Not that I'm interested in getting a blue ring or any other animal that contains TTX. It's just a random question that popped into my head while I was reading another forum where people were asking about keeping blue rings.
It didn't give me the exact answer, but it provided the information I should have been looking for. I recommend this article for anyone who has an interest in blue rings or TTX. It was very accessible to someone with a limited biology background, such as myself.
Here are two paragraphs I found especially interesting (edited for length):
Tetrodotoxin (TTX, CAS Number [4368-28-9]) is a potent marine neurotoxin, named after the order of fish from which it is most commonly associated, the Tetraodontiformes (tetras-four and odontos-tooth), or the tetraodon pufferfish. (*snip*) The members of this order include the fahaka puffer (Tetraodon fahaka), the Congo puffer (Tetraodon miurus), and the giant mbu puffer (Tetraodon mbu). Pufferfish from the genus Fugu (F. flavidus, F. poecilonotus, and F. niphobles), Arothron (A. nigropunctatus), Chelonodon (Chelonodon spp.), and Takifugu (Takifugu rubripes) also store TTX and related analogs in their tissues.
Other marine organisms have been found to store TTX and include the Australian blue-ringed octopus (Hapaloclaena maculosa, uses TTX as a toxin for capturing prey), parrotfish, triggerfish, goby, angelfish, cod, boxfish (Ostracion spp.), tobies, porcupine fish, molas or ocean sunfish, globefish, seastars, starfish (Astropecten scoparius), xanthid crabs (Eriphia spp.), a horseshoe crab (Carcinoscorpius rotundicauda), two Philippine crabs (Zosimus aeneus and Atergatis floridus), a number of marine snails, flatworms, sea squirts, ribbonworms and arrowworms (which both use TTX as a venom for prey), molluscs (Nassarius spp. and the Japanese trumpet shell "Boshubora"), and marine algae (Jania spp.). Terrestrial organisms include the Harlequin frogs (Atelopus spp.), Costa Rican frog (Atelopus chiriquiensis), three species of California newt Taricha spp., and members of the Salamandridae (Salamanders). The number of species continues to grow.