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71. A colony of Zoanthids


A small colony of zoanthids (the order is also called Zoanthidae, Zoantharia, or Zoanthiniaria) has colonised a piece of hard substrate on a Mauritian reef.

Zoanthids are anemone-like animals that belong to the hexacorallia in the phylum of cnidarians, like corals, jellyfish, hydroids, and other strange-looking stinging animals. Zoanthids can be found as a single solitary animal, so just one polyp or as a colony of several polyps, like in this short underwater video clip. Sometimes the polyps are attached directly to the substrate, usually a hard material like a submerged rock. On the occasions when the polyps are standing on a stolon, a fleshy stem-like structure that may contain pieces of sediment and sand. These stolons are anchored on a solid base like stones or other hard material that might be available underwater.

These Cnidarians rely partially on photosynthesis to provide them with nutrition. Photosynthesis is a process by which living organisms, most plants but also some marine animals like some nudibranchs and zoanthids, use the light provided by the sun to convert it into chemical energy. This energy is stored and used later as a food source. However, photosynthesis alone does not deliver enough nutrients to keep them alive, so they also need to capture plankton with their marginal tentacles.

These tentacles in certain species contain a deadly poison called "palytoxin," which is released from organelles called nematocysts. These release spiral thread-like structures with many little spines and needles that can pierce tissue upon contact. This toxin is a very potent vasoconstrictor that will retain its potency for several days. In large doses, it might be lethal. Some tribes in the Indo-Pacific region learned the effect of palytoxin through experience and use this poison to tip their arrows and spears to increase their effectiveness.

Palytoxin is not unique to zoanthids; other marine animals like the Gaudy Clown Crab (Platypodiella spectabilis), the Pinktail Triggerfish (Melichthys vidua), and the Scrawled Filefish (Aluterus scriptus), among others, can contain the toxin. These animals do not produce this toxin themselves, but they have received it through bioaccumulation. Bioaccumulation is the gradual accumulation of a substance through food.


This clip was filmed on a night dive. In the darkness, your video lights are magnets for all kinds of planktonic life. When your lights are positioned too close to your subject, these swarms of little critters can be a real nuisance and interfere with the quality of your recordings. Therefore, on night dives, you should place your lights as far away from your subject as possible.

If you are lucky enough to possess a red light, filming on a night dive becomes suddenly easier. The red light is not perceived by most marine life and won't attract these irritating swarms of zooplankton. Turn off your video lights and turn on your red light, place yourself in a good and stable position, and when you are ready to film, you can turn on your video lights. It usually takes a little time before the bright beams will start attracting these unwanted critters.

How to use your video lights correctly is explained extensively in our online Marine Wildlife Videography course. The course also dives deeper into how to film on night dives.

Filming location:

This short underwater videoclip has been filmed in Mauritius 🇲🇺

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