Polyp or Hydra
The Hydrozoan species Turritopsis nutricula is capable of cycling from a mature adult stage to an immature polyp stage, and back again, indefinitely. This means there is, theoretically, no limit to its life span, although no single specimen has been observed for any extended period and it is impossible to estimate the age of a specimen.
The Antarctic sponge Cinachyra antarctica has an extremely slow growth rate in the low temperatures of the Antarctic Ocean. One specimen has been estimated to be 1,550 years old.
Mollusk or Clam
A specimen of the Icelandic Cyprine Arctica islandica (also known as an ocean quahog), a mollusk, was found to have lived 405 years and possibly up to 410. Another specimen had a recorded lifespan of 374 years.
A koi fish (Cyprinus carpio) named Hanako lived to the age of 226, from 1751 to 1977.
The longest-lived higher animals include several species of tortoise (family Testudinidae), known to live 150 years or more. One owned by the Tongan royal family lived 188 years.
See the related link for more information about the animals with the longest life spans.