Brooklynella hostilis is a parasite of marine fish, found in wild fish, farmed fish and aquariums. In marine aquariums, it is most commonly found in clownfish. It feeds on dead skin cells and can cause severe damage to gills.
B. hostilis is kidney-bean shaped, and approximately 60–80 μm long, with bands of cilia.
The parasite spreads rapidly, and can easily transfer to a new host increasing the probability of cross contamination.
Affected fish have a gray discoloration, and may breath abnormally fast or abnormally slow. It can cause sloughing of skin, and congestion of the gills.