Are Shrimps Sentient?
What is the evidence for sentience in shrimps? And what is sentience, anyway?
This article will detail a definition of sentience, present the current scientific and legal consensus on shrimp sentience, and finally review the evidence of sentience in shrimps
What is Sentience?
Sentience (from the Latin sentire, to feel) is the capacity to have feelings . These may include feelings such as pleasure or pain. But these aren’t the only experiences that matter. An individual can have other positive or negative experiences such as warmth, boredom, contentment, frustration or joy*. A simple way to capture this idea is to ask: “Does it feel like something to be this individual?”. Sentience differs from self-awareness, which refers to awareness of awareness - or rather, awareness of your own sentience** .
The crux of disagreement around whether an animal is sentient (or can feel pain) often isn't around the facts (i.e. whether an animal can experience these positive or negative states), but rather around our understanding of the words we are using. For example, someone might claim that an animal can't feel pain because that person associates the word pain with human pain, and they are emphasising the difference between what a human experiences and what an non-human experiences. But note that this isn’t a disagreement in the facts, but around the meanings of words.
* Other examples include an individual suffering through severe hunger or thirst, or suffering emotionally (such as being separated from their mother).
** It has also been argued that being self-aware may reduce the severity of suffering . For example, a human can think about or discuss an emotional state, leading to the realisation that negative consequences may not be as bad as they feared, or that if they were suffering from something such as food poisoning, they know it will likely end after a day or two.
The Precautionary Principle
It’s worth discussing degrees and likelihood of sentience. Often the conversation around sentience is framed as if sentience is an “on/off switch”, but sentience may vary significantly among species (i.e. sentience in a shrimp is likely very different from sentience in a human, which in turn is likely very different from sentience in an octopus).
Additionally, we don’t need to be in a position to state with complete certainty whether an animal is sentient. Instead we should take into account how likely we think an animal is sentient. If there is a non-negligible chance that an animal is sentient, we should consider their interests when deciding how to treat them. In other words, we don't need to be 100% sure an animal is sentient to believe they are morally relevant, and to take actions to prevent their suffering.
This idea is sometimes referred to as The Precautionary Principle:
"Where there are threats of serious, negative animal welfare outcomes, lack of full scientific certainty as to the sentience of the animals in question shall not be used as a reason for postponing cost-effective measures to prevent those outcomes.” 
Scientific Conclusions on Shrimp Sentience
In November 2021, the London School of Economics and Political Science (UK) published a report titled "Review of the Evidence of Sentience in Cephalopod Molluscs and Decapod Crustaceans" . The report had been commissioned by the UK Government's Department for Environment, Food and Rural Affairs (DEFRA). The report’s central recommendation was:
We recommend that all cephalopod molluscs and decapod crustaceans be regarded as sentient.
Previously, in 2005, the EU’s European Food Safety Authority published their Scientific Opinion on the “Aspects of the biology and welfare of animals used for experimental and other scientific purposes” .
The report recommended that (pg 17):
All decapods should be in Category 1 and so receive "protection.".
With Category 1 is defined as (pg 20):
“The scientific evidence clearly indicates that those groups of animals are able to experience pain and distress, or the evidence, either directly or by analogy with animals in the same taxonomic group(s), are able to experience pain and distress.”
Shrimp Sentience in Legislation
The following countries cover decapod crustaceans in legislation:
Austria - The Austrian Animal Welfare Act 2004  includes decapods, requiring water quality parameters (such as temperature and dissolved oxygen levels) to be maintained within acceptable ranges.
Switzerland - The Animal Welfare Ordinance Act 2008  requires provisions for decapods such as stunning before slaughter, and adequate water quality during housing.
Norway - Legal protection for decapods was recognised by the Norwegian Animal Welfare Act 2010  including regulations on killing, confining and transport. The Act also requires that: “Animals shall be treated well and be protected from danger of unnecessary stress and strains.”
UK - In April 2022, the UK Animal Welfare (Sentience) Act  passed through the UK Govt., becoming law. The Act recognised the sentience of cephalopod molluscs and decapod crustaceans (including shrimps).
Evidence of Sentience in Shrimps
There is relatively strong evidence that larger decapod crustaceans (i.e. true crabs) are sentient . Unfortunately compared to larger decapods, penaeid shrimps have barely been studied . It is important, however, that protection of decapods should not be restricted in a way that privileges the most intensively studied laboratory species. Existing legislation, for example, extends protection to all vertebrates using evidence based on generalisations from intensively studied species (such as rats) to relevant species .
