I have tried Emerald crabs (Mithrax sculptus) before, but never really noticed a difference. They didn’t touch any bubble algae and I rarely saw them. With my previous experience in mind and some horror stories from other hobbyists, I was hesitant to try them again to help on a few small patches of Bubble Algae [...]
I have tried Emerald crabs (Mithrax sculptus) before, but never really noticed a difference. They didn’t touch any bubble algae and I rarely saw them. With my previous experience in mind and some horror stories from other hobbyists, I was hesitant to try them again to help on a few small patches of Bubble Algae (Valonia) that have recently popped up in the glassbox.
We picked up three small specimens at the LFS. After acclimation I placed each directly on their own patch of bubble algae. To our surprise within seconds they were stuffing their small mouths full of it; I was shocked. As hobbyists, we always hear that Emerald crabs do eat Valonia, but in my 10+ years in the hobby this was the first time I could actually vouch for what previously seemed like a myth. They were picking clumps with 4-5 small Valonia bubbles at a time and mowing it down quickly.
As days have passed there are many noticeable “bald” spots in the Valonia patches and they are quickly being eradicated from our reef aquarium. We are far from experts regarding this invertabrate, but we do have a few suggestions if you are considering adding an Emerald Crab to your reef.
Emerald’s can be nasty so the first thing to look at is size. Find the smallest specimens you can. I would advise staying away from larger clawed crabs in general, for example the Emerald shown below.
Via Flickr:JimmyRay
Some say that females are more likely to eat algae, and less likely to cause trouble. I cannot say this is true or not, but to play it safe we picked female Emeralds. There are two pieces that make up the crab, the carapace and abdomen. Generally the sex of Emeralds can be identified by their abdomen. The abdomen is only used in mating, and is what you want to look at on the underside.
On Males, there will be a disruption of the horizontal plates that looks very much like the Eiffel Tower. It starts at the rear legs and merges up the abdomen towards the mouth. Here’s a photo clearly showing the marking, some call it an “apron”.
Via ProjectDibs:Brian Plankis
Whereas females have a smooth abdomen with no disruption in their horizontal plates. Be aware that younger immature crabs can be difficult to sex, with some females even having the male “apron”.
When we added our Emeralds, we placed them directly on or near Bubble algae. They have since made that area their home and do not venture very far from there. This is not the first time I’ve observed this so for larger aquariums we suggest placing them where you want them. In previous systems where I have placed them randomly, they seemed to get “lost” and fed on other algae and detritus that could have been handled by other animals.
As I said before, we do not know if females are better candidates for the reef, but it is something worth considering when purchasing these oddball crabs. Be sure to choose small specimens and place them where you want them. You’ve likely heard some stories about these guys and as they grow, you may encounter problems. If you notice any overly aggressive or carnivorous behavior, pull them.
