Hi all,
The lighter shape you can see in the shrimp from the dorsal view is the "breeding saddle" of the developing ovary, so that shrimp is a female.
Usually you can sex RCS by colour and behaviour, the males tend to be paler and very active, whilst the females are brighter red and much less active, just staying in the same place continually feeding. When they are coming into breeding condition you'll be able to see the "breeding saddle"
cheers Darrel