[4] P. sedentaria is known to employ multiple feeding strategies and other interesting behaviors, including daily vertical migration.

, and both visual fields are largely overlapping, meaning that they have fine binocular vision in a narrow beam aimed directly above.

[8] Phronima sedentaria is found in temperate, subtropical, and tropical waters of all the world's oceans, including the Mediterranean Sea.

[9] Phronima sedentaria most commonly exhibits a symbiotic relationship with tunicates of the genera Pyrosoma spp., Doliolum spp., and Salpa spp.

[10][11][12] Females of P. sedentaria live in the barrel-like bodies of salps, pyrosomes, and cnidarians,[3] and use their strong pleopods to propel their homes through the water.

The cells on the tunic layer may serve various functions for the P. sedentaria, including protection from UV light, storage of acid, and defense against microorganisms, like bacteria.

[4]While certain prey (salps, pyrosomes, and cnidaria) have additional uses for P. sedentaria in hosting their young and providing feeding platforms, the species also pursues other zooplankton, such as krill, arrowworms, and other crustaceans.

[4][15] P. sedentaria use different feeding techniques depending on the food source, but the leading sets of pereiopods (front legs) are primarily used in all cases.

[4] Feeding preferentially occurs at nighttime when members of this species undergo a vertical migration of around 200–350 meters to the ocean's surface.

[5][16] P. sedentaria typically migrate to hypoxic areas (such as the Oxygen Minimum Zone) during the day, causing low metabolic rates and physical activity.

[16] Known predators of P. sedentaria include the longnose lancetfish,[17] European flying squid,[18] Pacific pomfret,[19] albacore,[7] and skipjack tuna.