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113. Hell’s Fire Anemone (Actinodendron arboreum)


On a sandy current-swept slope somewhere in the Bali Sea, a Hell's Fire Anemone (Actinodendron arboreum) is providing shelter for a school of little cardinalfish.

Unfortunately, the exact species of cardinalfish in this underwater videoclip could not be identified, as the number of different cardinalfish species runs into the hundreds. Today, science has described around 370 different species.

These anemones have a very potent sting, which can cause severe skin ulcers, hence their name. They are also known as the Pinnate Anemone, Tree Anemone, and Branching Anemone. However, when disturbed, these cnidarians will retreat into the sand, where they will be virtually invisible.

These very toxic anemones are found in the Indo-Pacific region on areas with sandy or rubble bottoms to a depth of a maximum of 30m/100ft or so. They are anchored with their foot on something hard, often a rock. The anemone also uses its foot, which is made out of muscular fibres, to displace itself. The big head with broccoli-shaped tentacles can grow up to 30cm/12in in diameter. The oral disc is situated in the middle of these tentacles and close to the foot.

Anemones from the genus Actinodendron have no symbiotic relationship with anemonefish (also sometimes called clownfish). However, they do have a symbiotic relationship with a photosynthetic algae known as zooxanthellae, and on a few rare occasions, they may host shrimp. How exactly the little cardinalfish are benefiting from the anemone's presence is not known, as cardinalfish have no protective mucus layer to prevent injury from the anemone's cnidocytes (little stinging cells that explode and penetrate or stick to the skin of the perpetrator).

Although the little cardinalfish hover near, above, and in front of the anemone for the protection it provides, it appears that these juveniles avoid the stinging tentacles.

Juvenile Cardinalfish are known to shelter amongst herds of sea urchins, and they even venture within the maze of defensive spines of these urchins, seeking protection from smaller predatory fish such as jacks and groupers.


As an underwater videographer, I almost never use my GoPro due to its limitations in settings. However, sometimes, on rare occasions, the use of these little cameras can be beneficial, as they can be left behind in a particular area to be retrieved at a later moment. A little underwater video camera on the reef will have less impact on the natural behaviour of fish or other marine critters than an underwater videographer with his/her big camera and housing. To do this, I positioned the action camera on a stand and left it alone for several minutes before I collected it to retrieve the footage.

When filming this scene, I took the direction of the current into account. Fish almost always face the current, and thus, by positioning yourself facing your subject in a +/- 45° angle, you will get the best possible images of a current-swept scene.

Make sure that when you are filming in water deeper than 6m/20ft, you install an orange filter on the action camera or put a light source nearby to avoid your image being too dark or having the wrong hue.

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