The green shore crab, Carcinus maenas, is found all over the world, scuttling around just shy of the shore. It’s known to conservationists as being a bit of an unwelcome presence, in that it’s an aggressive invasive species.
As reported in a brand new study, it also does something fairly unexpected: It can “eat” or absorb nutrients through its own gills, something previously thought to be impossible. To be clear, it doesn’t actually digest its own gills via a form of autocannibalism – that would be highly counterproductive, to say the least.
Writing in the Proceedings of the Royal Society B, a pair of researchers from the University of Alberta’s Department of Biological Sciences explain that estuaries are “enriched with dissolved nutrients such as amino acids.” Until now, the only invertebrate group that seem not to care or try to access this reservoir of deliciousness are marine arthropods, which includes the most populous group, crabs.
This struck the team as odd, so they decided to take a closer look.
Previous research on the crabs looking at their gills, hiding under their rigid exoskeletons, came to the conclusion that they simply weren’t designed to allow nutrients to be absorbed; rather, they were used to deal with oxygen and filter out hazardous compounds in the water column.
Instead, their diet was thought to only consist of a variety of prey, from clams and oysters to other crustaceans. Under new, careful observations, this intrepid team spotted green shore crabs taking in the amino acid leucine – vital for the synthesis of muscle proteins – through their complex arrangement of gills.
These nocturnal predators are already quite remarkable in their ability to flit from habitat to habitat, environment to environment. It’s thought that this newly discovered ability aids them in this regard, and the team suspect other crabs are able to do it too.
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.