Pea crabs are a type of tiny crabs that live mostly in bivalve shellfish such as mussels, oysters, and scallops. Scientifically known as pinnotherids, they hang out in the mantle cavity, a muscular bag which these molluscs use to pump water in and out of their body. Nestled in this flesh chamber, pea crabs get to enjoy a steady stream of aerated water and delicious phytoplankton, while protected by the bivalve's hard shell (although this does make hooking up a bit tricky). While some of these crabs can take up a fair bit of room in their host's body, they don't tend to feed on the shellfish's tissues. So are they just innocuous house guests hanging out with their bivalve homies?
 |
| Top left: A Pinnotheres pholadis pea crab in a scallop's mantle cavity, Top right: A female pea crab, Bottom left: A male pea crab, Bottom right: A female pea crab with eggs. Photos from Fig. 2 and 3 of the paper. |
Well, the study featured in this post shows that at least when it comes to Pinnotheres pholadis, their presence can have a detrimental effect on their hosts, especially among juvenile shellfish. Researchers in Japan collected both wild and farmed Yesso Scallops from twelve different locations across Mutsu Bay in Japan and examined them for pea crabs. They looked through 881 scallops, and found that close to one-third of them carried pea crabs. When they compared the conditions of the scallops with and without crabs, they found that the crab-endowed juveniles tend to be skinnier and have smaller shells, especially if they housed more than one crab.
So if the crabs aren't feeding on the scallop's tissue, why would they affect the host's condition? Like other bivalves, scallops feed on algae and other organic detritus in the water, which they suck up and trap in strings of mucus for easy swallowing. But this bundle of nutrient-rich mucus also happens to be the pea crab's favourite food, and the resident crab can intercept the mucus strings before they can be swallowed by the shellfish. So these crabs are literally snatching food out of the scallop's mouth, making them a kind of internal kelptoparasite.
While older and larger scallops might be able to offset their tenant's appetite due to their greater filtration capacity and larger reserves of energy, juvenile scallops need all the nutrients they can get to fuel their growing body. So they are less able to afford the crab skimming off the top from their phytoplankton slime smoothee. The researchers also found that crabs living in larger scallops tend to be bigger, possibly due to a combination of having more room in the mantle cavity, and also because bigger hosts are able to gather more food from the surrounding water. In this system, the available space with the bivalve places an absolute limit on how big the crabs can get.
So while for humans, having crabs may simply be itchy and embarrassing, for scallops, having crabs when you are young can potentially ruin you for life.
Reference:
