Does the Emerald Crab Earn Its Algae-Eating, Reef-Safe Reputation?

This emerald crab (M. sculptus) appears to be interested in bubble algae, but that's not the case for all specimens.

This emerald crab (M. sculptus) appears to be interested in bubble algae, but that’s not the case for all specimens.

Among the various and sundry marine life commonly sold to aquarium hobbyists for utilitarian purposes is the emerald crab (Mithraculus sculptus). Oft touted for its propensity to gobble up bubble algae and other irksome algal forms, M. sculptus has become a proud, card-carrying member of many a marine aquarium “cleanup crew.” But before adding this “benign” crustacean to your aquarium to control algae, be advised that it may not be as strictly herbivorous or reef-safe as you’ve been led to believe.

Physical traits

Aptly named for its shiny, emerald-green overall coloration, this Caribbean species reaches only a few inches in diameter and has a bumpy carapace, hairy legs, and relatively robust (for its size) claws. One could argue it even looks rather like the bubble algae it supposedly consumes with such enthusiasm.

About that bubble algae thing…

If you’re adding emerald crabs to your system in the hopes of eradicating a major outbreak of bubble algae or any other form of algae, you might be disappointed in the results. Not all specimens eat troublesome algae forms reliably, and even if they do, they may not eat enough of it to bring a severe outbreak under control.

Some years ago, I added a group to my 75-gallon reef tank when it was in the throes of a bubble algae plague, but they never really made much of a dent in the problem (it took a lot of handpicking and water changes to restore sanity).

About that herbivorous thing…

Also, as I alluded in my opening paragraph, M. sculptus isn’t strictly herbivorous. It’s an opportunistic omnivore that, in addition to grazing algae, will scavenge dead animals and may even attack and consume live ones—including coral polyps and small fishes. The scavenging behavior can be beneficial in aquariums, as it helps eliminate uneaten fish foods from little nooks and crannies in the rockwork, but the active hunting can obviously be problematic if the targeted prey happens to be one of your prized fish or invertebrates.

I can’t say how common attacks on fish actually are, but I can attest from personal experience that one of the emerald crabs that were (note the past tense) in my 75-gallon definitely made an attempt on my clownfish, actually shearing chunks out of the poor thing’s caudal and anal fins (the clown survived and still resides in that same tank today). Also, there are many anecdotal reports of rogue emerald crabs tearing apart and eating zoanthids and other soft corals in reef systems.

Suffice it to say, those claws aren’t just for decoration. So, if you choose to introduce one or more specimens of M. sculptus to a reef system, keep a close eye out for damage to livestock (which is most likely to occur after dark when these nocturnal crabs are active) and be prepared to remove the crabs if any is noted. If it turns out these crabs do have to be removed from a display tank, they can always be transferred to a refugium or sump where they can still be enjoyed but can’t do any damage to other animals.

Should you feed your emerald crab?

M. sculptus is not fussy and, as mentioned, will scavenge uneaten food, so virtually anything you offer your fish will be accepted by emerald crabs as well. Some authors suggest that keeping these crabs intentionally fed will help prevent them from turning on living invertebrates or small fish, but I would keep a watchful eye on them in this regard nonetheless.

Housing

Since they reach only a few inches in diameter, emerald crabs can be kept in virtually any sized system, provided there is plenty of live rock aquascaping for them to forage on. Just be mindful of their potential to snare small fish—or to be eaten by larger ones—and that they may not be entirely trustworthy around soft corals.

Photo credit: Jamie Henderson

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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.

Comments

  1. I have about a dozen small patches (1″ square or less) of bubble algae in my 75 gallon mixed reef, and have done for over 6 months now. They spread very slowly and manual removal has been a daily part of the routine (the superglue method has slowed the spread but ultimately proven unsuccessful). But after reading this article I went a purchased 2 x emerald crabs (specifically M. Sculptus) and placed them in the tank near BA patches. Im happy to say that both got straight to work on it! One cleaned off a 2″ x 1″ area of bubble algae in a few hours. Now they it if they find it, but other food sources usually grab their attention first. I think with the addition of a few more, and more manual removal, the BA might eventually be kept at bay. After 10 days the patch that got eaten has not grown back. Good article guys!

    • Thanks, Elliott! That’s great to hear! Just be sure to keep a close watch on those crabs to make sure they don’t “go rogue” on any of your livestock.

  2. Earl Moynihan says:

    I bought 4 emerald crabs for my tank but alas they took to feeding on my xenia. Removing them has been problematic. As I said I bought 4 and have now caught and returned to my LFS 7 and know I have at least 1 more parading around my tank. There fun to watch but are tough on soft corals.

    • Jeff Kurtz says:

      Ah, yes! An all too common complaint with these crabs. Have you tried catching the remaining specimen at night? If you’re quick, you might be able to snatch it up while spotlighting it with a flashlight. Thanks for sharing your experience, Earl!

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