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My male turtle is still under three inches. A few times when I've picked him up, he parts his mouth slightly. The other day I did, and he kind of opened his mouth and made a sound similar to a gasp.
The female's never done it. Is it a warning? Since I got him as a near-hatchling, I've always handled him to keep him friendly.

I know adults bite, and are extremely aggressive if rarely/never handled, seeing as to how I've been snapped at by one, but do your turtles attempt to bite you or another handler? If so, when did it start?

Biting or hissing when picked up is a natural response for a turtle. This is the only way they have of protecting themselves from predators (other than sliding into the water) and it's not intended to be taken as a sign of aggression, just that he feels threatened.

I think it has to do something with height. If I kneel and then pick my turtle up off the floor or something, he just pokes his head around. If I'm standing and pluck him out of his tank, he kind of acts timid.

Didn't know turtles hissed, though. That surprised me.

My female turtle doesn't like my daughter picking her up.. she hisses at her nearly every time she does. She's never hissed at me..

If your turtle retracted his head and you heard a hissing sound, that was the air being forced out of the lungs to make room for the head. My turtle will sometimes exhibit the behavior you described and I think it's instinctive to behave this way, particularly if they've been startled or feel threatened.

I must've scared him. I admit, I plucked him out of the water too fast. He did hiss at me, 'cause I saw him part his mouth.
I'll be waiting for the day he bites me. -shrug-
He's been acting a bit weird...any time he's on the island in the tank and someone walks by, he'll shoot off of it and sink himself to the bottom of the tank, usually behind a river rock.

that's very typical slider behavior. They "slide" into the water when they feel threatened. Nothing to take personal, just instinctual behavior

I know. But it just started one day, and no one's been able to near the tank without that reaction since. Hm.

you never know someone might have been less than coridial.

my husband told me of a sound Neptune made at him I called the vet to make sure he was fine, I found out that my husband was a creep and was making faces at him.

ya they can be very agressive at times i was fishing one time and i was useing a fishing lure 3/4th the size of the turtle i saw. i thought he was way to small to attck it and i left my lure there, after like 1 minute i saw the turtle again and he attcked it. thank god i pulled it up in time lol