Green shore crabs are used so extensively in laboratory experiments that they’ve been called the “marine biologist’s rat.” Indeed, in the U.S., rats and crabs used for research are both excluded from the Animal Welfare Act. 

It wasn’t until 2021 that the U.K. became the first Western country to legally recognize decapod crustaceans (including green shore crabs) as sentient — and, therefore, worthy of protection in research animal care protocols. To improve decapod crustacean care in laboratory settings, researchers studied how different laboratory conditions affect their survival rates and other physical vitality measures. 

The researchers specifically studied the effect of “stocking density” (the number of crabs per cage) and “environmental enrichment” (shelters made of plastic pipes) on wild male green shore crabs. They were especially interested in studying wild-caught crabs as they assumed these animals have different needs than crabs raised in laboratory settings. They trapped the crabs in September 2018 in Newfoundland, Canada. Then they secured foam tags on the crabs who would be tracked during the study.

To study the effect of stocking density, researchers housed tagged crabs in cages with three different levels of crowdedness: 1 tagged crab alone, 1 tagged crab plus 3 untagged crabs, and 1 tagged crab plus 7 untagged crabs. To study the effect of environmental enrichment, researchers added shelters to half of the cages (they note that in the wild, green shore crabs seek shelter to hide from predators and light). Overall, this means that there were six conditions — 1, 4, or 8 crabs with shelter and 1, 4, or 8 crabs without shelter. There were 24 tagged crabs in each condition, for a total of 144 tagged crabs in the study.

Researchers examined the crabs two times a week for six months (September 2018-March 2019). They tracked mortalities among all crabs plus five indicators of vitality (i.e., signs of an animal’s general health state) in the tagged crabs: limb loss, claw strength, blood protein concentration, “righting time” (the time it takes to return to a regular position after being placed on their backs), “leg flare” (a reflex of extending their legs horizontally), and “leg retraction” (a reflex of pulling limbs inward). Notably, only the tagged crabs were handled.

The authors found the following:

• Mortality Results: The tagged crabs in all conditions died steadily from four weeks onward. Neither density nor shelter affected their survival. One key result is that 67% of the tagged crabs died in the study period compared to less than 4% of the untagged crabs. According to the authors, this result suggests that repeated handling decreases wild crab survival in laboratory settings.
  
• Vitality Results: The tagged crabs’ vitality deteriorated over time, especially after 8 to 11 weeks in the laboratory. Density did not affect any vitality indicators. Tagged crabs who had shelter in cages alone or with three untagged crabs lost fewer limbs than tagged crabs without shelters at these low densities. As for leg retraction, crabs with shelters performed significantly better than those without shelters, but the only tagged crabs who maintained a strong response throughout the study were single crabs with a shelter. Overall, the effect of shelter on vitality was unclear.

In general, the crabs’ welfare declined over time. In other words, even minimal handling in research settings appears to reduce their survival and vitality. The authors emphasize that wild green shore crabs experience laboratory conditions differently from other animals, such as laboratory-bred vertebrates who are more commonly included in animal care protocols. They argue that these results can be used as a baseline for creating more inclusive animal care protocols.

One limitation of this research is that only two laboratory conditions — density and shelter — were directly studied. There are many other laboratory features (e.g., bright lights, noise, and bacteria present in cages) that could stress crabs. Another issue is the failure to report how many untagged crabs were used in the study. This number matters scientifically because untagged crabs who died during the study were replaced with new crabs, which may have impacted the tagged crabs being studied. This number also matters ethically because it recognizes crabs as individual, sentient beings whose lives are worth counting.

Animal advocates can use this study to raise awareness about green shore crabs, as well as other decapod crustaceans, in laboratory experiments. They can also point to the results as justification for expanding laws and animal care protocols to include all animals exploited for research, in addition to promoting the use of animal-free research alternatives.