Candiru

The candiru is a parasitic fish species found in the freshwaters of the Amazonian basin. Candiru, scientific name Vandellia cirrhosa, also goes by other common names such as the vampire fish, toothpick fish or cañero. It belongs to the family Trichomycteridae and is usually found in the countries of Peru, Columbia, Brazil and Ecuador. It earns its notorious reputation for its parasitic behaviour and its feeding on the blood of other fishes. Candiru fish attacks on humans have also been reported. 

In this article, we will learn about the candiru fish in detail, its morphology, behaviour, threats to humans, etc. 

Candiru Fish - Classification

Candiru definition differs among authors. The term is used either to represent the species, the genus Vandellia, the Vandelliinae sub-family or at times Vandelliinae and Stegophilinae subfamilies even. The complete classification of the candiru fish is provided below. 

Kingdom: Animalia

Phylum: Chordata

Class: Actinopterygii

Order: Siluriformes

Family: Trichomycteridae

Genus: Vandellia

Species: Vandellia cirrhosa

Description of the Candiru Fish

Candiru is a small, thin and scaleless catfish. The lack of scales in their bodies makes them translucent. Their bodies become coloured only when they feed. The average length attained by this fish species is 17 cm with most specimens much smaller. However, there have been reports of some candiru fish growing up to a length of 40 cm. They have a narrow cylindrical body with a head that is slightly flattened. Barbels that are lined with needle-like, minute teeth are present near the mouths of the candiru fish. They possess large eyes in comparison to their body size. The eyes are black in colour and placed atop their head. Their gill covers have backwards-facing, short spines that help the fish from getting dislodged during the process of feeding. 

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Habitat and Distribution

The geographic range of the candiru fish is exclusive to the Orinoco River basins and the upper Amazon River in the northern parts of South America. They inhabit acidic, slow-moving, shallow waterways with sandy or muddy bottoms in the benthic ecosystems such as rivers and streams of these tropical freshwaters. Riverbeds are where these candiru parasite fish are found most of the time. They emerge only for the process of mating or feeding. The candiru fish comprise a section of the greater Neotropical fish fauna. 

Diet of the Candiru Fish

As parasites, candiru fish are sanguivorous, known to feed on the blood of other fishes, hence earning them the reputation as the Vampire fish. The parasitic candiru fish locates its host by using chemical and visual cues. Once it locates its host, it heads in the direction of the gills of the host. Upon reaching the gills, the candiru fish either waits for the gills to open naturally or forces its way through the underside of the operculum. After it has passed the operculum, the candiru parasites latch onto the dorsal or ventral aortal arteries. The candiru fish stays attached to the gills of the host with the help of its opercular spines which also aid in the process of releasing blood.

Contrary to the previous notion that the candiru fish suck blood from the host, the blood pressure of the host allows for the pumping of the blood straight to the mouth of the candiru fish. A candiru fish’s single blood meal usually lasts anywhere between 30 and 145 seconds. Once the candiru has fed, it will sink and gradually burrow to the bottom of the river. Among the hosts of the candiru fish are the larger catfish species such as Pseudoplatystoma sp., Brachyplatystoma vaillantii and species of characins including Brycon spp., Pygocentrus nattereri, Salminus brasiliensis, Piaractus brachypomus and Colossoma macropomum). The species of Colossoma macropomum has, in fact, been observed exhibiting defense mechanisms against candiru fish attacks. These defense mechanisms include the use of their fins in order to sweep the candiru fish away and tightening of their operculum, thus, preventing the parasites from entering. 

Candiru Fish Behaviour

Apart from the feeding behaviour of the candiru, much is not known about the fish species. These parasitic fish are often buried in the substrate, although they are natatorial, i.e. specialised for swimming. They do not have any time preference for feeding and feed actively during the day as well as night making them a nocturnal, diurnal and crepuscular species. Although they are motile, they are seen preferring a sedentary and solitary lifestyle. Information about the home range of this species is also scarce.

Perception and Communication:

As already stated above, candiru fish locate their hosts by most likely using a combination of visual and chemical cues. One of the hypotheses states that the candiru fish can track the ammonia scent as well as other secretions released by potential prey. This hypothesis, however, has not been conclusively proven. Given the large structure of their eyes, it can correlate to a higher visual acuity. But the primary habitat of the candiru fish is in turbid waters where there is a limitation to vision. This is why their eyesight is unlikely to the primary mode of detecting their hosts. As in the case of most of the other fishes, the lateral line system of the candiru fish helps to be alert of vibrations or movements in the surrounding water. 

