Profile: Gorilla Crabs in Reef Tanks

Profile Gorilla Crabs in Reef Tanks

Gorilla crabs (also known as Mud crabs, Pebble crabs, or Rubble crabs) are arguably the most well-known example of invasive and destructive pests in reef tanks. The aquarium trade is the main reason for the spread of these crabs.

Gorilla crabs are hardy animals that can survive in polluted waters. They are pretty strong and can mess with your tank decorations, moving them around easily. Additionally, these crabs are opportunistic, aggressive, and territorial.

Gorilla crabs have an incredibly bad reputation. However, despite being so notoriously famous, we do not know much about them. It is like everyone has them, nobody knows them.

In this article, I’ve tried to summarize everything known about these crabs, including scientific research.

Taxonomy Problems

Gorilla crabs belong to the superfamily Xanthoidea.  Xanthoidea is globally one of the most speciose groups with more than 760 species in 170 genera. These crabs are one of the most dominant groups of decapod crustaceans.

They are distributed throughout the tropical and subtropical/temperate zones and occupy a wide array of marine habitats, from rocky, coralline substrates to soft, muddy, or sandy bottoms, and depths, from the intertidal zone to depths exceeding 500 m.

With such immense diversity, it becomes nearly impossible for the average aquarium enthusiast to accurately identify the species of crabs that make their way into our aquariums.

Are Gorilla Crabs Harmful to Reef Tanks?

Yes, they are. Gorilla crabs often exhibit destructive behavior, such as feeding on coral polyps, overturning decorations, killing other animals, etc.

The main problem though is that “Gorilla crabs” is essentially a general term for crabs that pose a threat to a reef aquarium. At the same time, it would be unfair to categorize all crabs from Xanthoidea family as pests.

The Xanthoidea superfamily includes a diverse range of crabs, and not all of them are inherently harmful to our tanks. Some species within Xanthoidea are even considered reef-safe and even beneficial.

For example, some cool crabs that people like to have in their aquariums such as Boxer Crabs (Lybiatesselata) have even symbiotic relationships with anemones.

Interesting fact: According to the study, some species are highly neurotoxic. They contain compounds such as saxitoxin, tetrodotoxin, and other paralytic shellfish toxins, which have been implicated in human deaths resulting from their ingestion. For example, the genus Demania is a group of Indo-Pacific xanthid crabs that contain poisonous species.

Description of Gorilla Crabs

Profile Gorilla Crabs in Reef TanksSize. Gorilla crabs can greatly vary in size depending on the species. Adult individuals can range from 1/2 to 1 inch (1 – 3 cm) in width and have a leg span of 3 – 4 inches (8 – 10 cm).

Carapace. These crabs have a strongly vaulted and deeply sculptured carapace.

Hairy. Their carapace, chelipeds, and ambulatory legs are entirely covered with short, stiff black setae.

Claws. A distinctive feature is the dark-colored tips of their claws.

Eyes. The eyes are usually whitish or dark in color.

They tend to grow quite rapidly.

Difference between Gorilla crabs and Emerald Crabs

  Emerald crab Gorilla crabs
Size Bigger Smaller
Setae (Hair) Only on legs Carapace, claws, and legs
Color Only green Can greatly vary from brown to yellowish
Claws Green Black-tipped claws
male Emerald Crab (Mithrax sculptus)
Emerald crab

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Typical Behavior of Gorilla Crabs

These crabs are not social. On the contrary, they are known for their aggressive and territorial behavior. They do not live together in groups and do not tolerate even their own species. Gorilla crabs have cannibalistic tendencies.

They are primarily nocturnal and skittish, especially small ones. However, large individuals may also exhibit some diurnal behaviors.

Although Gorilla crabs are not very fast, they are pretty agile and can be observed moving between rocks in search of food and shelter.

These crabs are not burrowers.

Features:

  • Social: No
  • Activity levelLow
  • Peaceful: No
  • Territorial: Yes
  • Burrowers: No

Diet of Gorilla Crabs

Profile Gorilla Crabs in Reef TanksThese crabs are omnivores and opportunistic (because they have adapted to eating animal, coral, and plant material dead or alive). It means that they will eat just about any food they manage to find on the bottom of your reef tank.

In their feeding habit, Gorilla crabs behave as a generalist, actively preying on various small invertebrates (such as snails, shrimp, crabs, etc.), fish, urchins, cucumbers, sea stars, as well as scraping alga-covered surfaces or grazing on debris. Cannibalism is also present between unequal in size individuals.

In reef tanks, these crabs can cause issues such as biting or/and nipping areas on coral colonies. Therefore, they pose a real danger to the hobby.

Note: As the Gorilla crab gets bigger, its taste in food changes too. In the beginning, when they are small, those little crabs might not look threatening at first, but eventually, they go on a rampage.

How Do We Get Gorilla Crabs?

Profile Gorilla Crabs in Reef Tanks
photo credit to JoJosReef

The most common way of getting Gorilla crabs (or any other pests by the way) is from live rocks.

These crabs are professional hitchhikers.

