Why do earthworms excrete more urea than marine annelids?

Earthworms excrete more urea than marine annelids primarily as a physiological mechanism to control their salt and water balance.1

While aquatic annelids typically excrete ammonia as their chief nitrogen-containing waste product, earthworms are adapted to life in the soil. Although they are terrestrial, earthworms maintain relationships with their environment that are characteristically similar to those of freshwater animals. To survive in these conditions, they utilize more complex nephridia than their marine counterparts to manage osmoregulation—the maintenance of an internal balance between water and dissolved materials. The production and excretion of urea, alongside the resorption of inorganic ions like sodium and calcium, allow earthworms to regulate their internal medium effectively.123