Red crabs will not breed succesfully in an aquarium, that we know for sure. The 'fry' are microscopic and need microscopic food, they require sea water to develop in and then move gradually towards fresh or brackish water. They are very cannibalistic also which is why each crab has millions of eggs.
When a female red crab is bearing eggs, you can very clearly see a large mass of red coloured goo under the legs, I've never seen a long stringy thing coming out of my crabs but it could be coming from the mouth parts (the crabs regurgitate waste back through the mouth).
You can tell if you have a female by looking at the flap underneath, if its wide and rounded, almost reaching the legs, then the crab is female, the male has a much thinner pointy flap. Thats the only way to know for sure, as a male crabs claws will be smaller if they have ever been lost previously.
