They reach the same maximum size as C. serpentina but they reach it more often. The average for an adult C.s.osceola around here (Tampa) is about 15 or 17 inches in carapace length. It would not be unusual for me to go into a reptile store down here and see a giant one that some local would bring in. I know Commons can get that big, but they typically max out smaller than that.
They are also a bit beefier in general. Florida snappers are often confused with alligator snappers down here because of their size and the shape of the papillae on their necks (pointed like an alligator snapper). They are also a bit more muscular than commons.
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