Green Emerald Crabs

So you want to get rid of bubble algae and think the eye-catching green emerald crab will take of it for you?

Like many others in the hobby, I can’t recommend them for this purpose, as their appetites can sometimes lead to mischief. So before you catch them nipping on your expensive coral polyps, let’s look at what they can and can’t do in your tank before you add one.

What Emerald Crabs Look Like

An Emerald crabs shell has a green hue that gives them their name, kind of like the gemstone. Their flat, compact bodies (up to 2.5 inches wide) let them squeeze into tight spots and press up against rocks to avoid predators.

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You’ll notice their hairy legs, tipped with sharp points for gripping, and large, powerful spoon-shaped claws.

They might be mistaken for green porcelain crabs, but they are smaller with smoother shells, while mithrax crabs are bigger and rounder.

Wait, did I say Mithrax? Yep! Green emerald crabs are in Mithrax crab family. The scientific name for green emerald crab is Mithraculus sculptus, and they are sometimes referred to as Mithrax crabs in reef-keeping shops (but verify the exact type of crab before buying).

There are other types of crabs that aren’t reef-safe and are similar in size and shape but lack the claw shape and green color of emeralds.

Diet And Feeding

One of the main reasons that you might consider getting an emerald crab is because you’ve heard that they are great for getting rid of bubble algae.

Their specialized, spoon-shaped claws are perfect for scraping both fleshy and stringy hair algae off surfaces, but in my experience, you’re better off managing that algae in other ways than relying on these crabs alone.

Here’s what you can expect them to eat:

• Algae (not reliable for bubble algae control)
• Detritus and organic debris
• Small invertebrates
• Leftover fish food
• Nori and zucchini

They are generally nocturnal scavengers that come alive after dark, but you may still see them do their thing when your tank lights are on.

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Will emerald crabs harm your corals? While they’re generally reef-safe, a hungry emerald crab might nibble on coral polyps. Keep them well-fed to help prevent any coral-snacking. However, I’ve found that once they get a taste for polyps, they will go back to picking at them, even when well-fed. At that point, you might consider giving it away or putting it in your sump.

Do emerald crabs eat Aiptasia? While emerald crabs may occasionally nip and snip Aiptasia, they’re not reliable Aiptasia controllers and shouldn’t be relied on as a solution for Aiptasia infestations.

Tank Setup And Acclimation

Emerald crabs require plenty of live rocks for hiding, climbing, and foraging. Lower lighting is preferred, as it reduces stress and helps maintain an environment similar to their natural, dimly lit coral reefs. As always, you should have stable water conditions with moderate flow.

Important: Never use dips, copper, or other treatments that are not designed for invertebrates, as it will likely kill them.

Before you dump your new crabs in, make sure that temperature and salinity match the bag water and your tank. I don’t think drip acclimation is necessary, but do it if you prefer.

Then, if you’re feeling ambitious, carefully scoop it out of the bag and place it in the water so that it will fall to your rockwork. This will allow it to hide immediately and keep its stress minimal.

I don’t like moving crabs with nets because they tend to stick to them, and I’m definitely not pouring bag water into my system.

How many emerald crabs can you keep in your tank? For a standard 30-gallon reef tank, one or two emerald crabs are usually the maximum. In larger systems, you can add more, but always provide enough rockwork and food sources to help prevent territorial disputes.

Behavior And Tank Mates

Emerald crabs are generally peaceful; they prefer to stay to themselves, especially during daylight hours. Once the lights go out, they transform into busy little scavengers, combing your tank for tasty morsels.

They can coexist with their own kind in larger tanks, but in tight quarters, they might get a bit crabby with each other (pun intended). So you’ll need to monitor aggression if you’ve got multiple crabs, including other types.

Suitable tank mates:

• Peaceful fish that swim in different areas
• Non-aggressive invertebrates
• Snails and some hermit crabs

Species to avoid:

• Big, bullying, or predator fish
• Slow-moving snacks (I mean, snails) they might overpower
• Other crab species that could start turf wars

How often do emerald crabs molt?

Emerald crabs typically molt every few months, with younger crabs molting more frequently. During molting, they may hide for several days until their new exoskeleton hardens, so don’t be alarmed if you don’t see them for a while.

Breeding And Reproduction

Breeding emerald crabs in your home reef tank? Good luck with that! It’s technically possible but incredibly rare, as they are notoriously finicky about reproducing in captivity.

If you’re lucky enough to witness any mating behaviors or egg-carrying females, consider it a fascinating glimpse into their natural life cycle rather than a sign of imminent success in breeding them.

The larvae need specific conditions to survive, and your average reef tank just isn’t cut out for providing for them. Water parameters have to be spot-on, and the larvae need special food right from the get-go.

More Emerald Crab Tips

Keeping your emerald crabs healthy is all about attentiveness and good tank habits.

Here’s what to watch for:

• Lethargy or hiding more than usual? Could be stress, poor water quality, or tank mate issues.
• Lost appetite? Might be molting soon or not feeling well.
• Pale or discolored shell? Check your water params and diet.
• Missing limbs? They’ll usually grow back, but keep an eye out for bullies.

Keeping your emerald crabs happy:

• Create plenty of hiding spots.
• Keep calcium levels up for shell development and smooth molting.
• Quarantine new tank mates to prevent hitchhiking nasties.
• Don’t expose them to dips or treatments

Can You Add An Emerald Crab To A Reef Jar?

Thinking about adding an emerald crab to a reef jar? Don’t.

Emerald crabs need room to roam. A 10-gallon tank is typically recommended as the minimum size for one crab. In a reef jar, they will feel cramped and not have enough algae to graze on, leading to stress or aggressive behavior. And their big size will lead to bulldozing over your tiny corals.

If you’re set on a pico reef, stick with smaller, less active invertebrates like certain shrimp species or tiny snails that are better suited to life in a jar.