I tried to look into specifically this, the best I could find is a similar family of creatures, called amphipods (a shell-less crustacean), were found to contain abnormal amounts of carbon-14 linked to Cold War bomb testing even at the bottom of the Mariana Trench (Smithsonian, 2019). I think the correct answer is that all animals contain traces of radioactive isotopes like Potassium-40 since it’s naturally occurring.