Do you see the lump on the animal on the right?
It is a quite common isopod parasite of this squat lobster species, Pleurocrypta sp. (longibranchiata or galateae). It is hidden in the branchial cavity of our squat lobster, and feeds on its hemolymph.
Isopods of this family are all ectoparasites of decapods (shrimps, crabs, etc…), and represent 7.7% of all isopods species. There is more exemples of parasitic isopods around the globe, as you can see below in the picture from a review.
If you don’t know what an isopod is, great news! I will introduce a common free-living species (non-parasitic) of our tidal pools very soon 🙂
Sources

- Doris
- J. D. Williams and C. B. Boyko, “The Global Diversity of Parasitic Isopods Associated with Crustacean Hosts (Isopoda: Bopyroidea and Cryptoniscoidea),” PLOS ONE, vol. 7, no. 4, p. e35350, Apr. 2012, doi: 10.1371/journal.pone.0035350.