"A crab and a turtle."

[LICA98]

why is it μία and not ένα?

[Djenthallman]

Because χελώνα is feminine.

Masc — ένας, fem — μία, neut — ένα.

[LICA98]

oh right, the hint should display μία first then tho

[Langenth]

'καβούρι' ends with a vowel but masculine? Is there a gender rule for words that end with a vowel that is not Α (feminine) or Ο, Ε (neuter)?

[D_..]

'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.