i remember reading this years ago on planet inverts - "There are many unknowns as to the exact science behind the mating process. It is believed that shortly after molting the female is ready to mate. The way in which the male knows that the female is ready to mate is unknown. Perhaps she releases a chemical signal or some other type of notification that only shrimp can detect. It is known though that when a female molts the males in the tank will swim around the tank in a drunken manner looking to find the ready female. When you see a bunch of shrimp swimming around in the tank against the glass make sure to sex the shrimp first. If you look closely you may notice that they are all males. If that is the case then everything is ok, they are just looking to mate."
and this is from wikipedia. -
Red cherry shrimp reach sexual maturity when they are around 4-6 months old. Breeding only requires a sexed pair of shrimp, stable water parameters, and a food source.[5] Eggs may be observed developing in the female's ovaries as a green or yellow triangular "saddle" marking on her back. When she is ready to lay the eggs, which occurs after moulting, she releases pheromones into the water to signal her availability to males. The male shrimp in the tank will often become agitated, swimming very actively about as they search for the source of the pheromones. After a brief mating process, during which the male deposits sperm onto the female's body, the female lays her eggs and affixes them to her swimmerettes. The eggs are not fertilized within the female; they are fertilized as they pass from the ovaries to the outside of the body. Therefore, it is certain that any shrimp carrying eggs has mated. A female carrying eggs under her abdomen is said to be "berried".
and this from another site "First a female will find a comfortable hiding spot in the aquarium. Once she has become comfortable she will molt (molting is the shedding of the exoskeleton to enable growth of invertebrates). After molting the female will release a pheromone into the water indicating to the male shrimp her readiness to breed.The pheromone in the water will sometimes cause the male shrimp to swim erratically in search of the female. Once the male finds the female he will mate with her. They will mate belly-to-belly, and the male will deposit sperm. This process does not last very long, and because the female is hiding most times it is rarely observed.After the mating process has occurred the female will pass her eggs threw the sperm and deposit them in her pleopods (swimming legs) under her tail. The female shrimp will carry the eggs until they hatch, normally in 20-40 days. The female will often be observed fanning and cleaning the eggs. Once the eggs hatch, there is no longer any parental care of the young shrimp"
still doesn't answer my question though :)