Hi there
Unfortunately there is no way to know whether they are male or female unless is during mating season which tends to occur If I am not wrong, in summer or at least when the water is hottest (around 70 degrees farenheit).
I have had mixed results with BSJ, same as everyone. Its all down to finding a hardy specimen and second a proper environment, which IMO is pretty hard to do.
For instance, this fish does not normally do well in a mixed environment, with this I mean with other fishes so be sure to pick up proper tank mates. This, wrong substrate and the wrong tank temperature are IMO the main reason of their highish failure rate.
Another problem you might have is exactly this. Being such as secretive fish I doubt they will mate in a mixed reef aquarium so make sure you chose the tank mates well.
The success I have seen was in an aquarium with no or very few fish and tank mates. But there is no rule about this, there are also plenty of articles out there about breeding them.
Also, to recreate their mating environment you need to increase the tank temperature, which in most cases means lowering it as you are probably running your aquarium way to hot for this fish (this fish is meant to leave on water temperatures on the 50s to high 60s low 70s.
With this I dont mean you wont be able to mate them, just that it would be tricky.
Just finding 1 fish that is hardy enough and get him/her to live for more than a year is hard enough so nevermind 2!
But back to their behaviour, dont worry about your jawfish not making a burrow, it could take a few days before he does, normally is an indication of either having the wrong substrate or him being too stressed or the opposite, not threatened! So hard to know as each individual is completely different to each other.
I am upgrading aquariums at the minute, and I decided to put my BSJ on its own tank at 65 degrees. He is very happy there, only has a crab to worry about and that's about it.
He was in a 60 gallon tank with Tangs who were very aggressive at feeding time but he still would stand his ground and always get his bit of food, moved him into the new aquarium (only him) and he didnt make a burrow for 2 weeks. I am not sure I am going to move him into my new 200 gallon, I think he will be better suited on its own, or maybe with 1 or 2 other subtropical fish like the catalina blenny.
This is a picture of him when I threw him into the tank
BSJ is not necessarily an aggressive fish, or at least IMO not in the way people think of aggressive fish. They are territorial and dont like others coming anywhere near their burrow, this as you know is bad as BSJ are quite brave and dont care of who the intruder is they will just make sure they tell him that this is his burrow, this normally means just opening their mouth as a warning, rarely goes any further, unless the other fish ignores this threat or decides to fight back... hence another reason to put him with peaceful tank mates, I have seen BSJ being killed but other fish such as triggers just because the trigger wouldnt wonder too close to its burrow and he would attack the trigger to scare him away, bad news for the BSJ as triggers dont tend to back off easily.
Give them time, hopefully they will both make it and who knows, they might even start breeding in the quarantine tank!
I would suggest a bit more substrate? maybe a mix of different size grain? I got mine on a mix of 0.3 to 0.5 cm gravel (40%), rest a mix of 0.2mm to 0.1cm and he seems to love it. Also created some PVC caves to help him hide. Its a temporary aquarium but he seems to enjoy it in there.
This is him 1 week later
Funny enough on the big tank he was much more active and he would swim out in the open much more often than he does in this tank.
If I decide not to move him to the new tank I will make him a more suitable tank and find a few good tank mates for him but I will never buy more BSJ ever, this fish IMO belongs in the sea. Only reason why I got this one was because he had been in the LFS for over 2 months inside a little box and I felt sorry for him.
I got mine over 1.5 years ago now
Good luck anyway, Jawfish are my favorite fish, post pictures