Final answer:
Marine invertebrates adapt to deep water conditions by increasing the unsaturated fatty acid content in their cell membrane phospholipids, which introduces kinks to maintain membrane fluidity at lower temperatures.
Step-by-step explanation:
When marine invertebrates like mollusks and tiny plankton descend from shallow surface water to deep waters, their cell membranes need to adapt to the change in external temperature and pressure. A critical adaptation involves the modification of membrane phospholipids to maintain proper membrane fluidity for cell function. At lower temperatures in deep waters, marine invertebrates increase the unsaturated fatty acid content in the phospholipids of their cell membranes. The unsaturated fatty acids have more C=C bonds which introduce kinks or bends in the hydrophobic tails of the phospholipids. These kinks prevent the phospholipid tails from packing too closely, which makes the membrane more fluid despite the cold temperature. More fluid membranes allow for better function of membrane proteins and proper cell operations. This adjustment is essential for poikilothermic, or cold-blooded, organisms that do not internally regulate their body temperature and must rely on the fluidity of their cell membranes to function properly in varying environmental conditions.