Caecobarbus

The Congo blind barb is found only in the Thysville Cave system, a part of the lower Congo River basin (notably Kwilu and Inkisi tributaries) in the Democratic Republic of the Congo.[1][2] The area where found covers about 120 km² (45 sq mi),[2] and is located at an altitude around 700–850 m (2,300–2,800 ft) above sea level.[4] It was initially known from seven caves in the Thysville system and an additional seven were discovered in the 2000s (decade).[2] The caves where found have high levels of calcium bicarbonate in the water and high variations in flood levels, meaning that animals such as terrestrial cave insects are absent.[2]

In the seven caves where first discovered, the total population has been estimated at perhaps 7,000 individuals in the 1950s; no estimates are available for the seven more recently discovered populations.[2] Among the first seven, one has disappeared entirely following quarrying of limestone that began in the 1930s.[2] The caves where it lives are considered sacred by the locals and this restricts access to most of them. A local law passed in 1937 limits fishing and the species has been listed on CITES since 1981, which limits international trade.[1][2] Before its listing on CITES Appendix II, the species was often caught and exported for the freshwater aquarium trade, but few exports have happened after.[2] Currently, the main threat is habitat loss: The human population in its range is increasing and farming near the caves can result in sedimentation and pollution.[1][2] The region was formerly covered in rainforest and grassland, which limited erosion. Other potentially serious threats are changes to the hydrology (for example, large-scale water extraction) of the small rivers that feed the caves and predation by Clarias catfish found in at least some places inhabited by the Congo blind barb.[2]

Like other cavefish, the Congo blind barb has reduced pigmentation and no externally visible eyes.[6] Because of the lack of pigment, it appears pale whitish-pink overall. The operculum and lateral line region are purplish-red due to the gills and lateral line veins, respectively.[2] The eye completely lacks a lens, and the retina and optical nerve are rudimentary and located deep inside the head. Despite this, the fish is photophobic, actively avoiding light.[2] Its maximum total length is 12 cm (4.7 in).[4] Minor differences have been described between the different cave populations; some have a spot on the opercular and one has a serrated dorsal fin spine.[2]

The Congo blind barb mainly relies on food items that flow into the cave during the rainy (flood) season, but it may also feed on small crustaceans.[2] Its longevity is estimated to be 9–14 years, but it may be able to live longer.[4] It is slow-growing and likely has a slow reproduction rate.[2] The adhesive eggs are spawned freely, ending up in crevices and pores in the bottom, which protects them from predation.[1]