A vacuole inside the Antarctic marine zooplankton Calanoides acutus changes its density and buoyancy by having a wax ester that changes from a liquid to a solid at the cold temperatures.

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The tiny Antarctic marine crustacean Calanoides acutus hibernates overwinter by descending to great depths. Once it reaches depths below 400 meters (one quarter mile), the cold temperatures cause a large pocket of waxy liquid within its body to transform to a dense solid, causing the organism to sink. As a buoyant substance, the waxy liquid is made up of saturated fatty acids, which are long chains of carbon atoms attached to each other by single bonds. To prepare for its descent and hibernation, the crustacean changes the waxy substance from saturated to unsaturated; that is, many of the single bonds connecting the carbon atoms to each other are converted to double bonds. This change allows the waxy compounds to fit together in a more tightly packed configuration. The increased density causes the crustacean to sink in the water column until it reaches a depth at which it is neutrally buoyant again. It can remain at the depth without additional energy input until it begins to consume the lipid when spring arrives.

Buoyancy of Calanoides acutus changes when wax esters change from saturated (single bond, top) to unsaturated (double bond, below) Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.

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"The lipid store can occupy over half of the body volume before winter in diapausing species such as C.[Calanoides] acutus (Fig. 1) and its influence on body density and hence buoyancy must be large." (Pond and Tarling 2011:1310)

"Levels of wax ester in the copepods were correlated with depth, with deeper animals containing higher amounts...The level of unsaturation can affect the physical conditions at which lipids make a transition between the liquid and solid phase. This in turn alters the specific volume of the lipid store, so altering buoyancy." (Pond and Tarling 2011:1311)

"[I]t was found that levels of unsaturation of the wax esters in the lipid pool differed significantly with depth, with animals deeper than 400 m consistently containing high levels of unsaturation (~50%)...wax ester samples with higher levels of unsaturation underwent a phase transition from liquid to solid state at typical deep-water pressures and temperatures." (Pond and Tarling 2011:1313)

"A transition from the liquid to the solid phase reduces the specific volume and hence the buoyancy of the large lipid pool, so assisting lipid-rich diapausing copepods in overcoming their positive buoyancy." (Pond and Tarling 2011:1314)

"A comparable mechanism of buoyancy regulation is utilized by sperm whales...sperm whales contain their lipid in a large spermaceti organ within the head space. To descend, the lipid is cooled to the point at which it solidifies, and the increase in density makes the whale able to sink without large amounts of downward swimming." (Pond and Tarling 2011:1316)

Journal article
Phase transitions of wax esters adjust buoyancy in diapausing Calanoides acutusLimnol. Oceanogr.June 21, 2011
David W. Pond, Geraint A. Tarling

Tiny marine crustaceans construct wax 'weight belts' to stay deep

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