Also you can avoid using recursion directly and use some basic abstractions instead:
val l = List(1, 3, 5, 11, -1, -3, -5)
l.foldLeft(0)(_ + _) // same as l.foldLeft(0)((a,b) => a + b)
foldLeft is as reduce() in python. Also there is foldRight which is also known as accumulate (e.g. in SICP).