Weirdo alert! If you’re looking for an unusual crustacean addition to your marine aquarium, you’ve found it. Let us introduce you to this spindly number: the arrow crab. With its extremely long, spindly legs and a unicorn-like horn on its head, this crab is hard to overlook.
Keep reading for everything you need to know about keeping arrow crabs in your aquarium!
| Name (Common, Scientific) | Arrow crab, arrowhead crab, yellowline arrow crab, spider crab, Stenorhynchus seticornis |
| Minimum tank size | 30 gallons |
| Minimum group size | 1 |
| Temperature | 72-78 °F |
| Salinity | 1.015-1.025 |
| pH | 8.1-8.4 |
| Difficulty level | Intermediate |
Arrow crab description & natural habitat
Description
As mentioned in the introduction, the arrow crab will make any visitor look twice at your aquarium. Also known as spider crab, this species is easily recognized from its very long legs and strange head protrusion.
There are various species in the Stenorhynchus genus, but when we refer to the ‘arrow crab’ we’re generally talking about Stenorhynchus seticornis. This particular variety is also referred to as the yellowline arrow crab, a common name derived from the fact that it has thin yellow lines running over the top of its rusty-colored extremities.
These crabs can grow relatively large, with a leg span of around 8”; that’s about the size of an adult person’s hand. The females do stay significantly smaller than the males.
Natural habitat
This crab species is naturally found in the western Atlantic Ocean. Here, it inhabits shallow coral reef environments, mostly hiding in and around Lebrunia anemones during the day and coming out at night to feed on feather duster worms and other small reef critters.
We spotted wild arrow crabs in the Caribbean Sea around the island of Curaçao, where they’re common. Despite the fact that they’re nocturnal and don’t really like light, it’s easy enough to spot them if you peek inside crevices and under rocks with a flashlight.
Arrow crab aquarium
If you’d like to keep an arrow crab in your aquarium, keep in mind that they can cause trouble in smaller tanks. Something of 30 gallons or up is preferable.
Because crabs like this one prefer to spend at least a big chunk of the day in hiding, make sure there’s plenty of cover available in the tank. Think rocks, corals and anything else that creates crevices and offers some shade.
Arrow crab compatibility
The arrow crab is easy to care for, but we’ve listed it as intermediate difficulty mostly because of compatibility issues that might arise with this species. Crabs are opportunistic omnivores, and many species can cause issues when they get hungry. The males are also somewhat territorial, so it’s usually best to keep only one.
The arrowhead crab is only tentatively reef safe, as hungry specimens don’t have qualms about munching on coral polyps (so keep yours well-fed if you grow corals!). They might even snatch small live fish if they get the chance.
Any fish species you introduce into an aquarium that also houses one of these crabs should be large and quick enough to avoid those snippy pincers. Small, friendly fellow crustaceans like shrimp, snails, and hermit crabs are at risk, and best avoided. Worms, such as feather dusters, will also be made quick work of.
Arrow crab diet
The advantage of the arrow crab’s ravenous omnivorous nature is that feeding it is an absolute breeze. They’ll feed on any bits they can find in the tank, whether alive or dead. Bristleworms make a welcome snack, which is why the species is often introduced into a tank to combat plagues of them.
To supplement your arrow crab’s diet, offer sinking foods like invertebrate tablets that the other aquarium inhabitants might not get to as quickly. Other than that, these crabs will just eat whatever leftovers make their way to the bottom of the tank.
Did you know? In the wild, arrow crabs have been spotted acting as cleaners for fearsome species that would normally happily eat a crab, such as moray eels. Picking parasites and dead skin bits off their clients earns the crabs some extra food.
Yellowline arrow crab Stenorhynchus seticornis (Brachyura: Majidae) acting as a cleaner of reef fish, eastern Brazil
Decorator arrow crab
As we’ve mentioned earlier, Stenorhynchus seticornis isn’t the only species in its genus that’s commonly kept in the aquarium. Stenorhynchus also contains a number of crab species that use pieces of their environment to camouflage themselves; these are collectively called decorator arrow crabs. Today’s subject, the “normal” arrow crab, doesn’t do this.
Don’t confuse the decorator arrow crab with other decorator crabs. There are quite a few crabs out there that will snag different pieces of aquarium décor, and not all of them are members of the genus Stenorhynchus. You’ll recognize the decorator arrow crab specifically from its spindly extremities. It has a more bland grayish color than the normal arrow crab and a layer of ‘hairy’-looking frills protruding from its body.
Not all decorator arrow crabs look the same, as there are a few varieties out there. Their care and temperament is the same as that of the normal arrow crab.
Breeding arrow crabs
Breeding these crabs in the home aquarium is not an easy feat. It can probably be done, but it’s a labor of love! You’d also need a relatively large aquarium (preferably 50+ gallons), because aside from when they join to mate, arrow crabs generally don’t get along with each other all that well.
Like most other invertebrates we keep in our tanks, Stenorhynchus crabs are born as tiny, extremely fragile larvae. These have almost no chance of survival in a normal display aquarium, so a special rearing tank will be needed. Feeding the larvae is difficult due to their size.
If you give breeding your arrow crabs a shot, the process will look a little like this:
- After the crabs mate, the female will produce eggs, which she carries on her abdomen until they’re ready to hatch.
- Once the eggs turn dark and are close to hatching, set up a larval trap with a light, which will allow you to catch the larvae as soon as they hatch.
- Move the larvae to the rearing tank; according to experiments, 77 °F is the ideal temperature.
- Feed the larvae twice a day using freshly hatched Artemia nauplii. Keep the rearing tank clean by regularly removing gunk and uneaten food.
- The larvae go through several developmental stages before finally metamorphosing into true mini crabs around the 21-day mark.
Conclusion
A marine aquarium inhabited by funky crustaceans like the arrow crab is an endlessly entertaining addition to your home, but requires specialized knowledge to thrive.
If you prefer leaving things to the experts, FantaSEA Aquariums can help! We design, build and maintain aquariums of all shapes and sizes for our clients. Contact us with your ideas.
PS: Love the arrow crab? Have a look at the list of our favorite aquarium crabs to meet more of the funky invertebrates our oceans have to offer.
Sources & further reading
Hernández, J. E., Palazón-Fernández, J. L., Hernández, G., & Bolaños, J. (2012). The effect of temperature and salinity on the larval development of Stenorhynchus seticornis (Brachyura: Inachidae) reared in the laboratory. Journal of the Marine Biological Association of the United Kingdom, 92(3), 505-513.
Medeiros, D. V., José de Anchieta, C. C., Reis-Filho, J. A., & Sampaio, C. L. (2011). Yellowline arrow crab Stenorhynchus seticornis (Brachyura: Majidae) acting as a cleaner of reef fish, eastern Brazil. Marine Biodiversity Records, 4, e68.
