Brown Ghost Knifefish

As a member of the family Apteronotidae, brown ghost knifefish are laterally compressed and sport a long, posteriorly tapered body form. They lack pelvic and dorsal fins, yet retain a pointed pectoral fin and a greatly reduced caudal tail, an identifying trait of Apteronotids among the Gymnotiforms. Brown ghost knifefish possess an elongated, ribbon-like anal fin which extends along the majority of its ventral side. This fin comprises around one hundred and fifty fin rays and is utilized as the species' main source of locomotion. Among teleost fishes, this species has relatively restricted gill openings. Brown ghost knifefish have a laterally compressed head, a terminal mouth, and small, laterally located eyes covered by a thin membrane. The urogenital papilla and anus are located beneath the head, posterior to the eyes. Generally, they possess 17–18 pectoral-fin rays, 151–156 anal-fin rays, 17–21 caudal-fin rays, 78–82 scales along the lateral line, 18 precaudal vertebrae, and 78–81 total vertebrae. Among collected individuals, brown ghost knifefish range in total length between 93.1 and 260.0 mm. Tail length ranges from 13.0 to 20.7 mm. Head length ranges from 15.0 to 46.7 mm.

Brown ghost knifefish can be distinguished from other genera within the family Apteronotidae based on a multitude of distinct synapomorphies. …

The brown ghost knifefish is endemic to the Essequibo River and its drainage basin in Guyana. Historically, this fish was thought to inhabit broad regions of northern South America, including the Pacific and Caribbean drainages of Colombia, Venezuela, and Peru. These populations were either misidentified or now belong to newly described, distinct species.

Within the Essequibo River and its drainage basin, brown ghost knifefish prefer deep channels where limited light can reach. They are active nocturnally and move from deep waters to the shore during dark hours. During the day, they seek refuge among roots and under logs to avoid diurnal predators. Intraspecific competition among shelter sites is determined by dominance hierarchies. These hierarchies are influenced by the size and electric signal intensity of males. Males take shelter alone while female fish group together within retreat sites. Electric organ discharges (EOD) are frequently produced by brown ghost knifefish, serving a variety of functions including prey capture, navigation, and social dominance. Six distinct signal types have been observed, the most common being short, low frequency "chirps". Chirp production is dependent on the close proximity between individual brown ghost knifefish. Individuals relay an "echo" signal in response to these hardly detectable chirps, indicating a substantially high sensitivity to electrocommunication within this species. Signals increase in frequency as individuals approach or are approached by a focal target. EODs do not seem to incur a significant metabolic costs when in oxygen-abundant conditions. …