A nocturnal Tube Anemone (order Ceriantharia) feasts on the zooplankton attracted by my video lamps.
Tube Anemones are animals belonging to the Cnidarians. They are solitary polyps that live buried in soft sediments such as sand or mud on the ocean floor, hence their alternative name; Burrowing Anemones. These Cnidarians are not true anemones because they lack a pedal disc, also called an adhesive foot, which true anemones use to anchor themselves on hard substrates. Tube Anemones live inside a tube made out of secreted mucus and threads of stinging cells (ptychocysts), in which they can retreat as a means of protection. The tube of some species can be 1m/3ft in length.
Because they prefer soft and muddy bottoms in which they can burrow themselves, they are not found on coral reefs. Although they usually stay anchored in a single spot, they have the ability to squeeze out of their tube and relocate themselves by slowly drifting to a more suitable spot where they can dig themselves in.
These flower-like animals have, in fact, two different kinds of tentacles. The large stinging tentacles of the anemone's crown, situated on the outside of the animal, are there to capture small prey like zooplankton and even very small fish or crustaceans. These tentacles are also used for defence. When prey is caught in the Cnidarian's tentacles, it retracts its arms towards the oral disk where the smaller tentacles are situated. These smaller tentacles are used to manipulate the catch and ensure that the catch finds its way inside the oral disk to their stomach (coelenteron) so that the Tube Anemone can digest its prey. Waste is removed through the same opening as these animals don't possess an anus. The mouth performs both functions of ingesting food and removing waste.
Tube anemones sometimes have beautifully coloured tentacles, and some even have fluorescent properties and can display dazzling greens, purples, and oranges. Identification is often very difficult as some species may exhibit different colours and different sizes.
This anemone was filmed during a night dive. Because you carry the only light source in the darkness of the night, it is important NOT to bring your lights too close to the subject to avoid over-illumination (which will result in over-exposed footage) of the subject, and this will also prevent unwanted critters, mostly zooplankton, attracted by your lights, from swarming in front of the camera. The frantically moving little critters are difficult to chase away, and adding another light a few meters away will only attract more plankton. Trying to wave them away with your hand will not work either. It is best to use as little light as possible. Another option is to use red light; most critters are not attracted to the wavelength of red light. Once you've positioned yourself perfectly, and your camera is ready to roll, you can turn off the red light and use your standard video lights to illuminate the scene. Unfortunately, the small critters will come back within less than 15 seconds or so.
This short underwater videoclip has been filmed in Mauritius 🇲🇺
For another Tube Anemone insight please go to these vlog posts 7: https://www.beyondscuba.com/post/tube-anemone
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