Gastrosaccus spinifer is a shrimp-like crustacean in the order Mysida, the opossum shrimps, native to the eastern Atlantic Ocean and the coasts of Northern and Western Europe.

As in other female opossum shrimps, the biramous (branching) pereopods (limbs on the thorax) form a marsupium or brood pouch.

[2] The chief distinguishing factor for this species is the fact that the fifth abdominal segment is slender and has a long spine on its dorsal surface.

In the North Sea, it often forms swarms and has sometimes been found at densities exceeding 1000 individuals per square metre (11 sq ft).

[3] It clings to the opossum shrimp's carapace near the junction of the cephalothorax and abdomen and resembles a fleshy growth[4] or may occupy the host's brood chamber.