How do annelids regulate water and salt balance?

Annelids regulate their internal water and salt balance through specialized excretory organs called nephridia, which function through filtration, resorption, and secretion.1

The specific mechanisms depend on the environment:

• Marine Annelids: Many marine polychaetes are osmoconformers, meaning their body fluids are isosmotic with seawater. Because they lack mechanisms to control water intake, they cannot tolerate significant salinity fluctuations. However, some species, such as Nereis diversicolor, can survive intertidal salinity changes by using enlarged nephridia to excrete excess water.23
• Freshwater and Terrestrial Annelids: Freshwater oligochaetes and leeches must actively regulate their internal medium to counter the osmotic influx of water. Earthworms, which function similarly to freshwater animals, use complex, multi-region nephridia to resorb inorganic ions like sodium and calcium from their coelomic fluid. They produce large quantities of dilute urine to eliminate excess water, a process that can involve up to 60 percent of their body weight daily. Additionally, earthworms may excrete urea to assist in controlling their salt and water balance.214

The coelomic fluid also plays a vital role in regulating fluid transfer through the body wall.2