String jellyfish are colonial animals that may reach 3 m in total length and have a diameter of 2–5 cm.

The colony is formed of a central string, bearing groups of pink and white tentacles, which clump together or extend.

[4] These ocean predators act like drift nets, spreading their tentacles to catch plankton.

[2][3][5] As of 2025[update] string jellyfish have been proliferating in seas around Norway and Scotland, and causing mass deaths of farmed salmon.

The increase has been attributed to rising sea temperatures, and to fish farms discarding waste upon which the jellyfish feed.