Elevated Levels of Trimethylamine Oxide in Deep-Sea Fish: Evidence for Synthesis and Intertissue Physiological Importance JASON R.TREBERG AND WILLIAM R. DRIEDZIC* Ocean Sciences Centre, Memorial University of Newfoundland. St. John’s, Newfoundland, Canada, A1C 5S7 ABSTRACT Tissue levels of trimethylamine oxide (TMAO) were compared for seven teleost and two elasmobranch species captured from three depth ranges: shallow (o150 m), moderate (500^700 m), and deep (1,000^1,500 m).Within the teleosts, the deep-caught species had signi¢cantly greater TMAO content than shallow- or moderate-caught species. In all teleosts, muscle had substantially more TMAO than all other tissues. Kidney or, in some cases, liver had elevated trimethylamine (TMA) con- tent, 2.20^9.65 mmol/kg, along with appreciable trimethylamine oxidase (TMAoxi) activity, suggesting active TMAO synthesis. No correlation was found between TMAoxi activity and TMAO content. The elasmobranchs in this study, Squalus acanthias and Centroscyllium fabricii from shallow and deep water, respectively, were both squaliform sharks.The deep-caught species had signi¢cantly moreTMAO in all tissues than the shallow species. Furthermore, urea was signi¢cantly less in the deep species in all tissues except liver, while the urea:TMAO ratio was signi¢cantly less in all tissues. As with teleosts, the TMAO content of muscle was substantially higher for both elasmobranchs than in all other tissues. TMAoxi was below levels of detection in both elasmobranch species, suggesting that TMAO is obtained solely from the diet.This study expands the trend of increased muscle TMAO in deep-sea ¢sh to a variety of other tissues. The accumulation of TMAO in various tissues in deep-sea teleosts and the accumulation of TMAO and concurrent urea decrease in a deep-sea elasmobranch in comparison to a shallow water species strongly support the contention that TMAO is of physiological importance in deep-sea ¢sh. J. Exp. Zool. 293:39^45, 2002. r 2002 Wiley-Liss, Inc. Trimethylamine oxide (TMAO) is a nitrogenous organic molecule commonly found in the muscle of marine ¢sh (reviewed by Hebard et al., ’82); how- ever, the importance of this organic osmolyte has yet to be completely explained with regards to adaptation to various environments. In marine elasmobranches, TMAO and other methylamine compounds are important osmoregulatory com- pounds (Perlman and Goldstein, ’88) and likely have signi¢cant roles in the counteraction of urea (Yancey,’94) and buoyancy (Withers et al.,’94). The function of TMAO in teleosts is somewhat more obscure. In some euryhaline species it has a substantial role in osmoregulation in muscle (Lange and Fugelli, ’65) but this does not explain the high occurrence of TMAO in marine teleosts that do not normally experience large £uctuations in salinity. Recently a number of ¢sh species found in two extreme environmental groups have been shown to have elevated levels of TMAO. These are marine teleosts adapted to near- or subzero tem- peratures (Raymond, ’98; Raymond and DeVries, ’98; Raymond and Hassel, 2000) and deep-sea ¢sh (Gillett et al.,’97; Kelly and Yancey,’99). Of the teleosts that are adapted to near- and sub- zero degrees that were examined for TMAO con- tent, several have what appear to be elevated levels in the serum and liver, and muscle in the case of certain Nototheniod species, when compared to the values tabulated in Hebard et al. (’82). In deep- sea gadiforms, some species have been shown to have elevated plasma TMAO, whereas all ¢sh spe- cies examined to date have shown elevated muscle TMAO in comparison to shallow water temperate species (Gillett et al, ’97; Kelly and Yancey, ’99). Furthermore, Kelly and Yancey (’99) found that in Grant sponsor: Natural Sciences and Engineering Research Council of Canada. *Correspondence to: William R. Driedzic, Ocean Sciences Centre, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada, A1C 5S7. E-mail: wdriedzic@mun.ca. Received 1 September 2001; Accepted 22 February 2002 Published online in Wiley InterScience (www.interscience.wiley. com). DOI: 10.1002/jez. 10109 r 2002 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY 293:39^45 (2002)