Arrow crabs look like the spiders of the ocean because of their long, spindly legs. Their legs are three times as long as the rest of their bodies. They help them to travel quickly across their territory.
Excellent hunters, arrow crabs use their long legs to grab prey from inside crevices. They also use them to pull off the sharp bristles of bristleworms.
Arrow crabs’ active nature and unique appearance make them popular pets. They are moderately easy to care for as they don’t need a lot of space. They are reasonably hardy, although they don’t like fluctuations in their water. The main concern is pairing them with tank mates they won’t eat.
What Are Arrow Crabs?
Arrow crabs get the name “arrow” from the peaked horn at the top of their head. The “horn” is their rostrum, or beak. It is long and pointed with serrated edges. Arrow crabs have two tiny eyes that sit on either side of their beak.
The yellow arrow crab (Stenorhynchus seticornis) is the most popular species available. Their bodies are golden-yellow to cream-colored. They have alternating red, yellow, and blue stripes, and their claws have purple or blue tips.
There are several other species of arrow crabs that are less common, like the decorator crab.
The most unique characteristic of arrow crabs is their long, spindly legs. The length of their legs is disproportionate to the rest of their bodies. The legs are often three times the length of their bodies, making them look like spiders.
An arrow crab’s legs measure up to 4 in (10.2 cm) long, while the carapace is much shorter.
They have ten legs, eight of which they use for walking. The other two legs are modified into pincers called chelipeds.
- Common Names: Arrow Crab, Yellowline Arrow Crab, Spider Crab
- Scientific Name: Stenorhynchus seticornis
- Origin: Western Atlantic Ocean & the Caribbean Sea
- Size: 6-10 in (15.2-25.4 cm)
- Lifespan: Up to 5 years
- Aquarium Size: 30+ gallons
- Temperament: Semi-Aggressive
- Ease of Care: Moderate
Habitat & Range
Arrow crabs live in the western Atlantic Ocean as far north as Bermuda and North Carolina. Their range expands southward to Brazil and the Caribbean Sea.
They live in the shallow waters of coral reefs, at depths of about 10-30 ft (3-9.1 m).
Behavior & Communication
Arrow crabs are small, shy creatures that hide in caves and crevices during the day. At night, they come out to hunt. Their long claws make reaching into small, deep holes to grab prey easy.
These crabs are fast movers with long legs. They spend much time scuttling about their environment.
The skinny legs of the arrow crab allow it to easily move amongst coral rubble.
Their thin bodies allow them to live among the tentacles of sea anemones. Arrow crabs use sea anemones for protection. The crabs often live in groups around their favorite anemone.
Molting
Arrow crabs must molt their shells because they don’t grow with their bodies. Molting refers to crabs shedding their shells and growing new ones. The entire process takes about one to two hours.
Molting is a vulnerable time for crabs; they most often do it at night. They usually hide inside a crevice or cave away from predators.
Their new shell is very soft for a time, leaving the crabs in danger of predation. Both fish and other invertebrates will attack molting crabs.
Leave the molted shell in their tank. Crabs eat their old shell as it contains vital nutrients and minerals.
Diet
Arrow crabs are carnivores that prefer to eat worms and small snails.
Arrow crabs are one of the few animals that can eat bristle worms. Their long arms prevent them from injury. They hold the worm far from their body and pick the bristles off with their claws. Once all the bristles are gone, the worm is safe for the crab to eat.
Reproduction
The courtship process looks like fighting. The male and female grapple with each other in an aggressive-looking dance.
After the ritual, the two crabs join their abdomens. The male fertilizes the female by passing a sperm packet into her body.
The female carries her eggs under her abdomen. After some time, the developing eggs become visible underneath her. Crabs carrying developing eggs are described as “berried.”
The mother carries her eggs for a few weeks until they are fully developed. As the eggs hatch, the female shakes her abdomen to help the larvae into the water column.
The larvae are called “zoea,” and they have round, transparent bodies.
As they go through the larval stage, they molt into their next phase, becoming “megalops.” Megalops are another larval form that looks more like an adult crab.
As the larvae continue to develop, they’ll molt again before becoming adult crabs.
Conservation
The International Union for the Conservation of Nature (IUCN) has not evaluated this species.
