Subject: Many cave and cavern dwelling fish swim upside down when they venture around the ceiling of their grotto. The higher part of the cavern is made of solid rock with cracks, fractures, gaps, and holes, thus hosting many invertebrate and vertebrate species that can easily hide or attach themselves to the hard substrate. Corals, sponges, bryozoans, tunicates and zoanthids all need a hard basis to grow on and thrive. These strange plant-like animals attract small critters such as shrimps and molluscs and form the base of the food chain in the little biotope these overhangs, caverns and caves are. The bottom however consists mostly of sand and broken off debris. For the denizen fish of caves and caverns and sporadic visitors, it is probably easier to find food or prey on the overgrown ceiling while swimming upside-down. Also bare in mind that fish live in a three-dimensional world and are not affected by gravity in the same way most other living things are. Most fish have a swim bladder, an internal with gas filled organ. This organ controls the fish’s buoyancy and permits the fish to stay at their current water depth without having to expend energy in swimming and not sinking to the bottom. Due to the dorsal position it gives the fish lateral stability. It is also believed that certain species of cave-dwelling fish start swimming upside-down when they move closer than 15cm/6inch to the substrate or ceiling. The rather to us humans strange behaviour of fish swimming upside down in overhangs, caverns, and caves is called Ventral Substrate Response. In this particular clip, filmed on an outer reef in Mauritius, besides the Lionfish (probably Pterois muricata) a cunning hunter of all kinds of little reef fish, there is also a juvenile Soldierfish (probably Myripristis kuntee) occasionally swimming upside-down close to the ceiling of this overhang. Fish swimming upside-down is a common sight under overhangs, in caves and caverns, and inside wrecks and is not restricted to certain fish species. Actually many fish species, including freshwater species, exhibit this rather bizar behaviour.
Technique: When filming under overhangs, it is essential to have adequate light. Video lights are crucial in underwater videography. Video lights must be placed correctly to minimise dark or too bright areas in the footage. Too much light or lights placed too close to the subject might result in overexposed zones. These burn out spots are extremely difficult, if not impossible to correct in post-production. If you have the option to engage the zebra pattern highlight warning detector feature in your camera, you should do so, as it warns the underwater videographer that the highlights in the image are overexposed. Also, adding a histogram on your screen when you film or take underwater pictures is good practice for all underwater videographers and photographers alike as it is an effective indicator of the exposure level in the image. Both these feature, zebra highlight warning and histogram, will help you to control all the exposure settings on your camera.
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