Allatostatin helps keep crabs safe when they shed their shells

Having a hard shell is excellent for protecting yourself, but it has its drawbacks. As animals such as insects, spiders and crabs grow, their protective covering doesn't grow along with them. So, what do these animals do when they've outgrown their shell? They break out of their old shell, having grown a new one underneath. But how does the animal know when their new shell is ready and when to break out of their older, smaller covering? Generally, there is a complex system of hormones that ready the animal to break out of its old shell and help the new shell harden. While this process is generally well understood in insects, much of the way this system works in crustaceans is still a mystery. In an attempt to decipher more of this mysterious cascade of events, Jodi Hoppes and Simon Webster of Bangor University, UK, teamed up with David Wilcockson of Aberystwyth University, UK, to see if the family of Allatostatin C proteins (which are normally involved in suppressing their appetite and stopping the crabs from growing) might also play a role when the green shore crab (Carcinus maenas) needs to shed its shell.

First though, Hoppes and colleagues needed to capture the crabs from the Menai Strait, UK, and bring them back to the lab. After the crabs were in their new homes, the team took samples of the crab's nervous system to see which part was producing Allatostatin. Interestingly, the researchers found three types of the hormone, Allatostatin-C, -CC and -CCC, but they didn't always find all of them in the same location. The crabs had all three types in their brain, suggesting that they play a role adjusting the activity of the nerve cells in the brain. However, when the researchers checked nerve cells near the heart and abdomen, they only found Allatostatin-C and -CC. The team also found these same hormones in the crabs’ immune cells, suggesting that they could play other roles as well.

Now that the scientists had figured out where the crabs were using Allatostatin, they needed to find out if the receptors for these proteins responded to all three types. After adding the three types of hormone to groups of cells containing the hormone's receptor, the team found something surprising: Allatostatin-C and -CC bound to the receptor, but Allatostatin-CCC did not. ‘Allatostatin-C and -CC are very similar in structure, but -CCC is a little bit different,’ explains Webster when discussing why Allatostatin-CCC didn't bind to the same receptor as the others.

So far, it certainly seemed like at least some type of Allatostatin was involved in shell shedding, but the scientists were unsure when the hormone comes into play. The researchers found that the crabs had some of the protein circulating in the hemolymph (a substance much like blood that transports nutrients around the crab's body) before the crabs finished shedding their shell. However, once they'd finished, the levels of Allatostatin spiked – along with two other hormones involved in the crabs coming out of their shells – and returned to normal 2 h later. If the Allatostatin levels don't rise before the crabs finish getting out of their old shells, then what is the protein doing? Because they found receptors for the hormones in the crabs’ immune cells, the researchers believe it's possible (even likely) that Allatostatin-C has a role in regulating their immune response. ‘This would make sense given that crustaceans are highly vulnerable to germs right after they shed their shells as their newly formed shell are soft,’ says Webster. Whatever the case, Hoppes and colleagues have certainly helped to demystify the series of events occurring when these crabs need to come out of their shell.