Soldier crab's weapon

I was wondering why crabs are not present in all the places where I expect to find them.  Fiddler crabs live in the same context in the adjacent creek, why not here?  The only difference between the creeks is that flats below are sandy where as the other flats are have firm mud below the surface.  In some creek mouths, silt is deposited by floods, buried by sand and over a few seasons becomes a firm plasticine-like material.  The consolidated mud being firmer than the sand is probably harder for stingrays to forage in and for some species, this may determine whether survival is possible.  The sandy flat below is cratered with stingray feeding hollows and whilst soldier crabs here seem to be just hanging on, it seems fiddler crabs cannot.

Stingray hollows in soldier crab colony

We have two species of soldier crab in Queensland, the regular one that swarms at low tide (Mictyris longicarpus - below) and a subterranean one that I have never seen above the ground surface (M. livingstonei).  To check which species is present, it is sometimes necessary to dig one up.  It is a case of finding the vertical hole below the sand balls and placing a finger in the hole to find the crab at the bottom.  My finger met with unexpected pain on finding the crab so I had to investigate.  Soldier crab nippers are useless for nipping fingers, so what was the crab doing?  The crab has a spine near where the nipper joins the body and crab was pulling its nippers back hard, spearing my finger against the spine.  It was a raptorial motion like a prey mantis and it was a new trick on me.

Spine soldier crab uses to fight

Same soldier crab escaping

Naturally, that set me wondering what kind of predator might be deterred by such a weapon.  The usual cast of predators would not be deterred at all (kingfishers, ibis, toadfish and predatory crabs), however a newly identified predator may just be feel the spines, the moon snail - Conuba sordidum.  Click here to go to a video of some moon snail and soldier crab encounters.  The scientific paper on the topic is here.