In this minute of relaxation, we would like to mesmerize you with the incredible collective performance of these small, but very interesting reef fish: Golden Sweeper or Parapriacanthus ransonneti. At first glance, Parapriacanthus ransonneti is just an unremarkable small reef fish that goes by many different names, depending on the region one encounters them. Yellow or Golden Sweeper, Pygmy sweeper, Golden or Yellow Bullseye, Rosy Sweep(er), and Golden Glassfish, just to name a few. Several species of Parapriacanthus are often huddled under the same banner of “Glassfish”, as these fish are partially transparent. However, since Parapriacanthus ransonneti rolls so nicely of the tongue, and in order to prevent any unnecessary confusion, we shall stick to this creature’s scientific name.

Performance of Golden Sweeper (Parapriacanthus ransonneti)

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Parapriacanthus ransonneti are reef fish that inhabit shallow coastal and offshore reefs between 3-30 metres of depth, throughout the West Indo-Pacific region. Although they can reach a maximum size of 10 centimetres, most of the specimens encountered are smaller than that. They have a rather compressed, silvery pink translucent body with a greenish golden head and large eyes, showing a faint dark horizontal streak starting from the upper part of their gill cover to below the rear end of their dorsal fin.

During day and night

During the day they can form large aggregations in reef cracks, caves, overhangs, as well as under ledges, and are often sharing these day retreats with nocturnal hunters, like moray eels. As darkness sets in, they disperse and individually feed in the vicinity of the “home” reef. During their nightly foraging, they float quite high above the seafloor, on the lookout for larger zooplankton, like copepods and ostracods. Feeding at night comes with its own set of challenges, which besides certain obvious evolutionary adaptations, requires an intimate knowledge and understanding of the prey that one’s after as well as the predators one tries to avoid. As a reward for its efforts and perseverance, P. ransonneti can wine & dine in relative safety.

For the night has fewer predators for Parapriacanthus ransonneti than the day time, and with its midwater feeding behaviour, this clever little fish has managed to steer clear of most of the night time predators that hunt on the reef. When daylight approaches, they converge again in a sheltered place on the reef, where the many combined eyes offer not only safety in numbers, but moreover provides the ability and opportunity to confuse would-be assailants with their wild flashmob choreography.

The special tricks of the Golden Sweeper (Parapriacanthus ransonneti)

Unfortunately, even feeding at night in midwater is not without peril. Those that do, still risk silhouetting against the dim-lit surface waters and subsequently be spotted by a nightly hunter looking up. This is probably why Parapriacanthus ransonneti have evolved their bioluminescent qualities, which gives them the ability to produce their own light. They possess 2 types of ventral light organs. Firstly a Y-shaped thoracic light organ that emits a V-shape pattern from the underside of their head, all the way to the base of their anal fins. Secondly, a linear anal light organ, which as one might have suspected, emerges from the rectum and anus of this fish. These organs allow P. ransonneti to camouflage themselves against the light above, through a phenomenon called “counter-illumination”.

In this process, the bioluminescent creature emits light from the underside of its body in order to break/smudge its silhouette and become less visible for any predator lurking below. Much in the way how the Bobtail squid (Take a Minute to Relax XIII) uses the same concept to avoid detection. Bioluminescence occurs when a protein named luciferin interacts with the enzyme luciferase, inside a particular light-emitting creature. The protein luciferin is usually obtained through the organisms these creatures consume. In contrast, the enzyme luciferase was thought to be exclusively made by the bioluminescent organism itself (endogenous) and is produced by transcription of DNA.

To shine or not to shine

Although the ability to shine a light from one’s arse might seem somewhat radical to non-glowing creatures like ourselves, bioluminescence is nothing new to the aquatic realm. There are more than 200 documented genera of fish, in addition to other marine life, that has developed these same qualities for camouflage, and/or communication. What sets Parapriacanthus ransonneti apart from the rest of these enlightened creatures, is that it’s able to acquire both the luciferin and the luciferase from the copepods and ostracods in their diet. Somehow P. ransonneti manages to extract the luciferase enzymes from their prey, without damaging and/or changing the enzyme’s DNA in any way, and then put them to work in their own bodies! The discovery of this extraordinary achievement was/is very interesting for a number of reasons, but mainly because it suggests that the acquisition of certain physical traits across species are not always the result of their genetic makeup.

Parapriacanthus ransonneti has made us re-evaluate much of what we thought to know about the evolutionary process of creating specific abilities, and how the environment in which a creature lives, is affecting said process.

There are more creatures to relax with

For more visual meditation, watch the whole playlist on our YouTube channel or browse through the different clips on our designated page „Take a Minute“ on this website.

Screenshot from video clip "Take a Minute to Relax": Golden Sweepers (Parapriacanthus ransonneti). Yellow/Golden Sweeper, Pygmy sweeper, Golden/Yellow Bullseye, and Golden Glassfish, just to name a few. Several species of Parapriacanthus are often huddled together under the same banner of “Glassfish”, as they're partially transparent. They have a rather compressed, silvery pink translucent body with a greenish golden head and large eyes, showing a faint dark horizontal streak starting from the upper part of their gill cover to below the rear end of their dorsal fin. They hurdle together and sweep in and out of the colourful reef with soft and hard corals in Komodo National Park in Indonesia.
Underwater ballet of Golden Sweeper (Parapriacanthus ransonneti) in colourful reef, Komodo, Indonesia, 2016.

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