[KenP]

Just observed a shrimp molt. Never a camera around when you need one. Within seconds 2 males became quite agitated swimming around the tank. The molted shrimp I presume is a female. Whenever one of the males got close to her she darted away. This activity lasted about 20 minutes. Males get close, she popcorns away. When a male would find the molted exoskeleton he would pause and inspect it then continue his search. After about 20 minutes the female disappeared and so did the males. The tank resumed its normal activities.
If she was fertile why would she release pheromones but then run away from the males? If she was not fertile why would she release pheromones causing males to chase her? I have only seen Halocaridina rubra's molt. With them I never noticed this activity. I figured that they were either males or non receptive females.

 

[GeToChKn]

All it takes is a second for them to mate. The male approaching the female and then her flicking away, that may have been enough time right there. She will hide for a few days regardless, to either let he exoskeleton harden from after molting, or let it harden and move the eggs down to under her. You will see her in a few days, probably with eggs.

And it's not like she has a choice to release pheromones, it happens. Happens in most animals, ever see a male dog catch a scent of the heat. They go stupid, pant for days till everything is soaked, they won't eat, won't go to the washroom, just sniff the air and whine and pant. Doesn't matter if the female wanted to mate, it just happens.