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The catfishes in the Neotropical subfamily Vandelliinae (family Trichomycteridae) feed exclusively on blood as adults, a very rare feeding specialization among vertebrates (the lampreys and vampire bats being the obvious examples). Their hosts are larger fishes. Little is known about vandelliines. The recently discovered juveniles are apparently predators on small aquatic invetebrates, as is the case for most other trichomycterids from other subfamilies. The structure of the mouth of juvenile vandelliines also resembles that of non-parasitic trichomycterids, but juvenile vandelliines undergo a metamorphosis during which their mouth structure transforms to the highly modified adult blood-feeding morphology. It is possible that they produce an anticoagulant as they feed since the large amount of blood taken in a single feeding bout (around twice their body volume) remains liquid during digestion. It has been speculated that some vandelliine species may be carried by large migratory fishes to upriver spawning grounds, with the large volume of ingested blood possibly inducing hormonal changes to synchronize their reproductive state with that of the host.
(De Pinna 2006)
Some vandelliines are reputed to invade and parasitize the human urethra, but researchers attempting to document this phenomenon have generally concluded that this rarely, if ever, actually happens. Spotte et al. (2001) and Zuanon and Sazima (2004) discuss the normal feeding behaviors of these fishes.