Kryptopterus vitreolus

Kryptopterus vitreolus, known commonly as the glass catfish, the glass cat, the ghost catfish or the phantom catfish, is a small species of translucent-bodied, social glass catfish in the family Siluridae. It is commonly seen in captivity and in the freshwater aquarium trade, although its official taxonomy is still debatable, and was only truly resolved in 2013. Glass catfish are endemic to Thailand, where they inhabit rivers and streams south of the Isthmus of Kra that drain into the Gulf of Thailand and river basins in the Cardamom Mountains. There are also unconfirmed reports from Penang, Malaysia.

Until 1989, K. vitreolus was considered to be the same as another "glass catfish", Kryptopterus bicirrhis, a larger species that is not commonly seen in captivity. Additionally, K. vitreolus was believed to be the same as K. minor until 2013, when it was established that captive specimens actually represented another species, which is now named as K. vitreolus. The true K. minor, which is restricted to Borneo, has rarely, if ever, entered the aquarium trade.

Description and taxonomy
Kryptopterus vitreolus is a small, transparent-bodied, freshwater-dwelling catfish with two long and sensory barbels. Standard lengths for mature fish may range up to 8 cm, but usually only reach around 6.5 cm in total length. Their bodies are transparent as, like all catfish, they are a scaleless fish; additionally, species within the genus Kryptopterus lack any body pigment. The species' skin is made of a "plywood"-like structure of collagen fibrils, which allows for light to enter and diffract through the muscles. The majority of their organs are located near the head; with a magnifying glass, the heart can be seen beating. When the light strikes this fish at a certain angle, it can create an iridescent, rainbow effect. During acute periods of illness or after death, the tissues turn a milky-white.

The specific name, vitreolus, is derived from the Latin vitreus, which means 'glass'. Among described species of Kryptopterus, only two other species, K. minor and K. piperatus, have clearly transparent bodies; these species are largely—if not entirely—absent from the aquarium trade. The bodies of other species in the genus, including K. bicirrhis, are only somewhat translucent or opaque.

In captivity
The aquarium trade in K. vitreolus generally relies on wild-caught specimens, and there are concerns that this may be unsustainable due to the volume and its limited range. Unlike many other aquarium fish, it is not known to be bred at commercial facilities.

Electromagnetic response
K. vitreolus reacts to electromagnetic fields owing to a protein encoded by Electromagnetic Perceptive Gene (EPG).