Aaron misses the shark tattoo, because Nick used to think of himself as a shark, all smooth lines and teeth, living in the water. Aaron knows a lot about sharks, though; knows that a shark's skin is just like thousands of tiny little knives, and if you run your hand over it from tail to head, you'll get cut. Knows that sharks have a hundred teeth and no bones, and that sharks are not people.
Nick is not a shark.
Nick's skin is soft to the touch, soft and smooth and it never hurts Aaron to touch him, except in his heart. His teeth leave red marks on Aaron's body, his collarbones, his hips, but they always fade with time and they never bleed except beneath the skin, red marks turning bruise-colored beneath his clothes.
When the bone in Nick's hand was broken, Aaron kissed the hard cast covering it, and Nick's face had scrunched up until tears ran down his face, salt water on smooth skin.
Nick doesn't live in salt water; it lives in him. Nick may pretend to be sharp and sleek, something that eats humans, but he's just pretending, because its easier than adknowledging that he's the one getting eaten.
Nick would like to be a shark, but he's hurt too easily--his skin is too thin, and not covered with knives. Aaron is more shark-like than Nick is, though he would never, never tell Nick that, never adknowlege his own predator-mind and Nick's lack thereof.
Nick won't tell him over the phone what his new tattoo is. He only says that the shark is gone, covered up by ink and Nick's refusal to face the past, to face the fact that he could never be a shark because he feels the harpoons that everybody shoots at him, and refuses to pursue when he smells blood.
Aaron wonders what it is that Nick thinks he is now.