Evidence of Pain Processing in Crabs Calls for New Welfare Laws

Decapod crustaceans such as crabs, lobsters, crayfish and shrimp are common ingredients in many culinary dishes. They can also be found in scientific research labs, where they are used as experimental models; the horseshoe crab served as a model for Keffer Hartline’s Nobel Prize-winning research.

For many years, decapods were viewed as non-sentient animals and deemed incapable of suffering. Consequently, they have been denied minimal care requirements – such as safe handling or humane slaughter – that are provided for other sentient creatures used in food and scientific research.

But in 2021, Professor Jonathan Birch and colleagues at the London School of Economics and Political Science (LSE) developed a novel framework for evaluating evidence of sentience.

The framework, which integrated the researchers’ expertise in comparative cognition, neuroscience, ecology, animal welfare and philosophy, was used to assess over 300 publications on decapods and other crustaceans. Such publications included a growing body of observational studies demonstrating pain-like behaviors in decapods after the application of electric shocks, acids or subjecting the animals’ tissues to mechanical impact.

This evidence called the non-sentience classification of decapods into question. If they demonstrate pain-like behaviors, they may possess nociceptors – receptors in the body that detect painful stimuli. How, therefore, can they be incapable of suffering?

Birch and colleagues argued that decapods and cephalopod mollusks should be regarded as sentient animals for the purposes of UK animal welfare law, which led to their inclusion in the Animal Welfare (Sentience) Act 2022.

This was a step in the right direction, Dr. Ben Sturgeon, CEO of Crustacean Compassion – an award-winning animal welfare organization that campaigns for the humane treatment and local protection of decapod crustaceans – told Technology Networks. “But no further legislative steps were taken.”

“Decapods are the only recognized sentient animals who are not covered by the Animals (Scientific Procedures) Act 1986, and, along with cephalopods mollusks, are the only sentient animals not covered by The Animal Welfare Act 2006 – a landmark piece of legislation that gives humans a duty of care of animals for animals under their control. They have some protections under regulations relating to transport and killing, but these are minimal, and unfortunately, woefully unenforced,” Sturgeon continued.

Now, new research from scientists at the University of Gothenburg calls for further action to protect decapods.

Shore crabs can process pain

Led by Zoophysiologist Dr. Lynne Sneddon, the study used electrophysiology methods to investigate whether electrical activity could be detected in the central nervous system (CNS) of shore crabs (Carcinus maenas) after different types of noxious and mechanical stimuli were applied to their soft tissues.

Pain is a complex physiological process that involves the peripheral and CNS. Nociceptors – pain receptors – are a key player in this process. “Nociceptors are receptors that preferentially detect potentially painful stimuli,” Sneddon explained. “We have nociceptors in our skin that respond to external noxious, injury-causing stimuli such as extremes of temperature, high mechanical pressure such as cutting and crushing, as well as chemicals, such as acids.”

“When we touch something very hot, for example, the nociceptors are excited and convert that external stimulation into nerve impulses that travel to the CNS. If the tissues are damaged then we experience pain, the negative affective state associated with the injured area,” she continued.

While observational studies have demonstrated pain-like behaviors in decapods – suggesting they might have nociceptors – Sneddon and colleagues’ study aimed to collect electrophysiological evidence of their presence in shore crabs.

They collected 20 crabs from Portugal and exposed their soft tissues to various concentrations of acetic acid. “We wanted to understand how crabs respond to external stimuli, especially noxious acetic acid, which is a valid pain test in other species,” Sneddon said.

Using a new electrophysiological technique, Sneddon and colleagues measured electrical activity directly from different regions of the crabs’ brains and an arrangement of nerves around the esophagus.

“Most studies in this animal group are conducted outside of the body where the nervous tissues are dissected out, placed into a saline solution at low temperature and, although we agree these are valuable studies, they do not reflect what is happening in the intact nervous system of the crab,” Sneddon explained.

Because there is little neurobiological information on how a crab responds to noxious stimuli, the researchers also tested the effects of mechanical stimulation of external soft tissues. This would allow them to explore any differences in the electrical recordings.

“Noxious stimulation of the antennae, antennules and the soft areas in between the joints of the walking legs and claws all elicited activity in the CNS,” Sneddon said. “This demonstrates that external stimulation using a potentially painful stimulus is conveyed to the CNS or brain.” What’s more, the activity of this pattern was different from when mechanical stimuli were applied.

“This phenomenon whereby CNS activity of painful stimuli is different to non-painful stimulation is seen in vertebrates such as fishes and mammals and is one of the key criteria for defining pain in animals,” Sneddon said.

When the researchers exposed the same areas of the crabs’ bodies to food odors or a drop of seawater, they did not detect any response in the CNS. “This demonstrates that chemically responsive nociceptors are present, since nociceptors do not respond to non-painful stimuli,” Sneddon explained.

Collectively, these data suggest that shore crabs possess a pain processing mechanism that transmits signals from noxious stimuli to the CNS for processing. Given that decapods share a similar structure and nervous system, the authors say it’s likely that shrimps, lobsters and crayfish do too.

“We have developed a means of recording CNS activity with reliable responses to noxious stimulation,” Sneddon said. “It would be possible to use the technique in other decapod crustaceans such as lobster, prawns, crayfish and shrimp or even in other crab species. It would be fascinating to understand if other decapods respond in the same way and increase our understanding of nociception and possible pain in these animals,” she added.

Safeguarding crustacean welfare

Sturgeon said that this study adds to the “now overwhelming burden of evidence” confirming decapod crustacean sentience: “The possession of nociceptors is the very first of the eight key criteria used to assess animal sentience. The paper demonstrates not just the evidence of pain receptors but also further sentience criteria – connections between nociceptors, peripheral and integrative brain regions (resulting in behavioral change), as well as self-protective behaviors in response to the injury and threat,” he continued.

We must, therefore, take a long and hard look at how we handle and slaughter these animals.

“They have some protections under regulations relating to transport and killing, but these are minimal, and unfortunately, woefully unenforced. All this means that they can still be subjected to horrific (and painful) practices while alive, including being sent alive in the post, sold alive for home killing and cooking, limbs torn off and chopped up like vegetables, stored on ice and asphyxiated,” Sturgeon said.

For Sneddon and colleagues, knowledge is power in this context. “If we know what excites their nociceptors, which may lead to pain, then we can avoid applying these stimuli to the animals to safeguard their welfare,” she said.

“This can also help us to understand how to humanely kill the animals providing them with a painless death,” Sneddon added. In a step toward achieving this goal, her team is looking to identify and characterize the nociceptors in shore crabs in more detail.

Reference: Kasiouras E, Hubbard PC, Gräns A, Sneddon LU. Putative nociceptive responses in a decapod crustacean: the shore crab (Carcinus Maenas). Biology. 2024;13(11). doi: 10.3390/biology13110851