Arvuna is a moon orbiting the planet Dranen. Arvuna is considered a water world, despite not actually being a planet, because 90% of its surface is water or ice. It is home to a considerable and diverse amount of sealife, as well as a myriad of venomous arthropods similar to the creatures found on Palaven. Their carapaces allow them to resist the radiation that comes from Dranen.
Humans have built several colonies, well-shielded from radiation, though the location in the galaxy has made them isolated and all but inconsequential to galactic political powers like the Citadel and, later, the United Federation of Planets. The Reapers, even, failed to reach this remote moon before they were stopped.
Arvuna is seen in Mass Effect 3.