Fig. 4. Trimethylamine N-oxide (TMAO) as a possible pressure counteractant in deep-sea animals (see also Fig. 1 for other deep-sea osmolytes). (A) Contents of TMAO (and urea in rajids, as shown) in muscles as a function of depth in shrimp, rajids (skates) and teleost fishes: gadid (cod) and related macrourids (grenadiers), plus scorpaenids (rockfish) (data from Kelly and Yancey, 1999; Yancey et al., 2004). (B) Effect of 250 mmol l^–1 osmolytes on NADH K_m of A₄-lactate dehydrogenase (LDH) from deep-sea grenadier (Coryphaenoides armatus). Measurements were made at atmospheric pressure (0.1 MPa) and 250 atmos (25 MPa), showing that TMAO counteracts pressure better than other common solutes. *Significant increase compared to 0.1 MPa water control; significant decrease compared to 25 MPa water control (modified from Yancey et al., 2004).