Candiru are South American catfish of a parasitic or scavenging niche. The species known as candiru aren't always closely related, though they all are of interest due to their impact on human health (both confirmed and anecdotal).

Cetopsidae (whale catfish)

Trichomycteridae (pencil catfish)

Phylogeny of Siluriformes based on Betancur et al. 2017, Trichomycteridae based on Fernández and Schaefer 2009, and Cetopsidae based on De Pinna et al. 2007.

Candiru species are of medical importance to people inhabiting the areas where they are native, such as the Amazon basin. The Cetopsis candiru-açu, as well as P. microps, are well known for their scavenging habit, boring into carrion and cadavers that drift downriver. Along with the vulture catfish, their combined action are able to deflesh an 80 kilograms (180 lb) cadaver within half an hour. The bites of whale candiru are strong enough to leave marks upon human bone; circular marks are often visible on the bones of the animal fed upon. Occasionally, Cetopsis may also attack live humans, though the cause of such attacks is still unknown. The undescribed human-biting candiru was first documented while feeding on blood from a boy's back, which is noted to be not dissimilar from the method other vandelliines use to feed off of fish blood.

A long-standing claim about candiru is their supposed propensity to insert themselves into human urethras or other orifices in an apparent attempt to parasitize; this alleged ability has been described in ethnological reports dating back to the 19th century. However, these accounts of human parasitism are often biased, arising from "imprecise, second- and third-hand accounts, misconceptions, and folk tales"; which lead to the spread of imprecise or dubious claims that may even be reported in scientific resources; there are no credible reports of candiru parasitizing human urethras.

Vandelliines are characterized by their hematophagy, or their habit of feeding on blood. Their parasitic habit on other fish is well documented, with Plectrochilus machadoi specimens found embedded in the belly of a surubí after apparently burrowing through its body wall, and species of Paracanthopoma such as P. parva boring into the flanks of armored Doras catfish, piramutaba, and gilded catfish (though these are apparently instances of phoretic "hitchiking"), and all species in the subfamily are known to enter the gill cavity of larger fish (such as characins, catfish, or stingrays) to latch onto the branchial artery which connects the gills to the heart; the host's arterial pressure pumping the candiru's stomach full of blood. Despite this known behavior, they may not comfortably fit the stricter definition of parasitism, leading some authors to use terms such as semi-parasitism, or more recently; micropredation, where the parasite is not relying upon a single host species or individual. Whatever the case, these fish are highly specialized hematophagic parasites, thought to possess adaptations to deal with high iron content (which are highly oxidative to DNA and proteins), mechanisms to obtain vitamin B lacking in blood meals, and an anti-coagulative agent to facilitate blood feeding and/or digestion.

Candiru reputed to parasitize humans include T. brevis, and most commonly, Vandellia cirrhosa, which is the type species of the type genus of its subfamily. V. cirrhosa were presumed to seek out urea excreted out of a potential host's gills, which would make them susceptible to following streams of urine to its source, the urethra, though experimental evidence disproves this; a 2001 study shows that these fish actually hunt by sight (the species studied being Vandellia cf. plazaii) and displayed no attraction to urine (or other potential chemical attractants such as ammonia, fish slime, or amino acids) at all. Despite their names, candiru-açu are not parasitic on any animal.