Subject:
A school of Golden Bullseye (Parapriacanthus ransonneti) is swimming rapidly on top of a shallow coral reef patch in a sheltered bay in Mauritius.
Bullseyes, also known as Sweepers (family Pempheridae), are small fish strongly resembling Cardinalfish and native to the Indian Ocean. Sharing looks, habitat, and behaviour with many small cardinalfish, they are easily mistaken for these species. Bullseyes or Sweepers, however, are very different fish from a completely different family (cardinalfish belong to the Apogonidae).
Bullseyes or Sweepers usually spend the daytime under ledges, caverns, and overhangs, only venturing out at nighttime to feed on zooplankton. When juvenile, they form big schooling aggregations.
These little fish exhibit something quite strange: bioluminescence. These fish obtain their glow from their food. By feeding on ostracods, very small crustaceans also known as seed shrimps, these fish accumulate a luciferase enzyme. The proteins of this enzyme are “stolen” from another animal, in this case, from a small crustacean. Proteins obtained from another animal are called “kleptoproteins”. If an aggregation feeds on enough ostracods containing enough luciferase enzyme, they will emit light that is used for countershading.
These fish possess two different light organs: a Y-shaped thoracic light organ and a linear anal light organ. Both are situated on the underside of the fish. This bioluminescence is used as a camouflage tool. The underside of the fish is illuminated in a subtle way to match the luminosity of the water column above the fish, making it very difficult for predators to distinguish their prey when viewed from below. This camouflage technique, where the underside of an animal is light-coloured and the upper side is darker, is called countershading and is applied by many different animals. Some shark species like the Great White Shark (Carcharodon carcharias) and most penguin species have a light almost white-coloured belly and a much darker back. However, countershading by using an active light source within the animal itself is more rare. This form of countershading is called counter-illumination and is a very efficient way of hiding their silhouette. Cephalopods like squids are known to use this very efficient counter-illumination technique. In contradiction to the luciferase enzyme used by the Bullseyes or Sweepers, the counter-illumination in squid is established thanks to a symbiotic relationship between the Cephalopod and bioluminescent bacteria (Aliivibrio fischeri), living in the squid's mantle’s light organ.
Technique:
This footage was really dark and dull in colours. When filming the school of Golden Bullseyes or Pygmy Sweepers, they kept a safe distance from me and my underwater camera, so I had to film through a lot of colour and light-absorbing water. I tried to remove the bluish haze from the submerged rock, but then the blue in the background turned grey. The only thing I could do to give it a bit of colour was boosting the reddish and yellowish hues of the little fish. I also tried to get the viewer’s attention to focus on the school rather than on the background by adding a barely noticeable vignette.
Filming location:
This short underwater videoclip has been filmed in Mauritius 🇲🇺
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