Most crabs are squat, unattractive crustaceans that spend all of their time hiding. Who would want a crab? Well, there are crabs and then there are Arrow Crabs!
These Caribbean crustaceans are active, interestingly patterned, and small enough to keep in most home aquariums. While they are still crabs, which are problematic when it comes to tank mates, Arrow Crabs are fairly safe for community tanks.
If you’re looking to learn more about Arrow Crab care, you’ve come to the right place!
Getting to Know Arrow Crabs
Arrow Crabs (Stenorhynchus sp.) are one of the most striking crabs you can buy for a marine aquarium! The Yellowline Arrow Crab (Stenorhynchus seticornis) is by far the most common species, and for very good reason.
It has a delightful striped pattern, with chocolate brown, cream, and yellow tones. The claws often have purple tips and the carapace has a peaked horn that gives them their name.
You may also see Decorator Arrow Crabs (Stenorhynchus sp.) on occasion. These Arrow Crabs are dull colored and slightly squatter as they prefer lying flat against the rocks and substrate. Decorator Crabs clip pieces of algae, sponges, and other bits to stick onto their carapace. This provides camouflage for them while they hunt for food.
The strange build of Arrow Crabs actually serves several purposes. Their thin form slides easily in between coral rubble and also allows them to live among the arms of crinoids and tentacles of sea anemones. In the wild, Arrow Crabs can be found in groups around a favorite anemone for protection.
Their long claws allow them to reach into holes for potential prey as well. Worms, small snails, and other live prey are their preferred food source. And if they happen upon something more dangerous, like a bristle worm, the Arrow Crab will hold the worm suspended between its legs far from its body. It then uses its claws like surgical tools to remove bristles and expose the soft portions of the worm to attack.
While Arrow Crabs are fascinating and offer a useful service if you’re worried you have too many bristle worms. But they can also be problematic if you have slow moving fish, delicate crustaceans like shrimp,and other easy prey for them…
- Common Names: Arrow Crab, Yellowline Arrow Crab
- Scientific Name: Stenorhynchus seticornis
- Origin: Caribbean Sea
- Size: Up to 6 inches across
- Aquarium Size: 30 gallons
- Temperament: Semi-Aggressive
- Ease of Care: Easy
Arrow Crab Care
Arrow Crabs are opportunistic predators, which can make them hard to keep with small fish and invertebrates. But with a bit of planning your Arrow Crab will make a great, long-lived show specimen!
Arrow Crabs have a small body but very long legs! This increases their space requirements; a 30 gallon tank is the minimum for an Arrow Crab. Arrow Crabs are not only very active but they can be territorial at times, both towards each other and other animals. Giving everyone a little space to avoid your crab is best for all, including you!
A larger tank can also potentially grow more bristle worms, detritus, and other tasty bits that accumulate in the live rock and substrate. Mature tanks rich with hidden biota are perfect habitats for Arrow Crabs, which spend all of their time reaching into crevices hoping to find something tasty. And the larger the aquarium, the more organic matter, worms, and other food it will accumulate.
Acclimating Arrow Crabs to a new tank should always be done with caution. They are hardy once established but like most invertebrates they are very sensitive to sudden shifts in water parameters. I recommend using a drip acclimation process to ease the transition to its new home!
Arrow Crabs are sensitive to high levels of ammonia, nitrite, and nitrate, making them poor choices to add to a new tank. They are some of the first animals to die of new tank syndrome, which can occur when tanks are not fully biologically cycled. Instead, wait until your tank is fully cycled, which takes 2-3 months, before adding an Arrow Crab.
Ammonia, nitrites, and nitrates in a fully cycled tank should test near or at 0 ppm, with some levels of nitrate being well tolerated (10 ppm or less). Temperatures for Arrow Crabs range from 72-78℉ and your salinity should fall between 1.023-1.026. A pH of 8.1-8.4 is recommended, which should be easy, assuming you’re using a crushed coral substrate.
Calcium and Iodine are of special importance to crustaceans as they require both for the maintenance of their shells and to aid in the molting process. As long as you’re using a standard marine salt blend that provides levels of these elements that matches seawater your crab will be fine.
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However, in reef tanks, there can sometimes be deficiencies as hard corals can use these elements very aggressively. Make sure you’re using a reef salt, which has elevated levels of essential elements if you’re keeping an Arrow Crab in a reef tank to avoid molting issues.
Molting is a fascinating process to see if you’re lucky enough to catch the Arrow Crab in the act. Usually, molting crustaceans wait until it’s dark out to begin the process. They will also hide in any cave or other spot that gives them enough space.
Crustaceans have a hard outer skin that acts like a suit of armor called an exoskeleton. While it provides them with a mobile defense it can’t grow with the insides of the animal. They therefore have to shed their skins periodically to grow larger.
The process takes anywhere from 1-2 hours and the crab is very vulnerable to predators during this time. It’s new shell is also soft and needs time to harden. Even peaceful fish may be tempted to peck at the crab during and after the molt, as will other crustaceans. But so long as you provide safe spaces among the live rock, your crab should be able to molt in peace.
