sponge Xestospongia muta >7m^3: tube worm Riftia pachyptila >3m: jellyfish Nemopilema nomurai >3m
||Smith et al., Body size evolution across the Geozoic, Annual Review of Earth and Planetary Sciences, Vol. 44 :523-553, 2016 DOI: 10.1146/annurev-earth-060115-012147 link p.532 top paragraph
||McClain CR, Balk MA, Benfield MC, Branch TA, Chen C, et al. 2015. Sizing ocean giants: patterns of intraspecific size variation in marine megafauna. PeerJ 3: e715 doi: 10.7717/peerj.715.PubMed ID25649000
||P.532 top paragraph: "Life in Water: The sizes of oceanic organisms span 23 orders of magnitude in biovolume, from the tiny thermophilic Archaea Thermodiscus to the blue whale, Rorqualus musculus (BNID 113313 Table - link ). A complete survey of size distributions within the ocean has yet to be attempted. Indeed, even sampling the diversity of microbial and macroscopic life in the oceans is difficult. However, a good deal is known about the maximum sizes of marine animals (primary source), in part because of the public fascination with the largest organisms and in part from long-term fisheries records. Though the largest species of many modern chordate groups are well known (e.g., blue whale, whale shark), there are many impressively large marine invertebrates, including Xestospongia muta, a sponge with a biovolume of more than 7 m^3, Riftia pachyptila, a tube worm more than 3 m long, and Nemopilema nomurai, a jellyfish with a bell more than 3 m across (primary source). Documenting the smallest marine organisms is challenging given the difficulty of comprehensively sampling biodiversity in the oceans and because many of the smallest animals are parasitic."