ParisJDar, una is used here as an indefinite article (a, and here it is una because tartaruga is feminine), and uno/una is also used as the number one. English also does that; we can say for eg a book or one book or in this case a turtle or one turtle, and we can use one to count.
Good reply. Just remember 'io sono ' is 'I am' 'Loro sono' is they are. If it were third person plural It would need to be (loro) sono tartaruge.or sono delle tartaruge. It is a feature in Italian that is rather marvellous. You know 'una ' is singular and feminine, therefore tartaruga is singular and feminine, and that there can only be one person being one.
Sono is indeed the conjugation of the verb essere for both Io and Loro, but when it means I am a... the noun that follows should logically be singular, because I am one person (also the indefinite article also indicates that it is singular), and when it means they are... the noun should logically be plural