It may appear to you as a giant spider inside the tank, but you are witnessing an exclusive aquarium pet, the arrow crab.

A prominent reason for fetching this unique name is the arrow-shaped carapace and angular head structure. It is also popularly known as an arrowhead crab and is easily distinguished by its exceptionally long legs.

These Crabs are resilient, cold-blooded invertebrates that can be kept in a big reef tank as an ornamental aquatic organism.

They also can change colors according to their surroundings, acting as camouflage to help them protect against predators.

About The Arrow Crab

Quick Species Facts
Scientific Name Stenorhynchus Seticornis
Other Common Names Yellowline Arrow Crab, Spider Crab, Caribbean Ghost Crab
Family Inachidae
Origin Atlantic Ocean (coral reefs)
Lifespan 10 years in captivity
Average Size Carapace- 2.4 inches;Pincers- 5 inches
Type Saltwater

Origin & Habitat

Arrow Crab prefers to dwell across the coral reefs along the tropical and subtropical regions of the western Atlantic Ocean. It exists from the southern states of the USA and Bermuda to Brazil, though most common on the Caribbean coasts.

They hide under the rock formations but can also be found near urchins, residing in soft-bodied aquatic creatures such as sponges and undersea anemones. Estuaries, mangrove swamps, and tidal flats are frequent places to locate them.

These are nocturnal by nature and spend most of their time in the ocean depths of 4 – 10 meters over the rock bottoms.

Lifespan

The arrow crab has a short lifespan in the wild as it is observed to survive for about 3 – 5 years. They live a harsh life in the corals and have a risk of predation.

If petted at home aquariums or in captivity, there is a higher chance that they exceed their average life of five years and stay alive actively for close to 10 years, but this higher number is rarely touched.

How Do They Look Like?

The carapace (shell)of the arrow crab varies from brownish red to golden or cream in color. They have brown, black, yellow, or gold stripes on their dorsal side.

The crab has a triangular body form and an incredibly pointed head (resembling an arrowhead). Their body is also bilaterally symmetrical. The legs are long like a spider, in red and yellow color with deep violet ends, while their claws are bluish-black.

The carapace looks pretty small compared to the limbs. The central body is about 6 cm, while the legs, when expanded, are measured to be approximately 10 cm long. Legs can sometimes be thrice the body size.

It has eight legs with two pincers. The claws are long, curved, and can throw poison toward the attackers.

A fascinating long nose or rostrum captures the attention as it looks similar to a javelin striking straight on the head, almost 3 – 4 cm in length.

How to Identify The Gender?

The male and female arrow crab can be easily distinguished by looking at their morphology. Here are some of the significant differences between the two genders.

  • Males are slightly larger with bigger and more powerful claws than females.
  • The abdomen of the male crab is triangular, while in females, it is broad and rounded.

Availability and Price

The Caribbean Sea is home to the most common species of arrow crab (Stenorhynchusseticornis). Still, the Indo-Pacific, eastern Africa, and California are also significant sources of importing these species.

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It is imported worldwide with higher shipping rates. This can be purchased from online stores and is generally rated between $17.49 to $19.99, but the price varies with the crab’s color, age, and size.

Care Stats

Quick Care Facts
Care Level Intermediate
Breeding Easy
Social Moderate aggression
Temperament Highly territorial
Diet Omnivores; Opportunistic

What do Arrow Crabs Eat?

These are omnivores and can eat both plants and other aquatic or non-aquatic organisms. It is an opportunistic feeder and shows hassle-free eating habits.

They scavenge on any dead aquatics in the water and don’t look for specific choices, but a balanced diet is necessary.

It may sometimes show predatory behavior towards other fishes in the tank, which could be due to starvation for a long time. So, feeding them 3 to 4 times a day is suggested.

You should also give some calcium supplements from time to time as it helps with the molting of crabs and forms a hard carapace which they need to survive.

In the ocean, they survive over seaweeds, algae, crustaceans, gastropods, small fishes, molluscs, urchins, sea cucumbers, or any other creature smaller than their size.

You can feed them with the following food items in the aquarium.

  • Zooplankton and Phytoplankton
  • Motile invertebrates
  • Shrimp (Opossum and Brine)
  • Squid
  • Feather Duster Worms
  • Enriched Flakes
  • Algae Wafers

General Behaviour

Arrowcrab is habitual of doing most of its scavenging and feeding activities during night. It moves slowly and gracefully on the seabed like a slow cellar spider, more commonly known as Daddy Long Legs.

