I, again, try to make the case against vegans eating oysters based on scientific literature and further examine the idea that simple nervous systems and differences in body plans are not synonymous with not being able to react and experience the world. I directly quote the studies and other literature so that what I write is completely transparent and not made up or based on ideology.
Unlike plants, but like most other invertebrates, oysters do have nervous systems. As we established in Part I and Part II, how developed those systems are does not automatically reduce them to the level of plants. In addition, as further discussed in Part I and Part II, because they have simple nervous systems does not mean that one can deduce that they are unable to respond to stimuli or have the inability to experience their own environment, particularly because we are incapable of truly understanding what pain and sentience are in other animals.
Just to mention the nervous system briefly, Carroll & Catapane (2007) state that, "Bivalve molluscs [this includes oysters] have a relatively simple bilaterally symmetrical nervous system composed of paired cerebral, visceral and pedal ganglia, and several pairs of nerves. The cerebral ganglia (CG) are connected to the visceral ganglia (VG) by a paired cerebrovisceral connective and the VG innervate each gill via branchial nerves."
Unfortunately, based on my review of the available data, there aren't that many studies focused on oysters. And those that exist seem to have an interest in human application or farming. As of this date, I could not find a specific paper devoted to the examination of nociception in oysters per se. However, that is not conclusive proof that nociception does not exist in oysters.
"The full length cDNA of a homologue of δ-opioid receptor (DOR) for [Met(5)]-enkaphalin was cloned from oyster Crassostrea gigas" by Liu et al (2015). These results, as outlined by Liu et al. (2015), "collectively suggested that CgDOR for [Met(5)]-enkephalin could modulate the haemocyte phagocytic and antibacterial functions through the second messengers Ca(2+) and cAMP, which might be requisite for pathogen elimination and homeostasis maintenance in oyster." Varga et al. (2004) describe, "delta opioid receptor (DOR) agonists are attractive potential analgesics, since these compounds exhibit strong antinociceptive activity..."
In addition, mu opioid receptors have been found in both blue mussels (Mantione et al. 2010) and oysters (Zhang 2012); these receptors are also antinociceptors.
Opioid peptides have also been documented in oysters. Liu, Chen, & Xu's (2008) described that, "The nervous and immune systems of invertebrates can exchange information through neuropeptides. Furthermore, some opioid peptides can function as endogenous immune system messengers and participate in the regulation of the immune responses." Their study concluded that their "data strongly suggests an involvement of opioid peptides in the regulation of the antioxidant defence systems of the Pacific Oyster." Endogenous opioid peptides have been described as inducing, "analgesia in humans and antinociception in animals. These peptides act in several regions of the CNS to mediate pain control, because antinociception is observed in animals whether endogenous opioid peptides are administered into the peripheral circulation; into spinal sites; or into various regions of the brain, such as the raphe nuclei, PAG region, or medial preoptic area. Many events or stimuli that are experienced as painful, stressful, or traumatic can induce the release of endogenous opioid peptides. These peptides then act to make humans and animals less sensitive to noxious events by inducing euphoria and analgesia or antinociception (Froehlich 1997)."
Why would oyters have any of these receptors or mechanism for antinociceptive activity? If they have antinociceptors, does that mean that they could have noticeptors as well? Regardless, it has been established above that opioid receptors have been found in oysters (topic further discussed in Part I), and “opiate systems may have a functional role in invertebrate nociception" (; ).
The following studies further show that oysters, although thought of as simplistic as plants by many, have nervous systems that are still complex and may use many of the same responses and regulations as other animal species.
