Fish Kidney

The fish kidney is a multitasker. Beyond filtering waste, it is the primary lymphoid organ - producing red and white blood cells throughout life (unlike mammals, where this shifts to bone marrow after birth). Its urine output changes drastically between freshwater and saltwater species: freshwater fish produce copious dilute urine to expel excess water; saltwater fish produce small volumes of concentrated urine to conserve water.

Elongated dark organ running dorsally along the body cavity, beneath the vertebral column. Divided into head kidney (immune and hematopoietic) and body kidney (excretory). Made up of nephrons - the functional filtration units.

Osmoregulation (water/ion balance), nitrogenous waste excretion (limited role - most goes through gills), blood cell production, immune function, and endocrine (renin-angiotensin system).

Freshwater fish: large kidneys, many nephrons, large urine volume. Saltwater fish: reduced glomeruli, smaller urine volume. Sharks: retain urea in blood for osmotic balance.

Nephritis, dropsy (fluid accumulation from kidney failure), polycystic kidney disease in some ornamental lines.

Because the fish kidney doubles as the primary immune organ, injecting a vaccine directly into the kidney (as in some salmonid RTA vaccines) triggers a stronger immune response than muscular injection.