Brooklyella hostilis (Lom and Nigrelli 1970) is a pathogenic dysteriid ciliate of tropical marine fishes.  Few reports have documented Brooklynella as a significant disease causing organism of marine fishes in captivity.  First described in 1970 from sporadic outbreaks in the New York Aquarium, the ciliate has been responsible for numerous mortalities at public aquaria, commercial importers and in retail stores.  Brooklynella was implicated as the causative agent of tropical fish mortalities at Steinhart Aquarium, San Francisco from 1977-73.  The ciliate was confirmed from three species of anemonefishes: Premnas biaculeatus (spine-cheek anemonefish), Amphiprion ephippum (red saddleback anemonefish), and Amghiprion percula (clown anemonefish).  In a separate occurrence, Brooklynella was confirmed from shipments of Hippocampus kuda (Pacific seahorse) that originated in the Philippines. In 1979, the ciliate was confirmed as the causative agent responsible for mortalities of A. frenatus (tomato Anemonefish) and P. biaculeatus, at a San Francisco importer.  In 1980-81, various species of anemonefishes, including P. biaculeatus and A. ephippium were found to be parasitized by Brooklynella at two Los Angeles fish importers.

Brooklynella, is a skin and gill parasite, although in the initial stages of an infestation it is primarily found on the gills.  Clinical signs include respiratory distress, hypersecretion of mucus, desquamation, hemorrhage and gill hypertrophy.  Outbreaks of Brooklynella are correlated with environmental stress due to trauma, high concentrations of ammonia, elevated temperatures, overcrowding, etc.  Eradication of the parasite is achieved using either freshwater dips, formalin baths or prolonged immersion in a mixture of formalin (25 mg/l) and malachite green (0.10 mg/l).  Chemotherapy with copper ion at concentrations of 0.15 to 0.20 mg/l is ineffective for eradication of the parasite.