Identification of Marine Crabs

Red legged crabs are great scavengers by feeding on algae, remaining fish food, and detritus. However, if the food supply gets too low (or they like a snail’s shell better than theirs), then they will kill snails. Because of this, keep enough food for the crabs (which is usually not a problem in most aquariums) and an available supply of various size/color shells.

We named this little fellow, “Godzilla,” for his tremendous strength and bulldozing capabilities. These crabs are not reef safe (at least in my opinion, but others disagree) since they grow quite large and will eat just about anything (including ripping one of my corals to shreds to eat the food the coral caught). However, these are great scavengers for a non-reef tank without predators, as they are able to survive a wide variety of temperature and salinity ranges. Plus, they’re a lot of fun to just watch!

Out of all crabs available for a cleanup crew, these are the only ones I feel at all comfortable recommending. They are great scavengers, and their small size allows them to maneuver throughout the aquarium rather well. They eat many types of algae, detritus, and leftover fish food. Provide plenty of extra shells in various sizes and colors so the crabs will not kill snails for their shells.

Although similar in size and shape to the more common blue-legged crabs, the Electric Blue hermit crabs are much more brilliant. They are also somewhat more aggressive, in my experience. But, like the blue-leegged crabs, they are still great scavengers, and their small size allows them to maneuver throughout the aquarium rather well. They eat many types of algae, detritus, and leftover fish food. Provide plenty of extra shells in various sizes and colors so the crabs will not kill snails for their shells.

This beautiful (and massive) hermit crab is far from reef safe as it can grow extremely large, be extremely predatorial, and also be very destructive. It is best kept in a fish-only (species limited to those that will co-exist, of course) or in a species-specific aquarium. They can also be housed in larger sumps with regular feedings.

This neat crab has a symbiotic relationship with its shell. The shell is composed of stinging creatures (similar to jellyfish), called hydroids and bryozoans. The shell of hydroids/ bryozoans protects the crab from predators. When the crab eats, pieces of food remnants are often caught by the hydroids/ bryozoans. As the shell grows, the crab trims the opening with its claws.

Although this is a very unique crab, it is not recommended for most reef aquariums as the shell itself requires special care. The organisms on the shell may die if the aquarium is not mature and stable enough. Additionally, these stinging creatures may harm corals, or corals may harm it. Lastly, due to the unique shape of the shell, the crab may not be able to right itself if it falls over.

Since there are several hundred species in the family Xanthidae, identifying this particular crab to the species level is rather difficult.  However, black-tipped claws are a good indicator that this crab is bad news (not all black-tipped crabs are bad though).  Xanthid crabs typically enter the aquarium world by hitchhiking on live rock or corals.  They use their strong claws to enlarge hiding spots in the rock, which ultimately weakens the rock.  Even worse is if they decide to nest in a coral since they will carve out a hole and destroy the surrounding tissue.  Xanthid crabs are known to eat almost anything and are very destructive in the process.  Recommend removal from a reef aquarium as soon as possible.  Occasionally soda bottle traps work, but typically homemade spears work best.  If removed safely from the reef aquarium, I recommend keeping these crabs in a species-specific tank or suitable sump as they are very interesting to watch.

This is another example of a Xanthid crab (the black-tipped claws are hidden under its body.)  This is one of the many crabs that falls under the generic name, “gorilla crab”.  Recommend relocation from a reef aquarium to a suitable sump or species-specific tank.  See above for more information.

This is another example of a Xanthid crab, and this is one crab that frequently falls under the common name, “Teddy Bear Crab.”  It is also frequently called a “Gorilla Crab”.  See above for more details/warnings on Xanthid crabs.

Like many crabs for the home aquarium, the emerald Mithrax crab may become overly aggressive as it grows. While small, the emerald crab is another great scavenger, and is one of the few crabs known for removing bubble algae (Valonia) (some eat it more readily than others). The emerald crab may also damage SPS corals as it
becomes larger by literally taking out chunks to eat. If you have a mature reef, you may not notice the damage, but if your tank is full of small coral fragments, the damage may be tremendous. Although the emerald crab is mostly available in green (hence the name), a red Mithrax variety is also sometimes available.

The arrow crab is quite unique and is ok for a reef aquarium without small fish (arrow crabs are known to catch small fish). Additionally, arrow crabs are known to eat the occasional bristle worm and may help control the population.

In the wild, I typically see them with corkscrew anemones (Bartholomea annulata) or branching anemonees (Lebrunia sp.), and sometimes even with Pederson shrimp.

Pom Pom Crabs are… well… just awesome.  They are beautifully colored, carry anemones for protection, and act like boxers about to take a swing.  They carry tiny anemones on the tips of their claws for protection.  If they feel threatened, they will sway their arms out and occasionally strike an opponent with the stinging anemone.  In exchange for the anemone’s protection for the crab, the crab helps feed the anemones.  The crab’s anemones may sting and damage typical reef inhabitants (e.g., corals, crabs, fish), so caution is urged.  However, this crab can be rather secretive, so potential damage is limited.

The Sally Lightfoot Crab is a scavenger and looks for detritus and dying matter to eat.  Its body shape allows it to fit into very tight places in the rocks and to run quickly to safety.  While the crab is great when smaller, as it grows it becomes more aggressive and may prey on small fish.  Keep in mind how hard this crab will be to catch as it grows if it becomes a problem.

The Zebra Urchin Crab, also known as the Zebra Thorn Crab, is an obligate symbiont of sea urchins. This means that both depend on each other for long-term survival.

Here is a female gall crab (notice the yellow eggs). Typically the females burrow into a coral and live out their lives in a small tunnel or hole (see image below with the red circle), while the males roam free. I often find these associated with struggling corals (as in the example shown), but I am not convinced that they cause the coral’s decline. The female does burrow and create a small amount of damage, which may lead to a bit of stress in the coral. However, a healthy coral should be able to tolerate this. I guess the jury is still out, but until then, I’ll appreciate the diversity they bring to a reef.

Do you see it? Good, neither do I. The Decorator Crab uses whatever is available (e.g., sponges, corals, anemones, rocks, shells, food) to adorn the small hairs covering its body as camouflage and protection. This crab can be reef-safe, depending on how it decorates. But, do not be surprised if small frags go missing, as this crab may take zoanthid polyps, pieces of leather coral, or other small snippets for personal use.