Analgesia of invertebrates (eg, mollusks, arachnids, crustaceans, insects, echinoderms) is a controversial and neglected topic because of the belief that they may not feel pain. The issue may be in differentiating nociception from pain; nociception describes the neurophysiologic components leading to pain sensation but not the central perception of the input leading to pain sensation. By strict definition, pain is a subjective experience resulting from nociception and occurs in the cerebral cortex. Whether invertebrates perceive pain and suffer emotional stress remains uncertain. The discovery of nociceptive pathways like those in vertebrates has challenged previous beliefs; even without a CNS, nociceptive cells and responses can be present and opioid systems can have a functional role in invertebrate nociception.

Until pain in invertebrates can be definitively determined, analgesics (preferably inhaled agent) should be administered to any patient subjected to painful procedures, noting that some drugs used for invertebrates may have muscle-relaxing properties but lack anesthetic potency. Because insensitivity to painful stimuli only lasts as long as the animal is anesthetized, administration of analgesics would be advisable if the procedure is associated with significant postoperative pain. Diluted lidocaine may be used for topical analgesia above a surgery site.

Commentary
Invertebrates represent a diverse group of animals used in biomedical research and held in zoos, aquariums, and increasingly as pets. Veterinary science is still far from understanding the anatomy, physiology, and pathophysiology of invertebrate species. Because data are lacking, this article provided an outstanding overview of anesthesia and analgesia relating to this group. As advocates for the health and well-being of all animals, veterinary professionals must be aware of the need for appropriate analgesia and methods of anesthesia for this group.—Anthony Pilny, DVM, DABVP

Source
Clinical anesthesia and analgesia in invertebrates. Lewbart GA, Mosley C. J EXOTIC PET MED 21:59-70, 2012.