Effects of gelatinous tissue and specimen damage on TMAO analyses from deep-sea fish

Hydrostatic pressure perturbs protein function, and while many proteins in deep-sea animals have evolved partial pressure resistance, many appear to require piezolytes: small organic molecules which counteract pressure’s perturbing effects on proteins via strengthening water structure in cells. The best known piezolyte, <b>trimethylamine N-oxide (TMAO),</b> has been found to <b>increase with depth--linearly with pressure--in muscle tissue of bony and cartilaginous fishes, squids, cnidarians and crustaceans</b>. For hadal fishes including the Mariana snailfish (Linley et al. 2017) and for amphipods (Downing et al. 2018), the linear trend extends all the way down to the greatest ocean depths at which these animals are found (reviewed by Yancey 2020, 2023). However <b>two recent studies have reported considerably lower TMAO levels</b> in a few specimens of hadal snailfish, plus an abyssal cusk eel:Mu et al. 2021 report TMAO in a snailfish from the Yap Trench at less than 50% of what our hypothesis would predict, but they also found multipe copies of a TMAO-producing enzyme gene which strongly supports our hypothesis.Xu et al. 2025 report TMAO in 4 snailfish and 1 cuskeel from the Mariana Trench at less than 30% of of what our hypothesis would predict. However, they did not include our Mariana grenadier and snailfish data from Linley et al. 2017, which make our hypothesis much stronger than they state.These low-TMAO values in hadal fish could indicate that TMAO does not increase linearly with depths below about 4000 m in bony fishes. While these data may be accurate, the data might instead be skewed lower due to 1) <b>gelatinous</b> tissue embedded in muscles of <b>cusk-eels</b> (see our analyses in Samerotte et al. 2007 and Gerringer et al. 2017) and 2) <b>physical damage</b> to fragile snailfish retrieved into very warm tropical waters over those trenches, leading to TMAO loss. For our detailed documentation of the latter problem, see pre-publication on BioRxiv:References:Downing A.B., Wallace G.T. , Yancey P.H. (2018). Organic osmolytes of amphipods from littoral to hadal zones: Increases with depth in trimethylamine N-oxide, scyllo-inositol and other potential pressure counteractants. Deep-Sea Res. I 138: 1-10 https://doi.org/10.1016/j.dsr.2018.05.008Gerringer M.E., Drazen J.C., Summers, A.P., Linley T.D., Jamieson A.J., Yancey P.H. (2017). Distribution, composition, and functions of gelatinous tissues in deep-sea fishes. Royal Soc. Open Sci. 4: 171063 https://doi.org/10.1098/rsos.171063Linley T., Gerringer M, Yancey P.H., Drazen J.C. , Weinstock C., Jamieson A. (2016). Fishes of the hadal zone including new species, in situ observations and depth records of Liparidae. Deep-Sea Res. I, 114: 99-110. https://doi.org/10.1016/j.dsr.2016.05.003Mu Y., Bian C., Liu R., Wang Y., Shao G., Li J., et al. (2021): Whole genome sequencing of a snailfish from the Yap Trench (~7, 000 m) clarifies the molecular mechanisms underlying adaptation to the deep sea. PLoS Genet 17(5):e1009530 https://doi.org/10.1371/journal.pgen.1009530Samerotte A.L., Drazen J.C., Brand G.L., Seibel B.A., Yancey P.H. (2007). Contents of trimethylamine oxide correlate with depth within as well as among species of teleost fish: an analysis of causation. Phys. Zool. Biochem. 80: 197-208Xu H., Fang C., Xu W., Wang C., Song Y., Zhu C., et al. (2025). Evolution and genetic adaptation of fishes to the deep sea. Cell 188, 1393–1408 https://doi.org/10.1016/j.cell.2025.01.002Yancey P.H. (2020) Cellular responses in marine animals to hydrostatic pressure. J. Exp. Zool. https://doi.org/10.1002/jez.2354Yancey P.H. (2023). Trimethylamine N-oxide (TMAO): a unique counteracting osmolyte? Paracelsus Proc Exp Med 2(S1):67-91; DOI: 10.33594/000000661