Strange example word, "splatoon"... but I highly doubt it is spelt like that in katakana. What it spells is more like "splato oon". Instead of トウ (romaji: to + u) it would need to be written トゥ (romaji: tu).
So, the ー lengthens the vowel sound of トゥ (tu), the same as if you had written トゥウ (tuu).
スプラトゥーン in romaji is "supuratuun", which an English listener would spell as "splatoon" if he had to write what he heard.
July 1, 2015 at 7:30am