Why did the research team set up four different treatments?

The European Green Crab is one of the world's largest ten most aggravating species. This tiny, very hardy coastal crab fights for prey and can destabilize the general equilibrium of the marine ecosystem.

Researchers investigated whether the predatory European green crab (Carcinus maenas) populations imposed similar selective pressures on their gastropod prey, the flat periwinkle (Littorina obtusata).

Periwinkles from the Gulf of Maine's southern sites have been predated by European green crabs for over 100 generations, at a rate of around one generation per year. Periwinkles from northern

Gulf locations have been interacting with invading green crabs for only a few generations, as the invasive crabs have only recently expanded to the northern Gulf.

The periwinkles and crabs were collected from places distant by 450 kilometers of seashore in Maine's northern and southern Gulfs. A single crab was housed in a cage with eight periwinkles of varied sizes. The study identified the destiny of the eight periwinkles following three days.

Four different treatments were devised, with crabs from northern and southern populations receiving periwinkles from northern and southern populations.

The scientist performs four different tests to see if the origins of the crabs' and periwinkles (northern or southern) influenced their behavior. As a result, all crabs were about the same size and had an equal number of males and females. Each experimental treatment was put through its paces 12–14 times.