They are moderately aggressive towards smaller organisms but highly territorial towards their species. You can not keep more than one of them in a tank, as both will constantly fight with each other till one crab dies off.

In south-eastern Brazil, this crab is most usually seen in association with anemones, where it can be found in groups of one to six individuals.

The nocturnal behavior is not strictly followed, and the crab also comes out during the day hours, if they feel comfortable and stress-free in the surroundings.

Mating, Breeding & Spawning

Warm temperatures in the environment mostly instigate the breeding in arrow crab, so you must prefer to use a separate tank for mating and prefer filling the tank with warm water.

The male sperms are deposited on the female body in a packet-like form during mating. The female carries her eggs under her abdomen after fertilization.

This study indicated that the weight of the eggs carried by the female is about 1/10th of her body weight, while they can be as large as 0.48 – 0.57 mm.

The eggs are carried until it is ready to hatch and become Zoea larva. It could range from 300 to 900 in numbers, which entirely depends on the size of the female.

After hatching of eggs, it is advised to remove the female from the hatchling tank as she could prey on them.

You can feed them with planktons to promote the molting process and form an adult body structure, with five limbs usually appearing in the beginning.

Research conducted by the Marine Biological Association of the UK found that the crab hatchlings, developing from the larval stage, survive comfortably at the following parameters.

  • The temperature of the water should be 25 °C.
  • 30% salinity in the tank water.
  • The photo-period of about 10 hours in light and 14 hours in the dark.
  • A pH in the range of 8.1 to 8.4.
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If the temperature is a bit high or low, the stability of the crab gets affected.

It took about 27 to 35 days for the larva to develop into crabs at 22 °C. Sufficient high-quality food should be provided to the hatchlings twice daily.

EXPERT ADVICE– If you are moving the hatchlings to a different tank, remember to acclimatize them (approximate at 1 degree/hour) to prevent thermal shock.

What is the Process of Molting in Arrow Crab?

Molting is a process of shedding the old exoskeleton and regrowing of a new section. It is a regular process as the crab grows bigger, and is a sign that it is healthy.

Molting takes only a few minutes, but it may take many days for the arrow crab to prepare for it. The experts advise the owners to feed them calcium supplements or a calcium-rich diet during this time.

After molting, the outer covering of the crab is quite soft, which makes them vulnerable at this stage. So, they need good hiding places to stay safe for a few days until the exoskeleton hardens.

QUICK TIP– Don’t remove their shed exoskeleton from the aquarium. Arrow crabs feed on them as they are rich in calcium which in turn helps them mature their soft shells after molting.

Diseases

Arrow Crabs are pretty tough, and they generally don’t encounter diseases, but some situations may hamper their health and make it difficult for them to survive.

So, always watch for the following points to keep the crab live longer.

  • Dirty and unhygienic water can cause many viral and bacterial infections, making the crab ill.
  • High nitrate or ammonia content can make the crabs sick very fast.
  • The tank’s unsuitable water parameters will not support healthy metabolism leading to death.
  • Another prominent factor is stress due to the environment or tank mates that may keep the crab unhealthy for a long time, disturbing the lifespan.

Tank Care

Quick Tank Facts
Water Temperature 72 ° F to82° F
Minimum size of the tank 80 L
Water Hardness levels 7 to 12 dKH
pH Level 8.1 to 8.4
Minimum/maximum population in the tank 1 Maximum
Salinity 1 to 3.5 pounds per gallon of water
Calcium Concentration 400 to 450 ppm (optimal)

Ideal Tank Size

Irrespective of the size of the arrow crab, that makes them suitable for small tanks, but because they roam around and scavenge, you must choose a tank of about 20 – 30 Gallons capacity.

These are sometimes aggressive and highly territorial; you need to provide them with a lot of open space for good health and peaceful existence with the fishes and other animals.

AUTHOR NOTE– If we want to keep two, we need at least a 50-gallon volume of tank to maintain a peaceful environment.

Tank Setup

You can introduce any aquarium plant in the tank with arrow crab. Seaweeds are a good choice as they can feed over it.

Since they normally stay near the water bed, the floating plants will only help provide shade from external lights.

Any lighting can be used, but dim or subdued lighting is advised as the crab is nocturnal and shows most activity in the dark.

If you enhance the visibility of the setup, then make sure to turn off the lights after some time. The crab prefers dark areas, and extra lights may increase the stress level.

