Candiru

The Candiru is a small, slender catfish native to the rivers of the Amazon basin, notable as one of the few vertebrate species with a true parasitic, blood-feeding lifestyle. It locates the gill chambers of larger fish, likely by detecting chemical or physical cues, and wedges itself inside using small recurved spines on its gill covers that anchor it in place while it feeds on blood from the host's gill filaments. Its near-transparent body makes it difficult to spot in the water, and its minute size — rarely more than a few centimeters — belies its specialized predatory strategy. Candiru are a natural, if unusual, part of Amazonian freshwater food webs, though they remain poorly studied due to their small size and secretive habits.

• Extremely slender, elongated, eel-like body, only a few centimeters long
• Near-transparent, faintly pinkish or greyish skin with visible internal structures
• Small, backward-curving spines on the opercle (gill cover) used to anchor to host fish
• Tiny eyes and reduced pigmentation typical of a specialized parasitic lifestyle
• No prominent barbels compared to most catfish
• Similar species: closely related trichomycterid catfishes share the slender parasitic body plan but differ in spine arrangement and host preference.

Candiru are found in the freshwater rivers, tributaries, and floodplain waters of the Amazon basin across Brazil, Peru, Ecuador, and neighboring countries. They occupy the same open water and near-bottom zones as the larger fish species they depend on as hosts, since their life cycle is closely tied to locating host gills for feeding. They are most often encountered in turbid, slow to moderately flowing water where host fish congregate, and their small transparent bodies make them nearly invisible in the water column outside of a host's gill chamber.

Candiru locate host fish, primarily larger catfish and characins, and enter the gill chamber where they use small recurved opercular spines to anchor themselves while feeding briefly on blood from gill filaments before detaching. This specialized hematophagous (blood-feeding) behavior is thought to be guided by chemical cues released by respiring gills. Unlike many parasites, Candiru do not remain permanently attached, instead feeding intermittently and swimming freely between meals. Little is documented about their reproductive behavior in the wild, though like other small trichomycterid catfishes they are believed to produce relatively small numbers of eggs. Their ecological role as a specialized micropredator makes them a distinctive, if rarely observed, component of Amazonian fish communities.