Yesterday I added a bunch (18) of green emerald crabs to my display tank. They are hard workers!
My experience:
I got 4 and added them to the tank. I was very concerned as to how they would treat my dear inhabitants so I started by putting one in the tank and the other three temporarily in a deli container that had holes in it, stuck to the side of the tank using an extra magnetic pump holder. So when the first one seemed doing fine and not going on a destructive rampage, I let the other three out of their temporary holding and get to work. I quickly realized I could use a bunch more, so I went and got another 14 of them.
Some more of my experience with them after observing them carefully for a few hours yesterday and today:
They basically go right to work where you place them. Don't just drop them in, place them carefully where you want them (they are slow movers).
They don't pinch (at least they didn't try to pinch me..), so you can grab one with your index finger and thumb, and place it wherever you need it to get to work cleaning the rockwork.
Of all the crabs I handled, not one dropped a claw. The claws are their tools they use to scrape the rocks clean and so you really don't want any of them to loose a claw, and I sort of expected to loose a few but not even one dropped its claw. That was very good and I was surprised.
They are not skittish and do not hide when you come up to the glass. This is good because if you want to grab one and move it elsewhere, it's pretty simple. They don't scatter far even when they are alerted you are chasing them. They should be called sloth crabs if you ask me, lol. But this is good because I was very concerned that if I had a problem that they would be hard to catch. Hence why I only let one loose at first, to see how it behaved.
They scrape the algae off the rocks very slowly and carefully and make a clean path as they move along. They work around the edges of the corals, so the encrusting corals now dont need to plow over algae, they'll have a nice clean area to grow now. The snails previously did a good job but didn't cut into those growth edges as good as the crabs do.
The crabs also get into the birdsnest corals and bases of the acroporas and clean really well, mainly and most importantly they get the areas that my tweezers previously were unable to reach. Those reading this that have old birdsnest colonies will know exactly what I mean.
As they move along and make a clean area, some of the asterina starfish I have in the tank followed along, seemingly finding scraps or fresh areas to graze further. One of the crabs decided to spend the night atop one of my large Duncan colonies, which made all the heads retract. Overtime this will make the coral use more energy to open and retract more than normally necessary but I don't forsee it to be a large problem.
There are tradeoffs when choosing clean up crew, with the way they will interact with the rest of the inhabitants. These are my first observations and I'll update the thread in the future with more.
Overall I am extremely happy with them and I would recommend them. I have an assortment of corals, those that have seen my tank know what I mean.. and these guys are hard at work now keeping it nice and tidy and overall not doing anything truly destructive or majorly undesirable.
I expect this addition of the flock of crabs to bring forth a new change to the dynamics and nutrient flow in my tank. No longer will micro (and occasionally macro) algaes be living on the majority of the surfaces of my rockwork. The crabs eating what grows there will return the nutrients to the water column after the waste is either dissolved or further broken down. So my refugium will certainly start to grow chaeto even faster. My skimmer will work a bit harder. And my phosphate removal media I will definitely want to change on an even more regular schedule (I'm pretty good about it anyway though.).
I hope this helps anyone considering Green Emerald Crabs (Mithrax Crabs) for their reef. Please share your experiences with them also.