Gill aneurisms are an acute response of the gills to injury. For an aneurisms to occur in the gills there is a localized break in a blood vessel and pooling of blood in the area. Gill aneurisms appear microscopically as discrete, often rather circular swollen areas filled with red blood cells in the gill lamellae.
Gill aneurisms can be caused by a variety of injuries to the gills including rough handling, other physical trauma or in response to exposure to toxic chemicals.