Unlike most pelagic crustaceans, the deep-sea shrimp Notostomus gibbosus (A. Milne-Edwards) (Oplophoridae) is positively buoyant, possessing a dorsally enlarged carapace which contains a low-density fluid. This fluid comprises 43% of the animal's wet mass, has a low pH, and gives a lift of 17.7 mg ml−1; when this fluid is drained, the animal sinks. Low density is achieved by the replacement of less buoyant ions with ions which reduce density via two mechanisms: a change in total solute mass by the use of ions of lesser mass, and an ion-specific disruption of the structure of water molecules (resulting in an increase in fluid volume) caused by ions having large, positive partial molal volumes. The presence of large amounts of trimethylamine (Me3NH+), a relatively large, heavy ion which, together with NH4+, replaces nearly 90 % of the Na+ in the carapace fluid, results in little change in the total solute mass of the carapace fluid of N. gibbosus (33.2‰) relative to sea water (approximately 34.1‰). Reduced fluid density is primarily a result of the large, positive partial molal volumes of Me3NH+ and NH4+, rather than a function of reduced solute masses.

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