"A crab and a turtle."
Translation:Ένα καβούρι και μία χελώνα.
'καβούρι' ends with a vowel but masculine?
It is not masculine, it is neuter! :)
Is there a gender rule for words that end with a vowel that is not Α (feminine) or Ο, Ε (neuter)?
There is not unfortunately and there is overlap between endings and genders but there are patterns. I suggest looking here http://www.foundalis.com/lan/grknouns.htm to see what the most common patterns are.