A trout of a different color
In the upper reaches of the Zerafshan River, a unique subspecies of brown trout (Salmo trutta) displays a brilliant, golden-yellow coloration unlike any other population. Locally known as the Zerafshan golden trout, this fish, Salmo trutta zerafshanensis, owes its unique appearance not to its own genetics, but to its highly specialized diet. The fish's skin and flesh are saturated with carotenoid pigments, specifically a class of xanthophylls that are normally found in crustaceans and algae. Fish cannot produce these pigments themselves and must acquire them from what they eat.
The source of the trout's color is a specific, single species of endemic snail. Analysis of the trout's stomach contents reveals it feeds almost exclusively on a small aquatic gastropod found only in these headwaters. This snail, in turn, has a diet that consists entirely of microbial mats growing near sulfur-rich springs that feed into the river. These mats are composed of dense colonies of anaerobic, photosynthetic purple sulfur bacteria (PSB). These bacteria, belonging to the family Chromatiaceae, utilize hydrogen sulfide instead of water for photosynthesis and produce elemental sulfur and carotenoid pigments as byproducts. The snails graze on these bacterial mats, concentrating the pigments in their own tissues. When the trout consume the snails, the pigments are transferred once more, bioaccumulating in the fish's fatty tissues and skin, resulting in their golden hue.