Jellyfish and other simple marine invertebrates lack the centralised nervous systems associated with pain in vertebrates. Understanding where the boundary of morally significant sentience lies is one of the frontier questions in animal welfare science.
Marine invertebrate welfare sits at the most uncertain frontier of animal ethics. The evidence for morally significant suffering in jellyfish and simpler invertebrates is much weaker than for vertebrates or even decapod crustaceans. However, the scale of human impact on these animals -- billions affected by trawling, ocean acidification, and aquaculture -- means that even small probabilities of sentience represent large expected welfare values. The most intellectually honest position combines rigorous investigation of the evidence with honest acknowledgement of uncertainty and precautionary consideration of the scale of potential suffering.