Are there any outward signs like there are with shrimp to tell male from femal cajuns. They seem a little small to turn them over and check their legs.
At the size you got them it's impossible to tell by just looking at them run around in the tank. They are actually large enough to be turned around and checked for male sex organs. Once they are adults, you can tell the males apart from the females by their slighly larger and broader claws. Also, the females are broader around their abdomen to accommodate the eggs. Since the animals are very small even as adults, it takes a trained eye to see these minute differences, though.
What is the best way to handle a tiny cajun to sex them? Can it be done at any age with the right magnification? Could you use a clear bottom beaker and just look through?
I just flip them over and check under the hood. I am pretty good at sexing them that way. On occasion i will double check with a fly tying magnifying glass under light. Just put under glass and look. The smaller ones are hard to sex by putting in a container and looking from underneath. Just put them in your fingers and roll them over. Each one you do just gets easier. Soon you will have a trained eye also. Do not sex them right after a molt though as they are a bit soft. They are pretty hardy and it is fun to watch them try to pinch you. Good Luck