Arrow Crab Tank Mates
The main issue with keeping Arrow Crabs is finding the right tank mates for them. Crabs in general are difficult to keep with other animals because they are known as opportunistic omnivores. If you keep freshwater tanks, Crayfish present the same problem: they will eat anything they can overpower.
They prefer eating small worms, snails, leftover food, and other detritus. But an Arrow Crab is very willing to snatch a Neon Goby or Firefish that’s too slow, a sick Clownfish, or other small tank mate. They are also aggressive towards other crustaceans.
Most saltwater shrimp species can be a problem because they have tiny claws that aren’t enough to defend against an Arrow Crab. In a larger tank there should be few issues. But if your shrimp are forced into contact constantly with the crab in a tank that’s too small, expect to see clipped antennae and missing legs.
And if the Arrow Crab happens to find your shrimp in the middle of, or just after a molt, it’s almost certainly going to be eaten. Those long claws are great for grabbing onto molting shrimp from inside their hiding places.
The best tank mates for Arrow Crabs are other invertebrates that have strong defenses as well as fish that are large and/or fast enough to not be eaten. But those fish need to also not see the Arrow Crab as food!
Defense oriented invertebrates include other crabs (Emerald, Porcelain, Hermit, etc), sea urchins, and starfish. Clams are a poor choice, though. While they do have a hard shell, they aren’t especially mobile and will stress and even starve if they get picked at every time they open their shell. Even Giant Clams aren’t immune to the attacks of a patient Arrow Crab.
While Arrow Crabs can be considered reef safe with caution, Decorator Arrow Crabs add another layer of complexity. They prefer using algae, sponges, and other material for their camouflage. But they may try to clip off coral tentacles or polyps as well. A seahorse keeper even reported a Decorator Crab clipping the tails from seahorses to add to its collection!
This behavior isn’t super common and not aggression. But it’s not out of character for these crabs, either. So be even more careful when choosing a Decorator Arrow Crab for a reef setup.
Good Tank Mates for Arrow Crabs:
- Small to medium sized, fast, healthy fish like Damselfish, Clownfish, Tangs, Pygmy Angelfish, Grammas, Basslets, and Cardinalfish
- Defense-oriented Invertebrates (Sea Anemones, Sea Urchins, other Crabs)
- Shrimp (in a large enough aquarium)
- Corals (with caution)
Poor Tank Mates for Arrow Crabs:
- Predatory Fish (Triggerfish, Pufferfish, large Wrasses)
- Small, slow fish (Gobies, Blennies)
- Corals and Anemones (Decorator Arrow Crab)
Feeding Arrow Crabs
While their appetites make them hard to keep with other animals it does make them very easy to feed! Arrow Crabs will eat absolutely anything organic and protein-rich. Prepared foods aren’t their favorite but they will eat flakes and pellets that your fish leave behind. But live, fresh, and thawed frozen foods are by far their favorite.
You can target feed them brine or mysis shrimp as well as chunks of krill or shrimp. In fact, I strongly recommend target feeding your Arrow Crab if you don’t see it getting enough food. Why? Because this will cut down on their aggressive behavior. Hungry crabs are much more dangerous to their tank mates.
An aquarium with a healthy bristle worm population may mean you rarely have to feed your Arrow Crab as they preferentially hunt for these. Still, it’s a good idea to offer your crab something if you don’t see him eating every now and again to be safe.
Breeding Arrow Crabs
Breeding Arrow Crabs is difficult but not impossible to do. The main challenge is their aggressive, territorial nature. They can also be cannibalistic if one catches another during a molt.
Sexing them is also challenging but there are a couple of clues you can use. Males are always larger than females when fully grown. So if you buy both the largest and the smallest adults for sale, you have a better shot of getting a pair. Also, you can try examining their “tail” (abdomen) which lies flat against their body.
Male crabs have a pointed abdomen while females have a more rounded one. You’ll have to look at them from under a clear container to get a good look at it. Assuming you have a pair and assuming they don’t fight each other, breeding is simply a matter of feeding and time.
Once the crabs settle in, you may see them engage in mating. The male and female will grapple in a way that looks like fighting. But the dance ends up with them joining abdomens to one another. The male fertilizes the female in this way and then departs.
Eventually, the female will show developing eggs that she carries under her abdomens. Female crustaceans are described as “berried” at this point as they look like a cluster of small fruits. She will hold onto the eggs for a few weeks before releasing the larvae into the water column once they begin hatching. She will vigorously shake her abdomen and body to help loosen the larvae from their shells.
Unfortunately, at this point, the fry usually die in most aquariums because crab larvae are so tiny they are planktonic. They float freely in the soup of zooplankton that drifts about the reef, eating and being eaten. It takes weeks for them to develop into miniature crabs that then drift down into the reef.
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If you’re serious about raising the baby Arrow Crabs, you can prepare a rearing tank for the larvae. Once the female berries, catch her and move her there because the exact amount of time it takes for Arrow Crab eggs to develop is unknown. Once she releases her larvae, you can move her back to the main tank and start adding a coral plankton mix to the water. Plankton are the best food for the microscopic crab larvae.
Over the course of weeks, they will feed, sink to the bottom. And if you’re lucky, one day, you may be surprised to see tiny, spindly Arrow Crabs hunting for prey!