Fish
Candiru
Vandellia cirrhosa · also called candiru, toothpick fish, vampire fish
The candiru is a finger-long, see-through Amazonian catfish with a genuinely gruesome way of life — it slips into the gills of a bigger fish, hooks itself in with spines, and drinks blood. It also has one of the most persistent scary legends in natural history, and that part is mostly not true.
Overview
Candiru live in the slow, muddy whitewater rivers of the Amazon and Orinoco basins, where they spend most of their time buried in the sediment of the bottom, nearly invisible. They are blood parasites of other fish, darting out when a host passes and slipping into its gill chamber.
Feeding on blood
Inside a host’s gills, the candiru uses sharp teeth to rasp open a blood vessel, then feeds on the blood, its transparent body swelling visibly as it fills. The feed lasts a minute or two; then it lets go and drops away, and the host’s wound usually heals. Backward-pointing spines on its gill covers lock it in place against the current while it feeds — which makes it very hard to pull out.
The human myth
The famous story — a candiru drawn to urea in urine, swimming up the stream of a person urinating in the river and lodging in the urethra — is almost entirely folklore. There is a single disputed 1997 case, widely criticised, and lab tests show candiru have no attraction to urine chemicals, hunting instead by sight. The real risk is effectively zero.
A real nuisance, though
Candiru infest netted, hooked, and penned fish, feeding on their gills and spoiling or killing them, and a fisher cleaning a catch can be bitten on the hand by one still in the gills.
Status
The candiru has not been assessed for conservation. It is common in parts of the Amazon and is regarded mainly as a pest by fishers and fish farmers.
Candiru: common questions
Can a candiru really swim up your urethra?
Almost certainly not, at least not the way the legend describes. The story — that a candiru is drawn to urea in urine, swims up the stream of a person urinating in the river, and lodges in the urethra where its spines make it impossible to remove without surgery — has circulated in the Amazon for over a century and has terrified travellers. But there is only one documented case, from Manaus in 1997, and that report has been strongly criticised on medical and physical grounds (the fish was reportedly larger than the urethra it supposedly entered, and the account has inconsistencies). Laboratory tests have found that candiru show no attraction to urea, ammonia, or other urine chemicals, and there is no plausible mechanism for a fish to swim against a urine stream into a body it cannot detect. The realistic risk is essentially zero.
How does the candiru actually feed?
It is a specialised blood parasite of other fish. A candiru lies buried in the river bottom and, when a suitable host swims over, darts out and slips in through the gap under the gill cover, following the water the host is pumping over its gills. Inside the gill chamber it uses sharp teeth to rasp open a large blood vessel or the gill filaments themselves, then feeds on the blood pouring out, swelling visibly as it fills. The whole feed takes a minute or two, after which the candiru releases its grip and drops away, and the wound in the host usually heals. Several candiru may feed on one large fish at once.
Why is it so hard to remove once it's in?
Along the edges of its gill covers, the candiru has short, stiff, backward-facing spines. When the fish is inside a host's gill chamber and feeding, these spines flare out and hook into the surrounding tissue, anchoring it firmly in place and against the current so it is not flushed out. Pulling it out forwards drives the spines deeper. This anchoring adaptation is entirely about staying attached to a fish host, but it is also the feature that made the human legend so frightening — the idea of a barbed fish that cannot be pulled back out.
Are candiru a problem for people at all?
Not as parasites of humans, but they are a genuine nuisance to fishers and fish farmers in the Amazon. Candiru infest netted and hooked fish and fish held in pens, feeding on their gills and reducing their value or killing them, and a fisher cleaning a catch may be bitten on the hand by candiru still lodged in the gills. Some Amazonian communities are wary of bathing in candiru-rich water and take practical precautions, but the specific urethra fear is disproportionate to any real danger.