Candirus (Vandellia) inhabit the Amazon and Orinoco basins of lowland Amazonia, where they constitute part of the Neotropical fish fauna.
Candirus are hematophagous and parasitize the gills of larger Amazonian fishes, especially catfish of the family Pimelodidae (Siluriformes).
Martius also speculated that the fish were attracted by the "odor" of urine.
Another report from French naturalist Francis de Castelnau in 1855 relates an allegation by local Araguay fisherman, saying that it is dangerous to urinate in the river as the fish "springs out of the water and penetrates into the urethra by ascending the length of the liquid column." While Castelnau himself dismissed this claim as "absolutely preposterous," and the fluid mechanics of such a maneuver defy the laws of physics, it remains one of the more stubborn myths about the candiru.
"They are a sub-tropical fish and they used to be very rare in Ireland but because of global warming they are moving further up north." Paul added that he has unfortunately been stung by one of these toxic fish before.
He said: "I was fishing a few years ago and I caught a weever, it was tiny and I didn't realise what it was so as i tried to un-hook it I got stung.
"The pain is at its most intense for the first two hours when the foot normally goes red and swells up, and then it may feel numb until the following day with irritation and pain that may last for up to two weeks.
There are short sensory barbels around the head, together with short, backward pointing spines on the gill covers.
Gudger, in 1930, noted there have been several other cases reported wherein the fish entered the vaginal canal, but not a single case of a candiru entering the anus was ever documented.
According to Gudger, this lends credence to the unlikelihood of the fish entering the male urethra, based on the comparatively small opening that would accommodate only the most immature members of the species.
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Irish Water Safety has advised that anyone heading to the beach needs to be wary of the weever fish and said they are most commonly found at low tide.