This organism was known by the name Costia necatrix for many years and the name Costia is still used as something of a common name. It is a frequent parasite of goldfish and koi but is often found on tropical fish as well.
Observation of these parasites is made difficult by their small size, less than 10 microns. Because they are relatively flattened their movement through the water gives a flickering appearance as the flat side or edge is alternately presented to the viewer.
Costia from the gills of a goldfish. These are loose live organisms moving along the coverslip, showing their dorso-ventral view with the single vacuole obvious in some.
Costia on the edges of a sample of skin from a fish. These are attached live organisms showing an edge view.
