The X-ray tetra, also known as the golden pristella tetra, is one of the very few animals that are transparent or translucent. Due to its faint yellow coloration, it is also called the water goldfinch. This rather obviously named fish is an extremely adaptable organism, and can be found in waters both marine and fresh, both acidic and alkaline, and both clear and brackish. This is an ability almost unrivaled among fish; most can only survive in one of all three variables. However, its ideal habitat includes freshwater with a slightly acidic pH (between 6 and 7) value. It is found in the Amazon and Orinoco rivers, including their coastal deltas. Due to their visual appeal, they are very popular aquarium fish. They are shoal fish, and thus, are always kept in groups of 8-10 individuals. They are very tolerant of other occupants of aquariums, and thus, can be housed with any species that reciprocates their gesture. Kingdom: Animalia Phylum: Chordata Class: Actinopterygii Order: Characiformes Family: Characidae Genus: Pristella Binomial Name: Pristella maxillaris Their most noticeable and popular feature is, of course, the translucent skin. Though this has helped the species become popular in aquariums, it has a much more basal purpose. The translucent skin helps the fish blend against rocks or plants, safe from detection by predators. Their lightly colored scales also help them remain inconspicuous, as the scales are barely noticeable in water. If it is caught swimming in open water, the X-ray fish is virtually defenseless. Their scales, as mentioned before, are light yellow, but can appear golden in suitable lighting. Their fins are notably tipped, and are rather conspicuously colored yellow, black, and white. This fish is also notable for having the Weberian apparatus, a bony structure that enhances their hearing ability. Growing to just 2 inches long, these are some of the smallest fish to have this structure. The Weberian apparatus helps them receive sound waves through the swim bladder, and direct them to their auditory canal through vertebrae. Like their translucent skin, which serves a protective function, the X-ray tetra's keen hearing allows it to sense and quickly respond to predators. Sexual dimorphism is noticeable in this species; the males are smaller than females, and can be identified on that basis. Though this tetra can survive in various water conditions, it seeks its native river's flood plains to breed. The female lays more than 300 eggs in the submerged vegetation. The fry (young fish) start hatching after just 24 hours. Larger fish, frogs, birds, and snakes are its main predators and account for many of the hatchlings. It reaches sexual maturity around 6-7 months of age, and lives for about 4-5 years, possibly more in captivity. The X-ray fish is a micropredator, i.e., it preys on various tiny invertebrates, including worms, tiny crustaceans, and insects. Mirroring the adult feeding habits, the fry of the X-ray fish usually feed on insect larvae. It also sometimes eats aquatic plants, but the majority of its diet is made up by prey. In captivity, it needs to be fed standard fish food for complete nutrition, as well as occasional blood worms and brine shrimp (Sea-Monkeys would work perfectly). In spite of its numerous natural predators, the greatest threat to the x-ray fish comes from water pollution, and consequent loss of habitat. Though this fish can survive in variable conditions, the conditions brought about by the interference of man in its natural habitat are much worse than it can tolerate. Like many other species, this weird piscine species is threatened due to human activities eating into their habitat and making it inhospitable. Let's hope that the X-ray fish is not soon counted among the ranks of the dodo!