My beloved hermit crabs are being cared for by a friend over break, but one of the things I’m most often asked about them is what gender they are. Not only is it impossible to tell, but according to my book, they can’t breed in captivity.
Not that I want itty bitty hermit crabs running around my dorm room… but why is it that certain species can’t breed in captivity? IIRC, sharks are another one who are unable to do so.
Maybe they’re shy 
“Shyness” is a factor.
In a controlled environment, we are often unable to recreate all the subtle environmental and social needs of a given species. Reproduction could be triggered by light levels, temperature changes, chemical changes in the population’s drinking water, the blossoming or fruiting of a specific plant, or any number of other factors.
Vicunas produce a superior wool to llamas or alpacas, but they require entire mountainsides for seperate feeding and mating areas. They have a rather complex ritual they need to perform before the female is accepting (three hours of begging {rimshot}). They have not been able to be bred at a commercially successful level. Other animals are even pickier about their needs.
I don’t believe Sally Lightfoot crabs have been bred in captivity, as they need to release their eggs into the ocean and they only release their eggs when practically the entire crab population is on the beach carryin fertilized eggs. The common land hermit crab may need something analogous, or just be so much easier to capture that no one has studied exactly what they need to be bred.
Many times it is not impossible to breed a given animal, it is just that no one has yet taken the time and expense to figure out just how to do it. You may find that crab-raising is your calling in life…
I think things like crabs are influenced a lot more than mammals by what others are doing nearby. All their hormones end up in the water, and they can coordinate with others of their species. Some fish do this to create great tides of mating fish, when they won’t mate at all in a tank with a filter.
It’s not that they CAN’T, it’s that they WON’T.
Several, if not many, species refuse to breed for our amusement and/or convienience. Usually it is assumed that the environment isn’t just right. (I knew a woman like that once. {rimshot repeat})
Don’t you hate it when someone is watching?
Actually, I find that quite arousing.
Breeding in bondage … it certainly doesn’t work for me either, maybe I lack the requisite sado-maschochistic tendencies?