Thermocline - <span>a steep temperature gradient in a body of water, that is marked by a layer above and below at which the water is at different temperatures.
halocline - </span><span> a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water.
pycnocline - </span><span>a layer in an ocean or other body of water in which water density increases rapidly with depth.</span>
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The answer to the first question is a.
They developed to be protected by the hard shell from parasites and ther predators.
