Scientists studying the parasitic Silver lamprey, which feeds on the blood of larger fishes in North American rivers and lakes, have found that it behaves like the candiru catfish by going for the gill area.
Philip Cochran and John Lyons of Saint Mary's University of Minnesota and Wisconsin Department of Natural Resources studied Silver lampreys, Ichthyomyzon unicuspis in the Wisonsin River and found large numbers of them attacking the gills of large Paddlefish, Polyodon spathula.
They recorded the attachment sites of 1578 lampreys and found that 6.5% of them had attacked the branchial cavity of the Paddlefish, which the scientists say is more than would be expected by chance, given the relative surface area of the gill chamber.
In some Paddlefish as many as four lampreys had attacked the branchial cavity and the authors believe that the phenomenon may have been underreported because the parasitic fish are concealed from view.
The scientists believe that the Silver lamprey may use a similar technique to the parasitic candiru catfishes of South America which bite into the gill tissue and gain a meal of blood from their host through the blood pressure of their prey.
They also suggest that Silver lampreys may attack the branchial cavity of the Paddlefish because they are then protected against dislodging when the Paddlefish jump out of the water in an attempt to shake them off.
For more information see the paper: Cochran, PA and J Lyons (2010) - Attachments by parasitic lampreys within the branchial cavities of their hosts. Environmental Biology of Fishes, 10.1007/s10641-010-9646-5.