Life in the Open Ocean. Joseph J. Torres

important to temperate species that must accommodate changes in temperature associated with seasonal cycles. In most instances, a need for change can be achieved through changes in the biosynthesis of lipids. An example is using enzymes that introduce double bonds into fatty acid chains to make them more suitable for use at cold temperature. Such enzymes are termed desaturases, and they can be up‐regulated quickly (Hochachka and Somero 2002).

It is most important to appreciate that not only do species’ membrane lipids vary greatly in character with the changes in habitat temperature typical of different zoogeographic regions, but considerable acclimation to temperature change by membrane lipids can also occur within a period of days to weeks. Such short‐term change can be considered part of the overall acclimation process that allows a species to adjust its upper and lower lethal limits (see Figure 2.2a, the tolerance polygon).

Table 2.2 Chemical formulas and melting points for a selection of saturated and unsaturated fatty acids.

Carbon atoms	Common name	Empirical formula	Chemical structure	Melting point (°C)
Saturated fatty acids
3	Propionic acid	C3H6O2	CH3CH2COOH	−22
12	Lauric acid	C12H24O2	CH3(CH2)10COOH	44
14	Myristic acid	C14H25O2	CH3(CH2)12COOH	54
16	Palmitic acid	C16H32O2	CH3(CH2)14COOH	63
18	Stearic acid	C18H36O2	CH3(CH2)16COOH	70
20	Arachidic acid	C20H40O2	CH3(CH2)18COOH	75
Unsaturated fatty acids
16	Palmitoleic acid	C16H30O2	CH3(CH2)5CH=CH(CH2)7COOH	−0.5
18	Oleic acid	C18H34O2	CH3(CH2)7CH=CH(CH2)7COOH	13
18	Elaidic acid	C18H34O2	CH3(CH2)7CH=CH(CH2)7COOH	13
18	Linoleic acid	C18H32O2	CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH	−5
18	Linolenic acid	C18H30O2	CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH	−10
20	Arachidonic acid	C20H32O2	CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH	−50

Figure 2.15 The relationship between adaptation temperature and percentage of unsaturated acyl chains in synaptosomal phospholipids of differently adapted vertebrates. Each symbol represents a different species. Open symbols denote phosphatidylethanolamine; filled symbols denote phosphatidylcholine.

      Source: Hochachka and Somero (2002), figure 7.27 (p. 372). Reproduced with the permission of Oxford University Press.

Figure 2.16 Temperature acclimation and phospholipid class. Time course of change in the ratio of phosphatidyl choline (PC) and phosphatidyl ethanolamine (PE) in gill cell membranes of rainbow trout acclimating to the indicated temperatures. *indicates a statistically significant difference (P<0.05) compared to the day zero mean.

      Source: Hazel and Carpenter (1985), figure 4 (p. 599). Reproduced with the permission of Springer.

Pressure

      Even though pressure is the most predictable variable in the ocean, increasing by 1 atm with every 10 m increase in depth, pressure is probably the variable most difficult to intuitively understand. Ocean pressure evokes thoughts of dark and forbidding depths, of submarine movies in which the captain and heroic crew must take their craft to depths far greater than she was built to withstand, to there lie on the bottom, evade the enemy, and hope to survive. The great pressure causes the sub to creak and groan, bolts to pop like bullets out of the hull, and leaks to sprout before the ordeal can be successfully ended. However, World War II submarines could not get very deep at all, <300 m, and even modern nuclear subs do not get out of the mesopelagic zone (200–1000 m). Our view on pressure from those movies is one where pressure is acting on gas‐filled spaces. A submarine is quite a large gas‐filled space and must be immensely strong to withstand even the modest pressure of a dive to 100 m: 11 atm, 162 psi, or 11 143 kPa. In point of fact, most of the species that live under pressure do not have gas‐filled spaces, and thus the effects of pressure are far more subtle, especially in the upper 1000 m where much of the ocean’s pelagic biomass resides. In our mind’s eye though, the pressure associated with even the average depth of the ocean must be a formidable challenge to