Glass Catfish

Kryptopterus vitreolus is a small, transparent-bodied, freshwater-dwelling catfish with two long sensory barbels. Standard lengths for mature fish may range up to 8 cm (3.1 in), but usually only reach around 6.5 cm (2.6 in) in total length. Their bodies are transparent because, like all catfish, they lack scales. Additionally, species in the genus Kryptopterus have no 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.

According to Thai folklore, this species of catfish is said to have originated from Phra Ruang, the legendary king who possessed supernatural speech. One tale recounts that after he ate a fish down to its bones, he cast the remains back into the water and uttered a wish for it to live again. Miraculously, the fish returned to life in the form of a glass catfish—hence its other name, pla kang phra ruang (Thai: ปลาก้างพระร่วง, pronounced [plaː kaːŋ pʰraʔ rua̯ŋ], lit. 'Phra Ruang's skeletal fish').