Harrison et al. (2008) found that their study confirmed and quantified, "histamine as an endogenous biogenic amine in C. virginica in the nervous system and innervated organs...Histamine is a biogenic amine found in a wide variety of invertebrates, where it has been found to be involved in local immune responses as well as regulating physiological function in the gut. It also functions as a neurotransmitter, especially for sensory systems1. Histamine has been well studied in arthropods and gastropods, but has been rarely reported to be present or have a function in bivalves other than the limited reports identifying it in ganglia and nerve fibers of the Baltic clam." The authors further stated that, "Bivalves, including the oyster, Crassostrea virginica, contain dopamine, serotonin and other biogenic amines in their nervous system and peripheral tissues. These biogenic amines serve as neurotransmitters and neurohormones and are important in the physiological functioning of the animal." They also stated that,"The mantle rim of bivalves is a sensory structure containing various sensory receptors. The involvement of histamine in sensory systems of invertebrates, particularly gastropods, coupled with our preliminary physiology research, strongly suggest histamine to be a sensory neurotransmitter in the mantle rim of C. virginica."
In addition, Park et al. (2007) were able to clone and characterize, "Lipopolysaccharide-induced TNF-alpha factor (LITAF) is an important transcription factor that mediates the expression of inflammatory cytokines" in the Pacific oyster Crassostrea gigas." Interestingly, Zhang & An (2007) describe that, "there is significant evidence showing that certain cytokines/chemokines are involved in not only the initiation but also the persistence of pathologic pain by directly activating nociceptive sensory neurons.
Like in mussels, it has been shown that oysters control the beating of their cilia to draw in water, which they do as filter-feeders. Carroll & Catapane's (2007) study demonstrated that there is a "reciprocal serotonergic-dopaminergic innervation of the lateral ciliated cells, similar to that of M. edulis, originating in the cerebral and visceral ganglia of the animal..." This, therefore, means that ganglia (their nervous system) regulates movement/behavior. Perhaps, like in mussels (see Part I), oysters also have the ability to actively control, based on a form of decision-making, why they employ the types of ciliary movements they do.
"Bivalves readily utilize chemical exudates that ema nate from predators and from injured conspecifics to evaluate predation risk (Caro & Castilla 2004, Cheung et al. 2004, Smee & Weissburg 2006b) (Robinson et al. (2014). A study by Robinson et al. (2014) found that in the presence of predators, "oysters grew shells that required more force to crush and resultantly were afforded greater protection from crab predators." This supports recent studies that "have shown that oysters react to gastropod and crustacean predators by producing thicker, heavier shells (Newell et al. 2007, Johnson & Smee 2012, Lord & Whitlatch 2012)"(Robinson 2014). Again, these are examples that oysters actively respond to their environment (predation in this case) as any other animal species would when threatened.
The studies that I've quoted above are only bits and pieces of a large body of data that is yet to be uncovered or even studied. What all this means when put together is yet unknown because few studies have been done. However, it shows that although oysters have simple, yet efficient nervous system to respond to the type of lifestyle that they live, they also have sensory structures and receptors like those found in other animal species. In essence, they are still nothing like plants regardless if they are sessile species (Review Part I and Part II). The fact that they are sessile still does not mean that they do not need to react to their environment if simply to protect themselves and carry out functions in order to survive.
Pain in Invertebrates
It is important to note that, "the clear distinction that once existed between the terms “pain” and “nociception” has become blurred recently, to the point that many neuroscientists and clinicians no longer make a distinction; that is, most accept that nociception is equivalent to pain." (Sladky 2014)
In his essay examining pain and analgesia in fish and invertebrates, Dr. Sladky, from the University of Wisconsin, asks, "can we recognise pain in fish and invertebrates? Is the perception of pain by a fish or an invertebrate equivalent to that of a mammal? We will never be able to fully and objectively answer these questions, because the animals simply cannot tell us...Could it be that recognition of pain in fish and invertebrates is impeded by our inability to empathise with species that do not convey distress through facial expressions, do not vocalise in response to distress, and are not warm and fuzzy?"
Dr. Sladky states that "our limited understanding of pain and analgesia in fish and invertebrates should not obscure our clinical decisions, and we should err on the side of fish and invertebrate well-being by making the assumption that conditions considered painful in humans and other mammals should be assumed to be potentially painful across all other vertebrate and invertebrate species."
