The lines get pretty fuzzy depending on the location and specimens involved (some specimens handle tropical to temperate, some temperate to cold, some just tropical, just temperate, or just cold). Regardless, though, there are some cool little shrimp, crabs, hermits, snails, chitons, limpets, and anemones that could all work (as well as some cool corals), and I know one company that sells coldwater micro brittle stars too. If you're up for dosing, you could also do inverts like clams, oysters, mussels, etc. and barnacles.
Some examples you can look:
- Dwarf Teardrop Crab (Pelia tumida); decorates itself with sponges, so it generally looks bigger than it really is.
- Flatback Mud Crab (Eurypanopeus depressus); typically found with oysters and hyroids.
- Broken-back Shrimp (Heptacarpus palpator); omnivorous scavenger.
- Grass Shrimp (Hippolyte clarki); feeds on algae and pods.
- Rough Limpet (Collisella scabra); generalist herbivore.
- Volcano Limpet (Fissurella volcano); feeds mainly on diatoms, cyanoprokaryota, and Pyropia spp. (nori).
- Common Periwinkle (Littorina littorea); feeds on algae (it likes Ulva spp.).