Arrow Crab Care, In A Nutshell
Consult this arrow crab care sheet for quick care tips:
| Aquarium Size | 30+ gallons |
| Water Temperature | 72-78°F (22.2-25.6°C) |
| pH | 8.1-8.4 |
| Carbonate Hardness | 8-12 |
| Salinity | 1.023-1.026 |
| Safe Water Levels | Ammonia: 0 ppm Nitrites: 0 ppm Nitrates: <10 ppm |
| Substrate | Crushed Coral Substrate |
| Decorations | Rocks — Create Crevices, Overhangs, & Caves |
| Tank Mates | Large, fast community fish (not predatory species)Some corals Anemones Some crabs & other inverts |
| Diet | Meaty foods Detritus |
How To Care For Arrow Crabs
Arrow crabs are fairly easy to care for once the tank is established. They have specific care requirements that can make their initial care challenging. Plus, arrow crabs are sensitive to poor water quality and fluctuations.
Tank Size
An arrow crab’s body is small, though it has long legs. Active critters, they need plenty of space to stretch out and explore their habitat.
Arrow crabs are territorial. They don’t appreciate other animals coming into their space.
The smallest tank for an arrow crab should be 30 gallons. The more arrow crabs, fish, and other invertebrates living in the tank, the larger the tank needs to be.
The larger a tank an aquarist can provide, the better. Arrow crabs and other inhabitants will appreciate the extra space.
Larger tanks are easier to maintain than smaller ones. The water parameters are less likely to fluctuate.
Plus, bigger tanks are more likely to produce bristleworms and other detritus. These are great, free foods for the arrow crab.
Water Parameters
Arrow crabs must slowly acclimate to a new tank. They are highly sensitive to water parameter fluctuations. The drip acclimation method works the best.
Once established in a tank, arrow crabs are reasonably hardy and forgiving.
Keep water parameters within these ranges:
- pH: 8.1-8.4
- Salinity: 1.023-1.026
- Carbonate Hardness (dkh): 8-12
- Ammonia: 0 ppm
- Nitrites: 0 ppm
- Nitrates: >10 ppm
Arrow crabs do not tolerate high levels of ammonia, nitrite, or nitrate. Avoid adding them to new, unestablished tanks. They should be one of the last additions to a tank because they cannot handle the nitrogen cycle.
Their pH should be between 8.1 and 8.4.
The salinity should fall between 1.023 and 1.026.
Maintaining the proper pH and salinity is pretty easy. The needed ranges are standard for most saltwater inhabitants.
The carbonate hardness of the water is important for the health of the crab’s shells. Their bodies pull nutrients, like calcium, from the water. The nutrients help with molting and maintaining the strength of the shell.
What Kind Of Salt Should You Use?
Use a marine salt blend that contains calcium and iodine. The nutrients in marine blends keep the arrow crab’s exoskeleton healthy.
Aquarists keeping a reef tank should use reef salt. Reef salt contains higher levels of essential elements.
Corals absorb nutrients quickly, leaving little behind for the other tank inhabitants. Reef salt blends provide enough nutrients for everyone in the tank.
Water Temperature
A tropical species, arrow crabs need water temperatures between 72-78°F (22.2-25.6°C).
Most caretakers will need a water heater in their tank unless they live in a warm climate year-round. Even if this is the case, using a water heater is best to prevent temperature fluctuations.
An adjustable heater keeps the water temperature stable year-round. Changing the temperature is also useful in case the animals get sick.
It’s rare, but aquarium heaters can malfunction. Keep a thermometer in the tank and check the temperature every day. A malfunctioning heater can cause death.
Substrate
Crushed coral substrate is an excellent option. It keeps the pH levels within the necessary range. The calcium in the coral slowly dissolves in the water.
Calcium is a necessary component in saltwater aquariums containing invertebrates. Calcium keeps the arrow crabs’ exoskeletons healthy, allowing them to molt properly.
Decorations
Arrow crabs spend most of the day hiding; create a diverse landscape of rocks for them to explore. The rocks should contain crevices, caves, and overhangs for the crabs to hunt for food.
Aquarists should not cover the entire tank in rocks. Leave enough open space for these active critters to move about their habitat.
Tank Mates
Arrow crabs are safe for some large community tanks because of their small size.
Still, they are opportunistic predators. They won’t hesitate to go after small fish and invertebrates.
Their usual prey comprises worms and snails. They will also snatch small, slow fish out of the water column.
Put arrow crabs with large, fast fish and large invertebrates with good defenses. Ensure the chosen fish will not try to make food of the arrow crab.
