No, this is not another recipe for baked clams or crab-filled treats. This is much better!
Where the shellback crab lives in old bivalve shells for protection, there are other crabs that live in LIVE clams, mussels, oysters, etc!! That crab is the
the polka dot pea crab (Pinnaxodes floridensis)
I was pretty excited when I learned about this guy (thanks Bryan!). There are a few species with the common name pea crab, but, as far as my little research review went, they all have one thing in common: they hunker down in those comfy hinged molluscs!
"Of the small crabs that live in a foreign host"
(from Rondeletius (1554) courtesy of Haines et al. (1994))
Female pea crabs seem to prefer this lifestyle compared to the free-swimming male crabs, but mating pairs have been found in mussels! Because of this sexual difference in preference, the relationship between male pea crabs and their hosts is considered amensalism, which means one species has a negative effect on another, but with no effect to itself. The male pea crab may harm its mussel host, but it gets no real benefit from the mussel.
Female pea crabs are simply parasites. Parasitism means one species benefits at the expense of the other species. Scientists saw negative effects to mussel growth when female pea crabs were present, so while the female pea crabs had a safe place to live, their mussel hosts were stressed and unable to attain similar lengths compared to unparasitized mussels.
"Hey! I'm livin' here!" - female pea crab
Bonus fun fact: pea crabs also parasitize sea cucumbers too! So if you're feeling like you want two snacks in one, keep an eye out for these little crabs on your next mussel/cucumber foraging expedition!
Pea crab papers:
Haines, C. M. C., M. Edmunds, and A. R. Pewsey. 1994. The pea crab, Pinnotheres pisum (Linnaeus, 1767), and its association with the common mussel, Mytilus edulis (Linneaus, 1758), in the Solent (UK). Journal of Shellfish Research 13: 5-10.
Tablado, A., and G. J. Lopez. 1995. Host-parasite relationships between the mussel, Mytilus edulis L., and the pea crab, Tumidotheres maculatus (Say), in the southwestern Atlantic. Journal of Shellfish Research 14: 417-423.