Module 1
01.
Lesson 1 : Definitions and Introduction to the 8 Criteria
This lesson defines nociception, pain, and suffering while introducing the eight criteria of sentience, essential for assessing animals' perception and response to pain.
What is nociception? What is pain? What is sentience?
How can we say that an animal is feeling something bad or good?
Nociception
Nociception is the detection by a nervous system of actually or potentially noxious stimuli achieved by means of specialized receptors called nociceptors. It is possible, in principle, for a noxious stimulus to be detected without any experience or feeling on the part of the system that detects it (Birch et al, 2021).
Sentience
Sentience is the capacity to have feeling and to feel for example pain, distress, anxiety, joy… We know that mammals and birds are sentient being. But what about shrimp? And what does it involve if they are?
Pain
Pain is a sensation that allows us to perceive the intensity, duration, and location of a harmful stimulus. It is not limited to a physical feeling but is also linked to an emotion influenced by the interpretation of the situation. Memory plays an important role in pain, helping to remember painful experiences to avoid them in the future.
Suffering
Suffering is an emotional state of distress associated with events that threaten the biological or psychological integrity of the individual. The state of suffering can occur in the absence of physical pain and be purely psychological (Ilana, E. (1979). Pain terms; a list with definitions and notes on usage. Pain, 6, 249).
What you will find in this lesson
References such as “Dupont et al., 2015” or “Martin & Durand., 2015” refer to the authors of the scientific publications on which the presented information is based. They ensure the accuracy and traceability of the data.
Criterion 1.
Criterion 1: Presence of nociceptors: sensory receptors of pain
Nociceptors are specialised sensory receptors that detect harmful stimuli and trigger a protective response against possible injury. In several species, these receptors are only activated by extreme stimuli (heat, cold, intense pressure or irritating chemicals).
Studies on pain perception in crustaceans, particularly Louisiana crayfish, show an increased response to thermal stimuli, suggesting the presence of nociceptors. The electrical activity observed in these experiments indicates that these animals react differently to hot water compared to a saline solution, providing neurophysiological evidence of their ability to detect harmful stimuli (Puri & Faulkes, 2010; Puri & Faulkes, 2015).
In shrimp, research has also identified the presence of nociceptors, particularly in marsh shrimp.
Criterion 2.
Criterion 2: Brain regions associated with pain perception
Shrimps have a central nervous system organised around a ventral nerve cord composed of ganglia and nerve fibres, as well as numerous sensory receptors (Sandeman et al, 2014; Strausfeld et al, 2020).
Their brains include structures dedicated to the integration of multimodal information, in particular visual and mechanosensory information, suggesting an advanced capacity for sensory processing (Meth et al, 2017).
The hemielipsoid body, involved in learning and memory, could play a role in perception and adaptation to painful stimuli. These elements indicate a potential for pain perception in farmed shrimp, although further studies are needed to better understand their reactions to noxious stimuli.
Criterion 3.
Criterion 3: Connections between brain areas and nociceptors
Shrimps have functional connections between their nociceptors and their brain structures via the nerve cord and ganglia. This nervous organisation suggests an integration of painful signals at the level of the central nervous system, reinforcing the hypothesis of a possible perception of pain (Sandeman et al, 2014; Strausfeld et al, 2020).
In farmed shrimp, this discovery underlines the importance of choosing slaughter methods that respect animal welfare, requiring consideration of the neuronal mechanisms involved in nociception (Birch et al, 2021).
Criterion 4.
Criterion 4: Behavioral response influenced by analgesics
The administration of analgesics influences the behavioural response of crustaceans by reducing signs of pain. In experiments on shrimps that had undergone eye-stem removal, behaviours indicative of pain were observed, such as erratic swimming in white-legged shrimps and tail flicking in large American shrimps (Taylor et al, 2004; Diarte-Plata et al, 2012).
Without anaesthetic, more than 80% of individuals showed these reactions, while the application of lidocaine significantly reduced their number. These results suggest that analgesics attenuate the pain perceived by these animals, thereby influencing their behaviour.
Criterion 5.
Criterion 5: Motivation trade-off between pain and reward
Hermit crabs make a compromise between pain and reward when looking for a better quality shell. When exposed to electric shocks, they leave their shells more quickly as the voltage increases, but this behaviour varies according to the quality of the shell (Appel & Elwood, 2009).
A better quality shell delayed their departure despite the pain, suggesting a decision based on a balance between discomfort and benefit. In addition, the presence of predator odour prolonged their waiting time before abandoning the shell, indicating that perceived risk influences their pain tolerance.
To date, no similar studies have been conducted on shrimp.
Criterion 6.
Criterion 6: Animal’s behavioral response to a painful stimulus similar to humans (e.g., avoidance; protection)
Pink shrimp (Palaemon elegans) react to painful stimuli applied to their antennae by adopting cleaning and rubbing behaviors against the substrate (Barr et al, 2008).
These behaviors are specifically directed toward the affected area and persist over time, suggesting conscious perception of pain rather than a simple reflex response (Elwood et al, 2017).
The application of acetic acid and sodium hydroxide significantly increased these behaviors, while the administration of an anesthetic considerably reduced them. These observations reinforce the hypothesis that shrimp feel pain and respond to it in complex ways.
Criterion 7.
Criterion 7: Behavioral response persists over time; learning occurs
Studies on the behavioural response of crustaceans to painful stimuli show that some species, such as the red-spotted crayfish, are capable of associative learning (Bhimani & Huber, 2016).
In experiments using different substrates, it was observed that these crayfish modified their locomotion after receiving electric shocks to certain areas, thus avoiding painful stimuli.
This behavioural adaptation, which persists over time, suggests a learning and memory process. However, no similar studies have yet been carried out on shrimps.
Criterion 8.
Criterion 8: The animal shows self-medication when injured
The study on the red swamp crayfish highlights a phenomenon of associative learning in response to substrate-induced pain. When exposed to different surfaces (smooth and hard vs. rough and soft), injured crayfish modify their locomotion, suggesting a behavioral adaptation to discomfort (Datta et al, 2018).
Additionally, experiments have shown that they receive amphetamine administration via a cannula, but the question of voluntary self-administration of an analgesic remains uncertain.
While these results indicate a form of associative learning and an active response to pain, no definitive evidence has been established regarding their ability to self-administer an analgesic when injured. Further research is needed to better understand these mechanisms in decapods.
What to remember
Some crustacean species, such as shrimp and crayfish, possess nociceptors—sensory receptors capable of detecting potentially painful stimuli. Their nervous system processes these signals, and studies have shown that their behavior may indicate an adaptive response to such stimuli.
The administration of analgesics reduces certain signs of distress in these species, and observations suggest forms of associative learning in response to aversive experiences. However, their ability to self-administer analgesics remains uncertain and requires further research. These findings highlight the importance of considering their welfare, particularly by adapting farming practices and slaughter methods.
The lack of specific studies on pain perception in certain species does not mean they do not experience pain. Instead, it underscores the need to apply the precautionary principle—assuming that these animals could feel pain and adopting welfare-conscious practices, even in the absence of definitive proof.
- Precence of nociceptors
- Brain regions associated with pain perception
- Connections between brain areas and nociceptors
- Behavioural response influenced by analgesics
- Motivational trade-off between pain and reward
- Animal's behavioural response to a painful stimulus is similar to that of humans
- Behavioural response persists over time; learning occurs
- Animal shows self-medication behaviour when injured