Taxonomy & naming
Lacepède described Cyprinus rubrofuscus in 1803 with the type locality given only as China. For most of the twentieth century East Asian carp were lumped into Cyprinus carpio. Later authors, notably Kottelat in the 2010s, restored C. rubrofuscus as the valid name for the East Asian carp, and the Catalog of Fishes lists it as valid. Whether koi belong under this name or under C. carpio is still stated differently from source to source.
Morphology
A deep-bodied, heavy cyprinid with a long dorsal fin, a spine-like serrated ray in the dorsal and anal fins, and two pairs of barbels on the upper lip. Wild fish are olive to bronze. Domesticated lines vary from gold and orange to white, red and black patterned koi. FishBase gives a maximum of 11 in SL, but pond and farmed fish are commonly much larger.
Habitat
The native range is East Asia, centred on China and including the Amur basin; FishBase gives it as the Amur to Red River drainages, with records from Laos and Viet Nam. The fish lives in slow rivers, lakes, floodplain waters and reservoirs over soft bottoms, and tolerates slightly brackish water. It has been introduced and farmed widely, and the line between native and introduced populations is blurred by centuries of culture.
Feeding
Carp are omnivorous bottom feeders. They root through mud for insect larvae, crustaceans, molluscs, plant matter and detritus, which stirs up sediment and uproots plants.
Mating
Spawning in carp is seasonal, with fish moving onto flooded vegetation in warm months. Males become tuberculate and several chase a female through plants as she scatters eggs.
Breeding
Eggs are adhesive and stick to plants. There is no parental care. Koi and goldfish-style carp are bred in ponds on a commercial scale; breeding wild-type fish in aquaria is not practical because of size.
In the aquarium
This is a pond fish, not a tank fish. It needs a large, deep, well-filtered pond, tolerates a wide temperature range and winter cold, and will eat or uproot most plants. In the specialty trade it appears under its scientific name when sold as a wild-type or amur carp; the ornamental koi trade usually lists it under common names and variety names instead.
Conservation
The IUCN Red List category is Least Concern, assessed in 2020. Wild populations in East Asia are affected by damming, habitat loss and hybridisation with farmed stock, and the species is among the most widely introduced fishes in the world.