Gill rot disease in fish is due to:
A. Aeromonas species
B. Branchiomyces sanguinis
C. Bacillus polymyxa
D. Bacillus subtilis

Hint: Fishes suffer from various bacterial and fungal diseases. Gill rot is one such fungal disease in freshwater fishes. The gills are swollen and become discoloured. The affected fishes die by choking. These diseases are one of the main causes of fish mortality.

Complete answer:
Aeromonas species belong to a family of bacteria called Enterobacteriaceae. They are rod shaped gram negative anaerobic bacteria. These organisms are known pathogens to the human intestine and extra intestinal parts. They are found in freshwater and in brackish water. In humans, Aeromonas species are known to cause gastroenteritis and have also been associated with respiratory diseases.
In fishes, this species generally affects catfishes and bass. They cause a disease like septicemia and Red sore disease in fishes. Commonly, this species of bacteria inhabits the gastro-intestinal tract of the fishes. In intensive fish farming culture systems, stress is undoubtedly the most important factor in causing diseases by Aeromonas species. Stress can happen due to overcrowding, poor water quality. Methods of prevention include non-crowding of fishes in ponds or aquaria, providing proper nutrition and excellent water quality.
Branchiomyces sanguinis is a common fungal pathogen of fishes. It causes gill-rot disease in mostly freshwater fishes. These fungi transmit the disease through spore formation. The spores are liberated water, and they adhere to gills of the fishes. It is in the gills, that the spores form hyphae and penetrate the blood vessels of the gills. Fishes become weak, show respiratory problems and gills exhibit necrosis. Prevention methods include water sanitation, removal of infected/dead fishes from the pond treatment with chemicals.
Bacillus polymyxa is also known as Paenibacillus polymyxa. These species are gram positive bacteria and are known to fix nitrogen. Various scientific studies have shown that Bacillus polymyxa is capable of secreting natural antimicrobial compounds that improve the quality of water and disease resistance in freshwater fishes.
Bacillus subtilis is a gram-positive rod-shaped bacterium found in the upper layer of soil. This bacterial species is considered to be non-pathogenic and is known to secrete antibiotics and enzymes with numerous medical, pharmaceutical and agricultural uses.

Hence, option (B) is the correct option.

Note: This disease is also known as Branchiomycosis. Gill rot disease is caused by fungal species like Branchiomyces sanguinis and Branchiomyces demigrans. Good water management techniques, usage of lime and potassium permanganate can reverse or prevent the disease. The exact causes of fish disease are still unknown, but natural droughts, pollution are said to be responsible for it. Proper sanitation, water management and nutrition are ways to prevent these diseases.