The wrasse - or Cleaner fish as it is known because of its habit of eating parasites from the skin, gills, and mouths of other fishes - is a brightly coloured fish living on Indo-Pacific coral reefs. In the words of the zoologist, R. Robertson (in "Sex Changes under the Waves") - "The ultimate ambition of female Cleaner fish is to become male." A male generally dominates a group of females: if however he abandons his harem the most dominant of the remaining females begin to change sex within a few hours. The tendency in females towards sex change can be accounted for on the basis of an inspection of their gonads - the females are actually hermaphrodites as they all have small amounts of active (but walled-off) testicular material scattered through their ovaries. This peculiar situation among the wrasse is said to afford a number of ecological advantages.