Random things you find on wikipedia:
“Due to the ambiguous nature of suffering, the issue of lobster pain may be argued by analogy—that lobster biology is similar to human biology or that lobster behavior warrants assumptions that lobsters can feel pain.
“The Norwegian Scientific Committee for Food Safety tentatively concluded that “it is unlikely that [lobsters] can feel pain,” though they note that “there is apparently a paucity of exact knowledge on sentience in crustaceans, and more research is needed.” This conclusion is based on the lobster’s simple nervous system. The report assumes that the violent reaction of lobsters to boiling water is a reflex to noxious stimuli.
“However, review by the Scottish animal rights group Advocate for Animals released the same year reported that “scientific evidence … strongly suggests that there is a potential for [lobsters] to experience pain and suffering,” primarily because lobsters (and other decapod crustaceans) “have opioid receptors and respond to opioids (analgesics such as morphine) in a similar way to vertebrates,” indicating that lobsters’ reaction to injury changes in the presence of painkillers. The similarities in lobsters’ and vertebrates’ stress systems and behavioral responses to noxious stimuli were given as additional evidence.”