Crabby Day

[mike31154]

Looks like a species of Mithrax crab. I literally have a small herd of these dudes in my 75 gal and several of them are a fair size. Prominent scoops on the pincers. From my observations they've mostly been scraping the live rock with their scoops, but I've also seen them go after small meaty scraps. One of them will hang out around my BTA and I've seen it reach in to remove the stuff coming out of the nem when it's expelling waste. It has also tried to take bits of silverside I feed the nem, but the nem is healthy and sticky so it's generally not successful in that regard. Whenever I feed the BTA, I make sure my cleaner shrimp and brittle stars get a small bit of meat first. That way the cleaner shrimp are less likely to come nosing around the BTA. When I first got the nem, it wasn't super healthy or sticky, so the cleaner shrimp stole its food. Now that it's healthy and much larger, the shrimp no longer do this but I still feed them first anyway, just to keep them busy.

From a quick web search I found some info on an 'emerald' mithrax and articles on these report they are generally reef safe as long as you keep them well fed with algae. It won't hurt to offer a few meaty scraps from time to time. Otherwise they may turn to corals. Mine are not emerald, they're quite black as those in your photos. Other than a colony of zoas, I don't have any corals and so far none of these crabs have molested or damaged any polyps. Dunno, so far they haven't been responsible for any deaths that I'm aware of so I'm just not sure I need to yank them out.

[untamed]

I agree. They look like emerald crabs to me... The flat surface of the main claws suggest that they are likely harmless.

[Snaz]

Originally Posted by untamed

I agree. They look like emerald crabs to me... The flat surface of the main claws suggest that they are likely harmless.

Really no crab is "harmless", all crabs are opportunistic omnivores. Some crabs are less harmful to reefs than others is all.