Specimen Showcase | A Superhero Killer of the Darkness

Hong Kong Biodiversity Museum
Mar 10
2 min read

We tend to perceive and describe our surrounding environment thanks to visual and audio cues, however many species rely on very different types of information, including some that we would associate with superpowers. Such is the case of our species of the day, the black ghost knifefish 𝘈𝘱𝘵𝘦𝘳𝘰𝘯𝘰𝘵𝘶𝘴 𝘢𝘭𝘣𝘪𝘧𝘳𝘰𝘯𝘴 (Linnaeus, 1766), a modest “hero” of 18 cm in length as an adult, with long ribbon fins and swimming like a knife slicing the water🔪🌊. This freshwater fish lives in floodplains and rivers of the Amazon and Orinoco basins in South America where it feeds on insect larvaes and tiny crustaceans😋.

Its hunts, however, occur in the darkness of the night within turbid waters and localizing its prey is key to successfully capturing them. So how does this fish achieve such a feat? Clearly visual cues😵‍ are of little use here in this environment, so instead the black ghost knifefish evolved the capabilities to generate and receive electrical signals to detect the environmental conditions through the development of a specific electrosensory system. The knifefish generates a weak electric signal which is known as an Electric Organ Discharge (EOD), a high frequency and low amplitude signal electric current which passes through the environment. Then, different objects such as rocks, or other organisms will have different electric impedance, altering the reflected electric current received by the electroreceptor organs on the fish skin. The fish is then able to locate and identify nearby objects, gauge their size and even the prey’s direction. Thanks to this special sensory system, the fish can “see” the prey without being discovered, hidden within the obscurity of the night.

But our superhero has some limitations. As the generation of electrical signals is energy demanding🔥, those fishes are restricted to well-oxygenated habitats for maintaining aerobic metabolism. Some study also suggests that the fishes would identify and avoid the hypoxic areas with low dissolved oxygen level thanks to their electrosensory system. Other challenges also occur in these areas as freshwater habitats are not like an aquarium only filled with water.

Indeed, it also contains many rocks, plants and other objects🦐 and creatures interfering or weakening the electric signals. To move efficiently in this complex environment in complete darkness, the black ghost knifefish developed a “special motor system”. Thanks to their long ribbon fins with wave-moving function, the fish are able to swim forward, backward, to the sideway and also stay in the same location just like a helicopter🚁. This allows them to access complex hunting grounds. Most importantly, their exact positioning allows them to work like an antenna to receive clear electrosensory signals reflected from different objects, and pointing their electroreceptor organs on the skin towards the detection sites. Ultimately, they are able to receive electric current from all directions, helping them to locate their prey in a complex environment🔍.