Reproduction in Candiru Fish

The mating behaviour of the candiru fish has not been observed in the wild. In only one instance that was recorded of a candiru fish in captivity, it was seen spawning. The male, in this case, was seen swimming around the female and eventually driving her down and towards the substrate. When the fishes were directly in contact with each other laterally, sperm and eggs were released. It has been predicted that the candiru fish mating behaviour, in addition to being promiscuous, is polygynandrous (a kind of polygamy in which the female mates with different males with each of the males, in turn, mating with different females).

The general reproductive behaviour of the candiru fish species is also relatively unknown, with very scarce information available. In the record of a candiru raised in captivity, there is a sole indication of a breeding season in late December when the female candiru fish was with ripe ovaries. In captivity, during spawning, the female candiru fish released 4 to 5 eggs at a time. The breeding was seen taking place a number of times during a 3-day period. However, of the eggs produced, none were viable. Thus, it has been hypothesized that the breeding is seasonal in the candiru fish where the separate sexes engage in sexual reproduction. The fertilisation is external and oviparous where the offspring are produced in groups of more than one (iteroparous). It is also estimated that the offspring are produced across multiple seasons or periods that are suitable for reproduction. The offspring range for this species is thought to vary between 4 and 15 in number.

The gestation period in candiru fish and the size of the fry at birth is not known. Information about the parental investment or care for the young ones in the species is also not available. The candiru fish that laid eggs in captivity did not display any nesting behaviour and there was no substrate preference either. The parents were also not seen providing any investment beyond fertilisation. The lifespan of the candiru fish, either in the wild or in captivity, is not known. 

Life Cycle of the Candiru Fish

The candiru fish lifestyle has not been established conclusively as presently there is no information available. In general, the eggs of catfish are spherical in shape and fertilised externally. Cell cleavage begins once the sperm enters the egg cell resulting in the development of the embryo. The gestation period is unknown but the young ones are generally seen hatch with a conspicuous yolk-sac. During the early stages of development, this yolk-sac acts as a source of food and is absorbed gradually. The individuals, post-yolk-sac, resemble small adults. 

Predators of Candiru Fish

There have been no reports on candiru predators but it is thought that the larger carnivorous fish are likely to feed on them.

Candiru Fish - Ecosystem Roles

Several fish species serve as hosts to the candiru fishes and the host fishes heal quickly after being parasitised by the candiru fish. It is only on rare occasions that the candiru fish have been found killing their hosts. It impacts the ecosystem as a parasite only. 

The several fish species that the candiru fish use as hosts include: 

• Amazonian pacu (Piaractus brachypomus)

• Tambaqui (Colossoma macropomum)

• Red-bellied piranha (Pygocentrus nattereri)

• Brachyplatystoma vaillantii (Class Actinopterygii, Order Siluriformes)

• Pseudoplatystoma sp. (Class Actinopterygii, Order Siluriformes)

• Golden dorado (Salminus brasiliensis)

• Brycon sp. (Class Actinopterygii, Order Characiformes)
  
  

Candiru Fish - Relation to Humans

The positive impact of the candiru fish on humans has been in the fields of research and education. Besides the same, there have been no known positive impacts for humans. The fish species is also of no interest to the fisheries.

Although there have been many claims about the candiru fish entering the urethra of males and females, reports documenting the same are less common. Naturalist Paul Le Cointe, in the year 1981, provided a rare first-hand account of a candiru fish entering a human body. In this case, the fish was logged in the vaginal canal of a woman. Cointe, in this case, was said to have removed the Candiru fish himself. He did so by pushing the candiru fish forward so as to disengage the spines, then he turned it around and then removed it headfirst.

In 1930, notes Gudger, there were several reported cases where it was said that the candiru fish entered the vaginal canal. However, there was not a single report documented about the candiru fish entering the anus. Gudger says that this contributes to supporting that the scenario where a candiru fish would swim up a male urethra is highly unlikely. Given the very small opening of the male urethra, it would only accommodate the tiniest or very immature member of the fish species.

In modern times, there has been only one case documented where a candiru fish has entered the human male urethra. This incident is documented to have occurred in 1997 in Itacoatiara, Brazil. The victim, in this case, was a 23-year-old boy. The victim reportedly underwent a urological surgery that lasted two hours in order for the candiru fish to be removed from his body. The case was later examined by an American marine biologist, who raised doubts over the instances narrated by the victim and the doctor who operated on him. 

IUCN Status of the Candiru Fish

The present population number of the candiru fish species is unknown and there have also been no reports of the introduction of the fish in other than native habitats. There are presently no conservation measures either for evaluation or maintenance of the current population levels of the candiru fish.