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, 111 (12), 4461-5

Marine Fish May Be Biochemically Constrained From Inhabiting the Deepest Ocean Depths

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Marine Fish May Be Biochemically Constrained From Inhabiting the Deepest Ocean Depths

Paul H Yancey et al. Proc Natl Acad Sci U S A.

Abstract

No fish have been found in the deepest 25% of the ocean (8,400-11,000 m). This apparent absence has been attributed to hydrostatic pressure, although direct evidence is wanting because of the lack of deepest-living species to study. The common osmolyte trimethylamine N-oxide (TMAO) stabilizes proteins against pressure and increases with depth, going from 40 to 261 mmol/kg in teleost fishes from 0 to 4,850 m. TMAO accumulation with depth results in increasing internal osmolality (typically 350 mOsmol/kg in shallow species compared with seawater's 1,100 mOsmol/kg). Preliminary extrapolation of osmolalities of predicted isosmotic state at 8,000-8,500 m may indicate a possible physiological limit, as greater depths would require reversal of osmotic gradients and, thus, osmoregulatory systems. We tested this prediction by capturing five of the second-deepest known fish, the hadal snailfish (Notoliparis kermadecensis; Liparidae), from 7,000 m in the Kermadec Trench. We found their muscles to have a TMAO content of 386 ± 18 mmol/kg and osmolality of 991 ± 22 mOsmol/kg. These data fit previous extrapolations and, combined with new osmolalities from bathyal and abyssal fishes, predict isosmotic state at 8,200 m. This is previously unidentified evidence that biochemistry could constrain the depth of a large, complex taxonomic group.

Keywords: chemical chaperone; deep-sea; osmoregulation; piezolyte.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
The hadal snailfish N. kermadecensis (25). The image shows the snailfish alive at 7,199 m, photographed by baited camera close to the site where the samples were retrieved in this study (8). N. kermadecensis is endemic to the Kermadec Trench off New Zealand and occupies a very narrow depth band of 6,472 (24) to 7,561 m (8). The samples recovered for this study represent the second time this fish has ever been captured (the first time in 59 y) and represents the second deepest fish ever seen alive (7).
Fig. 2.
Fig. 2.
Muscle TMAO contents (mmol/kg wet mass) vs depth of capture for teleosts. Circles, with standard deviation bars, are published data (11, 13, 16, 17), with a linear fit (black line) from 900–4,850 m. Solid red squares without standard deviation bars are new data (n = 1 each) for a snailfish C. melanurus (793 m), four eelpout, and two grenadier species. The solid red square with standard deviation bars is the hadal snailfish N. kermadecensis from 7,000 m (n = 5), with a new linear fit (the red line) for all new and old data for 900–7,000 m. *C. armatus (abyssal grenadier) at four depths (note that the specimen at 4,850 m was from the northeast Atlantic, whereas the others were from the northeastern Pacific).
Fig. 3.
Fig. 3.
Osmolalities (mOsmol/kg) of muscle fluid vs depth of capture for teleosts. Circles (n = 1 each; for the two osmolalities with standard deviation bars, n = 3) are new data for Monterey Bay fish: eelpouts, grenadiers, a morid, and a snailfish C. melanurus († at 793 m). A linear fit (the solid black line) extrapolates to isosmotic state at 8,450 m (dotted vertical black line). The red square with standard deviation bars is the hadal snailfish N. kermadecensis from 7,000 m (n = 5), included in a new linear fit (the solid red line) extrapolating to isosmotic state at about 8,200 m (dotted vertical red line). *A. microlepis (finescale mora) at three depths.

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