Candiru

Vandellia cirrhosa is a small, freshwater catfish. Members of the genus Vandellia can reach up to 17 cm (7 in) in standard length, but some others can grow to around 40 cm (16 in). The fish has an elongated body with an anterior dorsal fin and pelvic fin, and an anal fin slightly larger than the dorsal fin. The caudal fin is fairly small with a truncated shape. Each has a rather small head and a belly that can appear distended, especially after a large blood meal. The body is translucent, making it quite difficult to spot in the turbid waters of its home. Blood is often visible through the translucent body after feeding. The fish has small eyes with yellow irises. There are short sensory barbels around the head, together with short, backward pointing spines on the gill covers. These spines have been described as popping out in an umbrella-like fashion, which could be used to help lodge the fish into its host. All members of the subfamily Vandelliinae share the traits of blood parasitism, with parasitism in general being a shared ancestral trait of all members of Trichomycteridae. They have individual claw-like teeth for this purpose. Their bodies are very small and elongated to easily slip into the gills of host fish.

Candiru inhabits the Amazon and Orinoco basins of lowland Amazonia. This fish lives in shallow water with muddy, sandy, or rocky bottoms. It can be found in riffles. Its distribution is patchy and it does not seem to move very far from its spawning sites. One location that Vandellia cirrhosa is specifically known to inhabit is the Purus River of Brazil. This location is hard to study due to its geographic isolation, something that is common among the habitats of candiru.

Candiru are hematophagous and parasitizes the gills of larger Amazonian fish, especially catfish of the family Pimelodidae (Siluriformes) and members of the family Characidae. However, it has been known to parasitize many species in the same location, suggesting that its feeding habit is based more on availability than species preference. Vandellinae is one of only two groups of jawed vertebrates that exclusively feed on blood. The feeding mechanism of the candiru was not understood until fairly recently, but many theories had been proposed before. Some suggested that it uses its sharp teeth to latch onto an artery or vein and stays attached to the host until it has ingested enough blood. It then lets go of the host and continues swimming. Others suggested that it extracts blood from its hosts by latching onto the gill filaments, which bleed freely into the alimentary tract of the candiru. In one experiment involving Vandellia cirrhosa, no evidence of gill damage was found on the fish hosts. Thus, the experimenter suggested a hypothetical blood-pumping mechanism in which the candiru is able to quickly ingest large amounts of blood without permanently damaging the host. Modern experiments have shown that the candiru feeds by approaching a host fish and swimming alongside it until close to the gill cover. It then attempts to penetrate the gill chamber by forcing itself underneath. …