Fish and inverts that make good tank mates:
- Emerald crabs
- Porcelain crabs
- Hermit crabs
- Sea urchins
- Starfish
- Sea anemones
- Corals*
- Damselfish
- Clownfish
- Tangs
- Pygmy angelfish
- Grammas
- Basslets
- Cardinalfish
Arrow crabs are “reef-safe with caution.” They are usually safe around corals, though they may pick at polyps and soft corals. Stick with hard corals to avoid damage.
Arrow crabs love having anemones in their tank. They hide amongst their tentacles for safety.
Avoid these prey animals:
- Gobies
- Blennies
- Firefish
- Seahorses
- Other small fish
- Clams
- Shrimp*
Larger shrimp species may do okay in bigger aquariums with lots of hiding places. Shrimp that cannot hide from arrow crabs will likely lose body parts. Arrow crabs will always eat molting shrimp if they can find them.
Avoid these predatory animals:
- Triggerfish
- Pufferfish
- Large wrasses
- Other predatory fish
What About Other Arrow Crabs?
Aquarists should avoid keeping more than one arrow crab in a small tank.
Hobbyists should have at least 50 gallons for two or more crabs.
Arrow crabs are territorial and will fight with each other if there isn’t enough room.
Feeding
Arrow crabs have voracious appetites and are constantly eating. Their strong appetites make them easy to feed. Aquarists must always have lots of food available to prevent their crabs from starving.
It’s best to add arrow crabs to mature tanks because they will have plenty of food for the arrow crab to pick at. Newer tanks need more work, and the caretaker must provide more store-bought food.
Live, fresh, and thawed frozen foods are their favorites. They will also eat detritus left around the tank, like fish flakes.
An arrow crab’s favorite foods include:
- Brine shrimp
- Mysis shrimp
- Krill
It’s best to target feed arrow shrimp, especially if they aren’t finding enough food on their own.
Target feeding ensures they’re getting enough food and reduces aggression. Arrow crabs are much more likely to go after their tank mates when hungry.
Breeding
Breeding arrow crabs is difficult because of their aggressive and territorial nature.
Males VS Females
It can be difficult to tell male arrow crabs from females; it’s easiest to do when they are full-grown.
Full-grown male arrow crabs are larger than females. Aquarists often buy the largest and smallest crabs in the fish store. Doing so increases their chances of getting a male and a female.
Aquarists can also look at their abdomens for sexing. Males have pointed abdomens, while females have rounded ones.
Breeding
Assuming the male and female accept each other, they will breed without intervention. They begin their courtship ritual, and the female starts developing eggs.
Keeping The Young Alive
The hardest part of breeding arrow crabs is keeping the young alive. They hatch as microscopic larvae that drift through the water column on the current.
Move the mother to a separate tank shortly before the eggs hatch to keep the larvae alive. Once all the eggs have hatched, move the mother back to the main tank.
Providing the young with their own tank prevents tank mates from eating them. It also prevents them from being sucked up by the filter.
Add a coral plankton mix to the larvae’s tank, as plankton are an excellent food source for the developing young.
Over several weeks, the larvae will grow and develop into miniature crabs. As they take on the body shape of adult crabs, they drift down to the substrate.
FAQs
Are Arrow Crabs Good For Getting Rid Of Bristleworms?
An arrow crab’s long legs and hard shell make it ideal for getting rid of bristleworms.
Bristleworms are a common occurrence in saltwater tanks. While most are harmless, some species can kill aquarium inhabitants.
Bristleworm populations can also grow out of control, creating a nuisance.
There aren’t many animals that can eat bristleworms. They have sharp bristles that they use as a defense to keep away predators. Arrow crabs are one of the only animals that can eat them.
Arrow crabs are one of the best predators for getting rid of bristleworms. Still, they are expert predators and will attack other tank members. Arrow crabs are only a good option for those with animals that an arrow crab cannot eat.
Does An Arrow Crab’s Legs Grow Back?
Yes, an arrow crab’s legs may grow back if they lose them.
Losing limbs is not uncommon for arrow crabs. They get into fights and regularly sustain injuries. They may also lose a leg during the molting process.
Fortunately for them, the limbs usually grow back during the next molting process.
How Much Do Arrow Crabs Cost?
Arrow crabs are reasonably cheap; they generally cost between $5 and $30.
How much an arrow crab costs depends on its species, size, and colors. Their affordability makes them easy to add to many saltwater tanks.