The thing is that along with live rocks, various “illegals” may enter the aquarium, living within the rocks. Moreover, these hitchhikers usually come out at night, and sometimes they are challenging to notice, especially if the aquarium is large.

Therefore, if you do not want any unwelcome visitors in home aquariums, you need to quarantine everything that you are going to put in your tanks.

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How to Remove Gorilla Crabs From the Tank?

1. Avoid Chemical Methods:

Not known. Gorilla crabs are very resilient, and using chemicals in an active tank can harm other inhabitants before affecting them.

2. No Predators:

In the wild, these crabs occupy a relatively low position in the food chain and are often eaten by predators. However, replicating this in home aquariums is almost impossible.

The main problem is that these predators (such fish as pufferfish, timor wrasse, and triggerfish or mantis shrimp) would pose a threat to other aquarium inhabitants.

For example, mantis shrimp will definitely hunt for these crabs, but, in most cases, they are also considered undesirable guests in our tanks.

3. Manually with Red Light Flashlight:

Gorilla crabs are nocturnal, and they rarely come out during the day. It makes them challenging to capture.

Fortunately, it has been observed that these crabs do not perceive the color red. Therefore, by using red-colored flashlights can help detect the crabs during their nocturnal activities.

Note: Catch them with a net or spear them with something sharp.

4. DIY Trap (Plastic bottle)

Profile Gorilla Crabs in Reef Tanks trap

  • Cut 1/3 of the 67-ounce (2-liter) bottle.
  • Reverse it and place in the bottle.
  • Add some meat (fish, shrimp, etc.) and leave it overnight.

The crab may crawl in, finding it challenging to escape.

5. Long, Narrow-Necked Glasses:

This method can only work if you know the crab’s favorite hiding spot.

  • Place the glass with a long, narrow neck near its shelter.
  • Ensure the tilt angle is at least 30%, otherwise, it will get out easily.
  • Put a meat bait inside overnight and pray that it will fall into it.

Don’t expect an easy victory. Capturing these crabs can take weeks, especially if you have several of them in the tank.

Gorilla crabs are adept at hiding, and occasionally, dismantling parts of the tank may be the only way to catch them.

What Should I Do With Gorilla Crabs?

If you are a soft-hearted person and do not want to harm other animals, you can place them in the sump. As long as you give them to eat something they will be fine.

However, if you prefer a more humane approach to dealing with these parasites, you can put them in the freezer.

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How Do Gorilla Crabs Reproduce?

In reality, there is very little information about it even in scientific literature. The only things that are known:

  • According to the study, the only distinct, sexually dimorphic characteristics of significance in the Xanthidae are found in the abdomen of mature specimens. The claws of some male crabs are said to be larger than those of the females of the same species, but there is no strongly marked trend towards dimorphism.
  • Generally, these crabs have at least 4 zoeal (larvae) stages and one megalops stage before the first crab stage.
  • Females can carry from 1000 to 45000 eggs.
  • The eggs are attached to the pleopods and there incubated from 25 to 30 days.
  • Hatching occurs most frequently at night shortly after dark.
  • On average, it takes 5 – 6 weeks to complete the larval stages. The megalops stage lasts from 7 to 14 days.
  • The natural food of zoeae consists of planktonic organisms, such as diatoms and other microscopic algae, larval and small adult animals. Megalops can feed upon Artemia, zoeae, and small sessile algae.
  • The xanthid zoeae are photopositive and swim by means of the maxillipeds or with maxillipeds and telson.

In aquarium conditions, Gorilla crabs reproduce very rarely. Even if a female manages to carry eggs, the larvae have extremely low chances of survival.

This is mainly because, in the early stages of their life, the larvae face numerous predators that can consume them (small fish, corals, etc.). Large Artemia and Copepods can also prey on the zoeae. Additionally, they will eat their own. Many first zoeae were seen to eat the dorsal spine of other living first zoeae.

In Conclusion

Beginner marine aquarists should be aware that with live rocks often come the so-called hitchhikers, and some of them might have a very dubious reputation.

Gorilla crabs are tough little creatures. They are strong, aggressive, and territorial. In aquariums, they will snack on anything, from algae to corals, and they might even go for a live fish!

If you still decide to keep them, these crabs can be kept only in sumps.

References:

  1. Hyman, Orren Williams. “Studies on the larvae of crabs of the family Xanthidae.” (1925).
  2. Martin, Joel W. “Notes and bibliography on the larvae of xanthid crabs, with a key to the known xanthid zoeas of the western Atlantic and Gulf of Mexico.” Bulletin of Marine Science34, no. 2 (1984): 220-239.
  3. Knudsen, Jens W. “Reproduction, life history, and larval ecology of the California Xanthidae, the pebble crabs.” (1960).
  4. Mendoza, Jose CE, Kin Onn Chan, Joelle CY Lai, Brent P. Thoma, Paul F. Clark, Danièle Guinot, Darryl L. Felder, and Peter KL Ng. “A comprehensive molecular phylogeny of the brachyuran crab superfamily Xanthoidea provides novel insights into its systematics and evolutionary history.” Molecular Phylogenetics and Evolution177 (2022): 107627.

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