Oh No! Emerald Crab

Dear Caroline:

Most of the time, emerald Mithrax crabs do very well with seahorses, particularly if the Emerald crab is small and there is abundant algae in the aquarium for it to eat. Will Wooten lists them as a "2" on his seahorse compatibility guide, meaning they are generally safe with seahorses with the rare exception of the occasional "rowdy" individual, and I would agree with that assessment.

Emerald Mithrax crabs are primarily herbivorous in nature and are generally shy and inoffensive in the aquarium. I would say that 9 times out of 10, Emerald Mithrax make fine tankmates for seahorses and cause no problems at all. It’s just those rare exceptions and uncommonly cantankerous individuals you must be wary of, sir. Even the gentle Emerald crabs can very occasionally become problematic if they are not getting enough vegetable matter in their diet, in which case they may become opportunistic omnivores and are no longer averse to adding a little meat to their diet should they get hungry enough.

Remember, crabs and crustaceans in general are opportunistic predators that are liable to attack anything they can overpower. They may be entirely peaceful and inoffensive when they are small, but even a small crab can cause a lot of trouble as it grows. They may double in size following a molt (i.e., ecdysis) so they grow surprisingly fast, and even a tiny crab that’s completely docile at first can grow large enough to turn predatory almost literally overnight if it’s a species that reaches a respectable size. One day it’s a miniature crab that’s cute and entertaining in its own bumbling sort of way, and the next day following a successful molt, it can become a dangerous bully that regards its tankmates with a culinary eye.

Keep in mind that there are always exceptions to every rule, and large crustaceans are never completely trustworthy. Even the most harmless and seemingly inoffensive crabs can cause trouble under certain circumstances. For example, not long ago I heard from a hobbyist that had been keeping a decorator crab in his seahorse tank. All went well at first and there were no problems of any kind for months until, for no apparent reason, the crab suddenly began to quite deliberately amputate portions of the seahorses’ tails. It was not attacking the seahorses as prey or attempting to eat its mutilated victims, it was merely methodically harvesting portions of their anatomy with which to adorn itself! It was simply doing what all decorator crabs do — snipping off and gathering bits and pieces of its immediate environment to attach to itself as a form of natural camouflage. It just goes to show, with crabs you can never be sure how things are going to work out…

The bottom line is that it’s probably safe to keep the emerald Mithrax crab in your seahorse tank as long as there is algae for the crab to eat and it’s a relatively small specimen, Caroline. But if there is not algae for it to graze on or the Emerald crab is starting to become sizable, then it’s best to play it safe and relocate the crab to your reef tank.

Best wishes with all your fishes, Caroline!

Respectfully,
Pete Giwojna

Dear Caroline:

Yes, I think so — under the circumstances, I’m sure your Emerald Mithrax crab will be fine while you are away on vacation.

However, I am a little concerned about the hair algae in your seahorse tank. Hair algae is not directly harmful to seahorses, but it is an indicator of deteriorating water quality. The appearance of nuisance algae such as hair algae suggest there are excess nutrients in the aquarium (most likely phosphates and high levels of nitrates). So you will want to address that problem in order to restore optimum water quality to your aquarium again and get rid of the unsightly hair algae.

Search this forum for hair algae, and you will find discussions with lots of useful information for controlling and eliminating the hair algae. Let me know if you are having any difficulty locating the hair algae control suggestions, and I will be happy to post them again for you here if you can’t find them on your own.

Best wishes with all your fishes, Caroline!

Have a great time on your vacation in a few weeks!

Respectfully,
Pete Giwojna