  • Substrate
    You can put thick, hardened corals and rocks with holes for them to hide.
    Other such options are fan corals or soft sandy and gravelly bases with at least 12 inches of sand, gravel, and stones to hide under or shelter.
  • Tank Apparatus
    Highly efficient filters should always be used to keep the tank’s nitrogen levels at a bare minimum. Bubble air pumps can be used to mimic a somewhat sea-like environment. You can also put a thermometer to keep an eye on the water temperature.
  • Décor
    There is no restriction to decorations except for sharp or small objects that could potentially be ingested by the crab and may result in death. However, a log of wood, big stone, or coral with many holes is loved by them for hiding.
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Water Parameters

Arrow Crabs live in tropical and sub-tropical reefs, so they prefer water parameters that match similar conditions, but they are hardy creatures. They can tolerate alterations up to some degree.

The water temperature should be around 22 °C to 28 °C. It should be slightly alkaline from the 8.1 to 8.4 pH range.

The salinity of the water should match the ocean, so it should be in the range of 1 to 3.5 pounds of salt per gallon of water.

Water hardness must be maintained between 7 to 12 dKH, and the calcium concentration must be between 400 to 450 ppm.

Nitrate content should be kept close to 0 as much as possible.

Arrow Crab Compatibility

This crab is highly territorial and never wants to adjust with another of its kind in the same vicinity. Especially two males cannot live together, so only keep two arrow crabs if they are male and female.

In a community tank, they try to harass smaller fishes so that you can induce bigger ones but avoid species like Groupers, Lionfish, Eels, Dog-faced pufferfish, Picasso triggerfish, or Hawkfish, as they can hunt the arrow crab.

You must also put away small crustaceans like shrimps or snails as arrow crab regularly prey on them, or try damaging their shells or antenna through the sharp claws.

Few compatible aquatic species are fast& agile and can stay safe with them.

  • Sea urchins
  • Sea stars
  • Hermit crabs
  • Big snails

These crabs are popularly induced as reef cleaners in the fish tank. Crab and marine organisms like squirrelfish (Long fish, Holocentrus) and moray eels (Green, Spotted, Purple mouth) form a symbiotic relationship. Although they rarely live together for long as arrow crab is a standard diet of eels.

NOTE- If you want to keep two arrow crabs together, make sure both are not male. Or they will fight each other to death.

Frequently Asked Questions

Does arrow crab prey on Bristle worms?

They are known to be good bristle worm hunters and have frequently been seen preying on them, whether in the wild or in aquariums. Bristleworms are dangerous for fish, so putting this crab in the tank is a good measure to control the worms.

Are they compatible with Clownfish?

Even though they live together near corals and sea anemones, Clownfish is smaller in size and can become easy prey to the arrow crab. So, it is not advised to keep them together.

Do they eat Flatworms?

Flatworms aren’t very good for the corals as they tend to block the sunlight while populating the tank. So, this crab, with its long slender claws, can feed over it and keep the tank healthy.

Are arrow crabs reef safe?

They are considered reef safe even though they might sometimes nibble on soft corals or damage them with sharp claws, but there is nothing much to be concerned about.

They are scavengers and eat dead animals or fishes that fall to the ocean’s surface. So, they keep the reefs clean from matter.

Do their legs grow again?

These crabs often lose a limb during hostile reactions or even through the molting process. But nothing to worry about as they can naturally grow their limbs after molting.

Final Thoughts

Arrow crab acts both as an ornamental and utility organism and is a fantastic addition to the reef tanks. Along with other cleaning fishes and crustaceans, they can easily keep your environment clean without any extra water filters.

Their creepy, unique appearance has intrigued many aquarists worldwide and is attracting many buyers.

About the Author

Victoria Lamb

Victoria is a freshwater aquatics specialist, fish keeper, and amphibian enthusiast. She has had more than 6 years of experience caring for aquariums and keeping several fish species, and her home boasts of 3 aquariums and a garden pond. Her goal is to educate fish owners on raising healthy and happy aquatic pets.

Career Highlights:

  • Has worked with several aquarium manufacturers as a consultant
  • Organized and hosted workshops on freshwater fish keeping at retail stores, educational facilities, and libraries
  • Released content for the amphibian community through her writings

Educational Highlights:

Bachelor of Science in Animal Behavior and Welfare

  • University of Lincoln, Lincoln, UK (2014-2018)

Writing Experience

Victoria has done ghostwriting for many aquarium and pet websites in the past. She has also worked for Canada's largest natural health magazine- ALIVE, with 300,000 monthly circulations as a freelancer. She had six published articles on animal behavior and welfare during her graduation for her thesis.

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