Mediterranean strain is far more toxic and spreads much faster.

This has prevented any new predatory species from feeding on the algae, thus giving rise to an overpopulation problem.

Another means of defense is the plant’s ability to grow from minute pieces of detached plants.

This means that even when being fed on, if the smallest amount of algae is broken off and left behind, an entirely new area of Caulerpa taxifolia can grow.

This helps the plant survive, as well as spread long distances thanks to the aid of ocean currents.

Invasive Tendencies

The tropical and natural strain of Caulerpa taxifolia has no invasive tendencies and is well-maintained within its natural environment.

In the Mediterranean, a hybrid strain has quickly spread and given the algae its daunting name of “Killer Algae.”

Due to its high toxicity levels, rapid growth, reproduction by fragmentation, and lack of natural predators, the algae cover large areas of the sea bed, blocking off light from other plants and suffocating marine life.

Killer Alge spreads quickly and uncontrollably
Killer Alge spreads quickly and uncontrollably

Credit: Coughdrop12 – wikimediacommons

How Did Killer Algae Make it To the Mediterranean?

Although it is unconfirmed and not surprisingly denied by all parties involved, it’s believed that the hybrid strain that has become to be known as killer algae was accidentally developed in a saltwater aquarium in Stuttgart, Germany.

Wilhelmina Zoo was trying to identify a saltwater seaweed that could be cultivated for commercial use that was resilient yet aesthetically appealing when placed in a tank.

It’s believed that due to tank chemicals, high exposure to UV light, and other variating stress factors over multiple years, a more resilient strain was unknowingly created.

Samples were sent to various institutions, including the Oceanographic Museum in Monaco, where it is believed (but denied) to have been leaked into the Mediterranean.

Invasive Control

Experts believe that the algae are too well established in the Mediterranean to be eradicated. However, some methods have been put into place in order to curb the population.

Some of these methods include:

  • Covering areas of algae to prevent sunlight
  • The use of copper sulfate
  • Manually removing the plant
  • Divers have used vacuum devices to remove plant material

Although the plant can be removed manually, any piece that becomes dislodged can potentially cause another outbreak. 

This has made removing the algae an extremely difficult task.

Furthermore, natural predators have not been introduced to the environment as there is not enough research on their potential ecological effects.


How did Caulerpa Taxifolia get its name?

Caulerpa taxifolia derives its name from its leaf life fronds that closely represent those of a fern, hence the “-folia” aspect of the name.

Why is it called ‘Killer Algae?’

The plant is known as killer algae because it tends to spread rapidly and form large blankets across rock, reef, sand, and sediment.

This coverage kills off natural vegetation and drives off other sea life as the plant is toxic and unfavorable for feeding. This can often leave grazing invertebrates to starve.

How fast does Killer Algae grow?

The invasive strain on Killer Algae can grow rapidly in favorable conditions, often up to an inch per day.

Is Killer Algae toxic to humans?

Although killer algae may be lethal to some fish species, the toxins have no negative effect on humans.

How does Killer Algae spread?

As killer algae reproduce via fragmentation, the spread of the plant is extremely easy. 

This can be done over large distances by water discharge from boats and disturbances from anchors, divers, and swimmers. Fragments are then carried off by ocean currents.

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