"Although peripheral nociceptors have not been identified in cephalopods, there are no published reports that anyone has investigated peripheral nociception in cephalopods. On the other hand, nociceptors have been identified in anemones, sea cucumbers, leeches, nematodes, Drosophila, and many other insects (Kavaliers 1988; Tobin & Bargmann 2004; Xu, et al. 2006; Smith & Lewin 2009; Puri & Faulkes 2010)...Many invertebrate species (earthworms, roundworms, molluscs, Drosophila) possess endogenous opioid receptors (Dalton & Widdowson 1989; Tobin & Bargmann 2004). Immunohistochemical staining indicated the presence of endogenous opioid receptors in nematodes (Prior et al. 2007). Mussels possess benzodiazepine and opioid receptors in their nervous systems (Gagne et al. 2010). In addition, there is genetic and physiologic evidence that invertebrates and vertebrates may have similar capacities with respect to pain and analgesia..." (Sladky 2014)
"Pain-associated behaviour of invertebrates has been described in multiple species. In sea anemones, crabs, crayfish, sea slugs, snails, flatworms, crickets, praying mantis and Drosophila, withdrawal responses are observed with thermal and mechanical noxious stimuli..."(Sladky 2014).
The paper by Dr. Sladky is definitely worth the read because it is a nice summary of all the discoveries that have been made about fish and invertebrates with relation to pain. Read it here: http://anzccart.org.nz/wp-content/uploads/2014/08/Sladky.pdf
Albeit slowly, science has shown us that invertebrate species are not as simple as we once thought. So I ask, what basis is there for not erring on the side that potentially oysters, and other invertebrates, that have yet to be studied in detail, also have the ability for these mechanisms and behaviors?
Environmental Impacts of Oyster Farming
Refer to Part II for a look at the negative effects associated with oyster farming.
The author of this post has a B.S. in Biological Sciences with an emphasis in Marine Science and a M.Sc. in Conservation & Ecology with an emphasis in research. Other experiences include, but are not limited to, aquaculture, molecular biology, fungal and plant symbiosis, and invasive species ecology. The author is also vegan, which means the author does not consume or consciously exploits any species of animal.
Carroll & Catapane (2007) The Nervous System Control of Lateral
Ciliary Activity of the Gill of the Bivalve Mollusc, Crassostrea
virginica, Comp Biochem Physiol A Mol Integr Physiol, 148(2): 445–450.
Froehlich (1997) Opioid Peptides,Neurotransmitter review, Vol. 21, 2.
Harrison et al. (2015), The Presence of Histamine and a Histamine Receptor in the Bivalve Mollusc, Crassostrea virginica, In Vivo, 36(3): 123–130.
Liu et al. (2008), Effects of Leucine-enkephalin on Catalase Activity and Hydrogen Peroxide Levels in the Haemolymph of the Pacific Oyster (Crassostrea gigas), Molecules.
Liu et al. (2015), The immunomodulation mediated by a delta-opioid receptor for [Met(5)]-enkephalin in oyster Crassostrea gigas. Dev Comp Immunol. 2015 Apr;49(2):217-24.
Mantione et al. (2010), Seasonal variations in mu opiate receptor
signaling in the nervous system of the blue mussel, Mytilus edulis:
temperature controls physiological processes, ISJ 7: 141-145.
Park et al. (2008), Cloning, characterization and expression analysis
of the gene for a putative lipopolysaccharide-induced TNF-alpha factor
of the Pacific oyster, Crassostrea gigas. Fish Shellfish Immunol. 2008 Jan;24(1):11-7.
Robinson et al. (2014), Eastern oysters Crassostrea virginica deter crab
predators by altering their morphology in response to crab cues, Aquatic
Biology, Vol. 20: 111–118.
Sladky (2014),“I’ll have the fish and shrimps”: pain and analgesia in
invertebrates and fish, http://anzccart.org.nz/wp-content/uploads/2014/08/Sladky.pdf
Varga et al. (2004), Agonist-specific regulation of the δ-opioid receptor, Vol. 76, Issue 6, Pages 599–612.
Zhang & An (2007), Cytokines, Inflammation and Pain, Int Anesthesiol Clin.;