Complex sentence: There have been no confirmed cases of tetrodotoxicity from the Atlantic pufferfish, "Sphoeroides maculatus"; but in three studies, extracts from fish of this species were highly toxic in mice.
Split into simpler sentences: There have been no confirmed cases of tetrodotoxicity from the Atlantic pufferfish, "Sphoeroides maculatus". However, in three studies, extracts from fish of this species were highly toxic in mice.