Why do green spotted puffer's belly swell up every time they eat?

Answer 1

Well, Princess is heading downt he right road with her claims, not the best explanation but maybe its the best answer, heh.

Tetraodontidae, or puffer fish swell by swallowing large amounts of air and holding it in. This act is performed when the fish feels threatened. As you likely know by owning one, they range from freshwater specimens, to brackish water specimens onto saltwater specimens. The relevance of this is that dolphins are somewhat "known" to play with members of this familae. Not exactly what a fishkeeper would call playing though. The dolphins will nibble at the fish, causing it to inflate and release minor amounts of Tetrodotoxin, which is suggested that the dolphins enjoy as it numbs their mouthes. Eventually the fish will deflate again, and the dolphin will once again, nibble at it to make it inflate.

This behavior is more innate than most would expect. As you do things in your sub-concience, fish perform primitive behaviors, such as a puffer fish inflating when exposed to specific stimuli.

This taken into account, it would suggest your "puffer" is inflating during feeding time as a behavioral habbit to guard its meal, as inflation of puffers is readily accepted as an innate defense mechanism.

Answer 2

b/c they're puffer fish...and they are supposed to do that? hehe. i'm not sure but i wanted you to smile so i thought i'd leave a cute little message. might win best answer. hehe.