Koi

Koi are domesticated ornamental varieties of the Amur carp, developed primarily in Niigata, Japan, beginning in the early 1800s from food-grade common carp stocks. They belong to the family Cyprinidae and are prized worldwide for their striking, selectively bred color patterns rather than any wild trait. Hundreds of named varieties exist, including Kohaku (red-and-white), Taisho Sanke, and Showa, each defined by specific pattern and color rules. Koi are kept almost exclusively in ornamental garden ponds and specialized breeding facilities rather than in the wild, and they hold deep cultural symbolism in Japan representing perseverance and good fortune. They are among the longest-lived freshwater fish kept by hobbyists.

Koi share the classic carp body plan but are identified by their vivid, irregular color patches rather than uniform coloring.

• Body: thick, elongated, slightly arched profile with large, smooth-edged scales (fully scaled, partially scaled 'doitsu,' or scaleless leather forms exist)
• Barbels: two pairs framing the mouth, used to detect food
• Fins: single long dorsal fin, forked tail
• Color: patches of white, red-orange, black, cream, or metallic gold/platinum in named combinations
• Size: typically 60-90 cm in mature pond specimens, occasionally larger

Unlike wild common carp, koi never show plain olive-bronze coloring over the whole body.

Koi are kept in freshwater garden ponds, ornamental lakes, and specialized aquaculture ponds rather than natural wild habitats, though their ancestral common carp stock originates from the Amur River basin of East Asia. They tolerate a very wide temperature range, from near-freezing water in winter dormancy to around 30°C in summer, and prefer calm to slow-moving water with some aquatic vegetation, biological filtration, and areas of deeper water for shelter. Ponds are typically built with depths of at least 1-1.5 meters to protect koi from temperature extremes and predators. Because koi are entirely a human-bred ornamental form, their present-day distribution is defined by ornamental ponds across Japan, East Asia, Europe, and North America rather than by any natural range.

Koi are social, non-aggressive fish that often form loose groups and quickly learn to associate humans with feeding time, surfacing and gathering at the water's edge. They are bottom-oriented omnivores, using their barbels to sift through substrate for insect larvae, plant matter, and other small food items, though pond-kept koi are commonly fed prepared foods. In spring, when water temperatures rise, koi spawn in a boisterous fashion: males chase females through shallow vegetated water while females scatter thousands of adhesive eggs onto plants and pond edges. Koi are exceptionally long-lived, with many individuals reaching several decades of age and some famous specimens reported well beyond 50 years, making them one of the longest-lived ornamental animals kept by hobbyists.