Although it’s likely a feeding mechanism, there’s a chance that it helps the crab maintain the fluid pressure behind its gills.
If the salt content of the water column is too high, then the water passing through the gills may be far too saline for the crab to handle. One way to regulate this would be to absorb more amino acids, which can be used to prevent more salt from entering the gills.
This solvent-based pressure is known as “osmotic pressure”, and it could be that the absorption of nutrients via its gills is a form of osmoregulation. At this stage, it’s not clear.
In any case, this odd mechanism is the first example of it in crustaceans. It’s already a hardy crab, but now it’s just showing off.
“This type of crab is so readily adaptable to extremely harsh environments, and that's why they're everywhere,” co-author Tamzin Blewett, a postdoctoral fellow at Alberta, said in a statement. “They're super-tolerant to low oxygen levels and changes in salinity, and now we know they also have this ability to consume nutrients through their gills. This ability may come in handy between meals.”
Rather beautifully, the researchers don’t shy away from the somewhat uncomfortable atmosphere the curious capabilities of these crabs create.
“This ability kind of makes them the superhero of the marine world – or supervillain, depending on your perspective,” Blewett notes.