Since mermaids are fishes or at least part fish they would have a lateral line. Since the lateral line helps the mermaid to perceive their surrounding would there be any social implications on touching a mermaid's lateral line? Would it be an intimate zone? Would it be mean to touch it? Would the mermaid even be able to feel something different as if any other place of their tail were touched? Would it affect their perception of their surroundings as if you covered someone's eyes? Would it be more (or less?) sensitive that other zones of their body?
closed as primarily opinion-based by Frostfyre, Hohmannfan, DaaaahWhoosh, Thucydides, JDługosz Aug 9 '16 at 20:19
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The lateral line consists of cells that react to being displaced by pressure changes and movement in the water. Also, it's a pretty long... well, line, of these cells.
So since they react to pressure, yes they would probably be more sensitive to touching than other parts of the body. And not just touch, even just moving any bodypart or object near it would be noticed, since that moves the water - to which the cells in the line react.
It wouldn't be like covering someone's eyes since it'd hardly get completely covered by anything less than a tarp or the entire body of another mermaid. It'd probably be percieved like a kind of "shadow" effect, like having a leaf or someone's hand near your eye, in the field of vision, but not covering all of it.
As for whether or not it'd be intimate... Well, for humans any part of the body that's covered with enough nerve cells to be sensitive ends up being someone's fetish. Whether the line is commonly or rarely considered thus by mermaids is something that I can't answer without more in-depth knowledge of fish mating habits, especially as they concern the ancestors of your mermaids :P