A shoal of Soldierfish (Myripristis vittata) hangs around in the middle of the water column near the entrance of an underwater cavern system. Caves, caverns, and crevices are the preferred habitat of this fish. This Indo-Pacific soldierfish is mainly found on oceanic islands starting from the east of Africa, such as the Seychelles and Mascareignes in the Indian Ocean, to French Polynesia and the Tuamoto Archipelago in the Pacific Ocean.
These nocturnal feeders of large zooplankton are extremely noisy. The emitted sounds are of such a low frequency, though, that they are inaudible to the human ear. These sounds are used to delimit their territory and to communicate with each other. They are created by the vibration of muscles inserted on the swim bladder, which acts as a soundbox. Although the swim bladder's main function is to regulate the fish's buoyancy, this gas-filled chamber has the ability to produce sound like a drum. This happens through muscles called the "sonic muscles." These muscles, which are the fastest contracting muscles known in vertebrates, contract and expand the swim bladder at a rapid rate. These fast movements create low-frequency drumming sounds.
Many fish species produce sounds, hence their names, such as drums, grunts, croakers, and more. However, not all fishes produce sound through the use of their swim bladder. Some sounds are produced by striking or rubbing adapted bony structures or skeletal components together, similar to how humans gnash their teeth. This way of producing sound is called stridulation. Another way of producing sound is by quickly and suddenly changing speed and direction while swimming. These are the so-called hydrodynamic sounds. Comparable to the cracking of a whip. Hydrodynamic sounds are often produced by wounded or injured fish, and the typical frequency of this sound attracts sharks and other opportunistic predators. In the case of predation by sharks, you could call it the ringing of the dinner bell....
The underwater world is actually a very noisy environment; however, most of it is inaudible to humans.
Filming in the water column is always more difficult than on or near the bottom. With little or no real reference to your actual depth, it is quite difficult to remain hanging motionless at a certain given depth. When you are looking at your underwater camera's screen or peeking through a viewfinder, it is almost impossible to watch your dive computer's depth gauge. Stability is also a big issue, as it is nearly impossible to lean on something to increase your stability. On a sandy bottom, it is easy to lie down and put your underwater camera rig on the substrate.
Sometimes you could stand in an upright position on the bottom. Although far from ideal, this posture will give you slightly better stability than just hanging in the water column. As an underwater videographer or marine wildlife filmmaker, it is extremely important to master your buoyancy to near perfection.
To obtain nice footage in similar situations, you should always film slightly upwards and try to be as neutrally buoyant as possible, avoiding jerky movements. In post-production, you can always stabilise your footage, provided that the camera movements are not too erratic.
This short underwater videoclip has been filmed in Mauritius 🇲🇺
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