Introduction

Below you will find a companion article to our recent one on snails for the reef aquarium. This one is on crabs, and it is, by no means, an exhaustive treatment of the subject, but rather an introduction to the ones most commonly found in the marine aquarium trade and ones you may have heard of.

What scientists call "true" crabs are decapod crustaceans belonging to the infraorder called Brachyura. Decapod means literally "10 feet," however, decapod crustaceans can have many more than 10 appendages. And decapod refers to the order, Decapoda.

True crabs are on the left, and some common crustaceans that we'll mention from Anomura are on the right. Any crab that isn't from Brachyura may look like a crab and act like a crab, but it's not technically a crab.

So, greatly simplified crab taxonomy looks like this below. Brachyura, on the left, and Anomura, on the right, are, for example, just two of 12 infraorders under the suborder, Pleocyemata. There are currently estimated about 7000 species of true crabs within Brachyura of which about 10 percent are found in freshwater.

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These are screenshots from Wikipedia, cited at the bottom, and courtesy of @Seawitch.

Aside from the issue of which crabs are actually true crabs, there are enough species that crabs are often incorrectly named or labeled at your local fish store (LFS). Aquarists use common names for crabs, but the common name may refer to crabs of more than one species or genus.

Crabs in the reef aquarium are controversial. Some aquarists feel strongly that the only reef-safe crab is a filter-feeder-type crab. Other aquarists, such as our highly respected forum member, @Lasse, who has a whole method of starting a reef aquarium named after him, recommend having several types of crabs as part of your clean-up crew (CUC). So, you will have to be your own judge, especially of ones that hitchhike in, and you aren't even sure what they are.

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Captivating Crustaceans

Crabs and other captive crustaceans are often seen as a controversial consideration for your clean up crew. This is due to their possible consumption of their cohabitants, and here we hope to clear up and clarify some of your concerns regarding these crusty crustaceans.

Alliteration aside, it’s because of these worries that some aquarists have sworn crabs off all together declaring them not worth the risk. However others will insist that discounting crabs entirely is an unfair judgement, and they need to be considered on an individual basis rather than as a broad group.

All types of crustaceans commonly labeled as crabs are omnivorous and could, under some circumstances, attack your corals and fish. But hopefully this beginner's guide to crabs will help you determine whether they should be a fixture of your aquarium, or whether the troubles in keeping them are better left to others.

· Anemone Crabs: An Anomuran rather than a proper crab, they are filter feeders and have very distinctive colors, being white with reddish brown or black spots. They live in close association with anemones, as they don’t get stung by them. They will often spend their entire lives on one, crawling around the base or up and down the tentacles. If they aren’t sold with an anemone you will likely need to provide one or they will remain hidden. Unfortunately, the anemones can often be more difficult to keep alive than the crabs. Carpet anemones work well and are relatively hardy compartively speaking. They will eat plankton if available but will also eat fish food if offered, and they won’t harass other tank dwellers.

Arrow crab.
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Photo is from the Reef2Reef archives courtesy of @jonny, ©2019, All Rights Reserved.

· Arrow Crabs: At first glance these critters resemble an underwater daddy longlegs but they have a distinctive snout, called a rostrum, that protects the front of their shell. These crustaceans are carnivores that will go after any sort of meaty food including other small invertebrates, bristle worms, feather duster worms and have been known to go after small sleeping fish when given the chance. They are fine in fish-only aquariums with fish big enough to take care of themselves but should probably not be put in a tank with other small invertebrates. If insufficient food is provided they also may pick at polyps and mushroom coral.

· Coral Crabs: These small crabs can be found nestled between the branches of stony corals and are often found in Acropora and Pocillopora. These crabs will occasionally arrive with these corals so you may get some unintentionally. While they are carnivores they are thought to dine on coral mucus and don’t do any actual harm to the corals though there are similar-looking imposters that will feed on coral tissue so keep an eye on them, removing them immediately if you see this occurring. Proper coral crabs will clean and defend their hosts, aiding in stronger growth and better survival of the coral.

· Decorator Crabs: These crustaceans are recognizable by their habit of taking small bits of algae, sponge, tunicates and other soft coral polyps and sticking them to their own shells for camouflage. Some types are omnivores, but most seem to be straight carnivores. Most need a ready supply of “decorations” and often will cut up what they need if they can’t find what they need as detritus. Some have very specific diets and will starve in captivity, and they are not really recommended for most aquarists due to the difficulty in keeping them.

Emerald crab.
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Photo is courtesy of and used with permission from World Wide Corals, ©2019, All Rights Reserved.

· Emerald Crabs: A green crab within the Mithrax genus. There are also red ones. See Mithrax.

· Fiddler Crabs: These very recognizable crabs are unsuitable for typical aquariums as they usually live in the intertidal zone where they dig burrows and spend much of their time out of the water. The males have a greatly enlarged claw which they use to attract mates and threaten competitors. All fiddler crabs are omnivorous and eat both meaty foods and algae and should only be kept in special aquariums that mimic the intertidal zone.

· Gorilla Crabs: Gorilla crabs are any hitchhiker crabs that have a dark "hairy" appearance. Generally not considered reef-safe and may come from several different genera.

Polka dot hermit crab
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Photo is courtesy of and used with permission from World Wide Corals, ©2019, All Rights Reserved.

