The Amazon River Basin is home to the infamous blood-sucking creature – the candiru – a type of vampire fish. However, it seems that these fish do not only suck blood but also use their host’s body as a “means of transportation” or protection from predators.
Vandelliinae Fish.
The candiru is a small catfish that is somewhat transparent and lives in the murky waters of the Amazon. It wriggles its narrow head into the gills of much larger fish. There, it firmly attaches with its strong teeth and sucks blood. This fish is perhaps most famous for stories in which it is attracted to human urine in the river and swims up their urethra, a terrifying ordeal. However, there is no clear evidence that candiru prefer to attack human genitalia.
So far, there has only been one report considered evidence of a candiru invading a human body. In 1977, in Manaus, Brazil, a male patient was admitted to the hospital with a candiru in his urethra. After several hours of surgery, doctors pulled the fish out from the victim’s penis.
There are about 9 species of candiru (subfamily Vandelliinae) that are definitely vampire fish, excellently adapted to nibbling on the circulatory system of their hosts.
This fish is excellently adapted to nibbling on the circulatory system of the host.
In April 2019, Chiara Lubich – a hydrologist at the Federal University of Amazonas in Manaus, Brazil – and her colleagues were surveying and collecting fish species in the Rio Negro, a major tributary of the Amazon. While the team was removing and measuring the fish caught in their nets, they saw something bizarre clinging to the sides of a thorny catfish (Doras phlyzakion). The several-inch-long candiru — later identified in the genus Paracanthopoma — were residing on the sides of the catfish, a peculiar position for a parasite that typically lives in the vulnerable gill area. The research team found more: a total of 9 thorny catfish, with over a dozen candiru parasites.
The researchers took the candiru to the laboratory and examined their stomach contents under a microscope to see if they were feeding on the larger catfish from the sides of their bodies. However, they found nothing. No blood, skin, flesh, or mucus.
The findings — published in the journal Acta Ichthyologica et Piscatoria — suggest that the candiru may not have eaten anything from the host’s body but merely clung on like remora fish often do on sharks.
The candiru only clings on like remora fish often do on sharks.
Lubich said: “It seems that candiru cling and move with their host for reasons other than food.”
This benign interaction between candiru and larger fish might indicate that vampire fish have a more diverse and complex relationship with their hosts than previously thought, and they may derive some benefits beyond simply satisfying their hunger.
The candiru may be exploiting the size of the host and their strong swimming abilities, using their bodies as taxis to travel distances in the river that would be unattainable if they swam alone. Additionally, because candiru are somewhat transparent, hiding within the body of a larger fish could make it harder for predators to detect them.
Larry Page, a fish biologist at the Florida Museum of Natural History in Gainesville, who did not participate in this study, is intrigued by the idea that candiru are using larger fish as a means of transportation.
Paracanthopoma Fish.
“To me, that seems like a reasonable explanation,” Page said, noting that Lubich’s team does not necessarily rule out the possibility that candiru sometimes feed on the host’s skin. “But it seems that they are using larger fish to travel long distances, or perhaps they are doing both: feeding and hitching a ride.”
Lubich mentioned that there may be a “clear difference” in the feeding habits of the candiru subfamily, of which Paracanthopoma is just a part. Some species have been found with their heads deeply embedded in the abdominal cavity of their hosts. Others appear to also consume scales, mucus, and even small amounts of the host’s flesh. Lubich noted that Paracanthopoma has the longest and strongest snout among candiru species, which — along with its tightly gripping teeth — may help it cling to the sides of larger fish.
However, Lubich pointed out that this might also relate to how thorny catfish are caught. The fish were stuck in the net for hours before the research team removed them. Candiru may sense when the host is injured or careless and take advantage of that situation, Lubich explained.
Whatever the case may be, it is clear that there is much more to the behavior of these peculiar little parasitic fish than just bloodsucking.
“I believe there is still much to be answered and learned about this relationship, not only with thorny catfish but also the relationship between vampire fish and other species that we have yet to encounter and report,” Lubich said.