Let’s have a look at how shrimps respond in relation to indicators of sentience. Unfortunately, studies of P. vannamei tend to focus on feeding behaviour and production practices such as stocking densities and mating. Therefore, research on motivational trade-offs, and information processing that go beyond habituation and sensitisation are currently missing . The lack of this research is due to an information gap, and does not reflect absence of this phenomena.
The following is a framework we can use to assess sentience in decapods :
Possession of nociceptors - Let's say you accidentally touch a hot stove. Instantly, you will withdraw your hand from the stove. A simple circuit of neurons saved your hand from a noxious stimulus (the hot stove) and prevented you from a burn. This is referred to as nociception, named after nociceptors, the sensory receptors that detect the noxious stimuli.
Possession of integrative brain regions - Being able to integrate information from multiple different sensory sources.
Connections between nociceptors and integrative brain regions - Neural pathways connecting nociceptors to the brain (though it should be noted that the nervous system is organised very differently in elongated species such as shrimps, compared to compact species, such as crabs).
Responses affected by potential local anaesthetics or analgesics - If you have been to the dentist, you have probably been applied a substance to numb an area of your mouth so that the dentist can perform a procedure that otherwise would be uncomfortable or painful. That’s an anaesthetic, a substance that creates a lack of feeling. When used locally, anaesthetics reduce any sensation (not only pain) in a target area, while the individual remains conscious. If an animal responds differently to stimuli while under the influence of anaesthetics, this suggests the lack of feeling is mitigating an animal's experience of pain or distress.
Motivational trade-offs that show a balancing of threat against opportunity for reward - An animal can make a decision, like avoiding negative stimuli, such as an electric shock, rather than experience a positive stimuli (i.e. their actions cannot be explained by reflexes alone).
Flexible self-protective behaviours in response to injury and threat - Guarding, grooming or otherwise tending to an injured body part are protective behaviours . These are typically long term responses to injury, seen over periods of hours and days rather than seconds and minutes. In humans, protective behaviour is often controlled by the conscious feeling of pain, such as avoiding using an injured limb.
Associative learning that goes beyond habituation and sensitisation - If an animal can experience something, learn from it, and then make different decisions in the future, it provides good evidence of sentience.
Behaviour that shows the animal values local anaesthetics or analgesics when injured - For example, learning to self-administer anaesthetics or analgesics, or could learn to prefer a location where these can be accessed, or prioritise obtaining these over other needs (i.e. food) when injured.
Shrimps react in a way that suggests nociception when encountering a predator , or when subjected to physical pitches or electric shocks . The animals flip their tail, allowing them to escape from a potentially dangerous stimulus.
Physiological responses to negative stimuli have also been documented in shrimps. For example, extreme temperature changes   or injections of noxious heavy metals  (e.g. copper and mercury) lead to the release of the crustacean hyperglycemic hormone (CHH). This hormone primarily regulates blood sugar, and it is typically released when crustaceans are subjected to physical stressors.
If an injured animal reacts similarly to a human to a local anaesthetic, we may conclude that the substance reduces sensation in that area. That sensation is probably pain if we observe, for example, that the injured animal does not groom or show other nociceptive behaviour–while their regular activity is not altered.
It has been observed that when whiteleg shrimps undergo eyestalk ablation, the local anaesthetic Xylocaine reduces behaviours potentially indicative of stress like levels of feeding and unusual swimming behaviour . Another local anaesthetic also proved to inhibit tail-flicking and rubbing responses to irritant substances on shrimp antennae .
It has also been argued that for species which inhabit three-dimensional space, such as water, the complexity of inhabiting this world may demand special neural networks that allow a sophisticated level of cognition for negotiating space in three dimensions .
In the case of shrimps, it has been documented that mutilating the animals’ eyestalk caused them to rub the affected area . Similarly, when one of the antennae was pinched, shrimps also specifically groomed this antenna . The same study reports that brushing an irritant substance onto one of the antennae caused a similar prolonged rubbing reaction in shrimps .
When shrimps have an eye mutilated (eyestalk ablation), the animals do not only tail-flip but also show a relatively long-lasting unusual behaviour. For example, shrimps avoided sheltering  and swam erratically  right after having one of their eyes ablated and for at least half an hour.