· Hermit Crabs: These are Anomurans that are often available in stores and are often found in both small and big varieties. They make great algae eaters and don’t generally cause problems. However some can be purely carnivores and can kill other small invertebrates.

If you cannot positively ID a small crab and be sure its okay to keep in your aquarium, keep a close eye on them. All species of hermit crab will scavenge meat if the opportunity arises, and you may find them feeding on a dead fish or snail. Most of all be sure to offer shells for your hermit crabs so they don’t attack your snails to get new homes.

Many hermits can get much larger and fancier in appearance but can be a threat to many invertebrates and small fishes in your tank. Even more mild-mannered large species will get large enough to knock things over as they crawl around and are generally unsuitable for reef aquariums. In fish-only aquariums they can be excellent as there is nothing for them to damage and they can be very engaging tank denizens as often they will crawl around rather than staying hidden like many other crabs.

· Horseshoe Crabs: These are not true crabs and while they may look small when being sold they can grow to over a foot in length and spend most of their time buried in sediments. They are omnivorous but primarily consume worms and clams found in the sand bed. However, they require a lot of space and can be harmful to reef aquariums through their decimation of any beneficial worms and small organisms in the sand. They aren’t really suitable for any aquariums and should be avoided as tank additions.

· Mithrax Crabs: These are one of the most commonly available types of crab. While they are omnivorous these crustaceans are popular due to their algae-eating habits. They tend to eat bubble algae and other types but be sure they get plenty of “green food” so they don’t go after things like snails and fish, as they have been known to become predatory when hungry. These crabs have a wide range of sizes, from the small Mithraculus to the large Mithrax but be wary of some of the larger types as those have been known to try to catch small fish.

· Pea Crabs: These are tiny crabs that may hitchhike in on a giant clam or other bivalve mollusk. They steal food from the mollusk to survive but don't typically kill the mollusk. Whether you want to keep them or not is up to you.

Pom pom crab.
r2rwwcpompom.jpg

Photo is courtesy of and used with permission from World Wide Corals, ©2019, All Rights Reserved.

· Pom Pom Crab: These crabs are white to tan in coloration and have darker markings covering their bodies which serve as camouflage. They carry anemones on their claws for defense which is where they get their name. This type of crab aids in cleaning the aquarium of leftover food and detritus--though be warned as the anemones could sting some invertebrates and corals though damage should be minimal. You should be sure to supplement their diet with meaty foods such as mysis shrimp, chopped fish, and clams.

· Porcelain Crabs: Despite the name, these are Anomurans, not true crabs, though they are a sister group and incredibly similar. While they aren’t seen for sale that often, from time to time they will enter an aquarium on live rock, especially rock from Florida. These crustaceans are filter feeders that use special appendages to capture plankton. For this they use limbs that look like little fans that they sweep through the water and pass what they catch into the crab's mouth. They have relatively large claws but these seem to be just for show rather than possessing any predatory function. They will occasionally eat fish foods but won’t bother any other tank denizens so they should be fine with any aquarium setting.

· Sally Lightfoot Crabs: Also called spray crabs these guys are primarily algae eaters but like many crabs they will eat meat and may go for small fish if the opportunity arises. However, they should be fine in fish-only aquariums provided that the fish are large enough to take care of themselves. But you would be taking a chance putting them in with other invertebrates.

· Shame Face Box Crabs: From genus, Calappa, these crabs have claws that fold up to cover their faces. This type of crab often gets large and they have claws purposely built for breaking open shells. They’ll also bury themselves in sediment and stir things up in addition to bowling over rocks and rearranging things. They are not suitable for reef aquariums.

· Stone Crabs: Hitchhiking juveniles are occasionally found on live rock, but they can get very large and can be outright destructive, eliminating all types of creatures in no time at all if given the chance. They have large, heavy claws that they can use to crush the shells of snails and clams and have a bad habit of knocking over and rearranging rocks, digging burrows all over the place. No type of stone crab is a suitable addition to a reef aquarium.

· Strawberry Crab: Also known as the Red Boxing Crab, this crab originates from Hawaii and can be identified by its bright pink coloration with white spots. Peaceful and personable, generally getting along with other invertebrates and fish, these crabs have claws that are specialized for picking algae off of rocks. They not only control unwanted algae but also eat uneaten food but you should supplement their diet with both herbivorous and meaty foods in order to maintain health.

We hope that our coverage above has provided you with useful information in regards to these controversial crustaceans and whether or not they are right for your tank.

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References

https://www.advancedaquarist.com/2010/12/inverts

https://www.liveaquaria.com/category/501/

https://en.wikipedia.org/wiki/Anomura

https://www.aims.gov.au/web/creefs/true-crabs-false-crabs-and-crabs-twice-described

http://www.mesa.edu.au/crustaceans/crustaceans05a.asp

https://academic.oup.com/mbe/article/31/5/1173/996855

https://en.wikipedia.org/wiki/Crab#Classification

https://en.wikipedia.org/wiki/Anomura

http://www.marinespecies.org/aphia.php?p=taxdetails&id=106670

https://www.ncbi.nlm.nih.gov/pubmed/24520090

https://www.pangeashellfish.com/blog/what-are-pea-crabs-and-why-are-they-in-my-oysters

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Author Profile: Peter Steckley

Peter Steckley is a freelance science writer based in Calgary, Alberta, Canada. When he's not studying reef aquariums or testing out new recipes on his family, he's usually reading or enjoying the latest video game releases.
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