The Arrow crab or Spider crab (Stenorhynchus seticornis) is one of the most weird-looking crabs in the marine aquarium industry. Nonetheless, whatever you can think of its appearance, there are certain debates about this species (aggression, feeding habits, breeding, care, etc).
The marine ornamental species trade exploits Stenorhynchus seticornis because of their hardiness in captivity, unique coloration, and morphology. Besides being easy to care for, the Arrow crab is one of the crab species known by controlling predatory bristleworms. Ornamental crabs of the genus Stenorhynchus seticornis are good scavengers and can be a part of a group of animals in a reef aquarium, “cleaning crew”.
In this detailed guide, I have gathered all information about this species based on existing studies, experiments, researches and experience of aquarists.
Quick Notes about Arrow Crabs
| Name | Arrow crab |
| Other Names |
Yellowline arrow crab, Majoid crab, Caribean spider crab, Caribean ghost crab |
| Scientific Name | Stenorhynchus seticornis |
| Tank size (minimum) | 20 gallons (~80 liters) |
| Keeping | Easy |
| Breeding | Difficult |
| Size of the carapace | up 3 – 4 cm (~1.4 inches) |
| Size across the legs span |
15 cm (~6 inches) |
| Optimal Temperature | 24 – 27°C (~75°F – 79°F) |
| Water type | SG = 1.021 – 1.025 |
| Optimal PH | 8.1 – 8.4 |
| Optimal KH | 7 – 12 |
| Nitrate | Less than 20 ppm |
| Diet | Mostly Carnivore / Omnivore |
| Temperament | Semi-aggressive |
| Life span | up to 5 years |
| Color Form | Whitish or light tan with fine dark/brown lines |
Natural Habitat of the Arrow Crab
The Arrow crab lives in a variety of bottoms—rocks, corals, seaweed beds, pebbles, sand, or sand mixed with broken shell, wharf piling and rock jetties. This is one of the most diverse crab species when it comes to the environment.
It is a common and widely distributed inhabitant of the Caribbean Sea and the western Atlantic Ocean —from the intertidal zone to 200 m in depth and ranges along the west Atlantic coasts, from North Carolina to Argentina.
Description of the Arrow Crab
At first glance, Arrow crabs almost appear more like underwater spiders than crabs. That is why the species is appreciated by aquarists owing to its extremely long legs which resemble those of a spider and small triangular body the size of a dime. Its spindly legs can reach up to 4 – 6 inches (10 – 15 cm) long.
Arrow crabs have spear-like rostrum (spike on the head) that sticks out between two beady eyes.
The carapace is more whitish or light tan with fine dark/brown lines from the median line to the margin of the carapace. Full-grown, an adult Arrow crab’s shell (carapace width) is only around 3 cm long (a little bit more than half an inch). It has a long and narrow rostrum (up to 3 cm or ~1 inch) drawn out into a tapered point.
Their long and slender legs are often spotted or banded with orange or red. Arrow crabs have blue to violet claws that look more like gloves.
In captivity, Arrow crabs can live up to 5 years.
The Behavior of the Arrow Crab
Both in the wild and in the aquarium, the Arrow crabs are nocturnal. At daytime, they prefer well-protected locations in crevices, holes or between the tentacles or arms of crinoids, actinians and sea anemones. In the evening, they start climbing up the coral head, looking for exposed locations. At night they can be found on exposed locations, on soft corals or gorgonians.
Arrow crabs actively feed only at night and dawn, but not during the daytime. At dawn, they descend from the soft corals and return to their daytime location. This diurnal rhythm seems to be a sort of negative phototaxis.
Note: Once Arrow crabs get adapted to a new environment and feeding regime, they can slightly change their night/day behavior and start coming out during the day a lot more.
Although Arrow crabs are pretty slow-moving crabs, it should not confuse you. It is important to remember that Arrow crabs have a semi-aggressive temperament, a characteristic that may turn off some potential owners. Arrow crabs, especially males, are territorial and very aggressive to each other. They will fight for territory and dominance. Therefore, if you have a small tank, it will be better to have only one crab.
In addition, their aggression is latent until they grow into their adult size. At this point, they may become aggressive and can attack small prey in the tank, such as invertebrates and even small fish.
Arrow crabs are not shy and can be easily approached, obviously relying on camouflage as a defense.
In nature, adult crabs seem to migrate into deeper water. There are no obvious predators on Stenorhynchus seticornis. The arrow crabs clean the reef of organic matter and serve as scavengers of the coral heads.
Interesting Fact: Arrow Crab as Cleaner
The paper reports the Yellowline arrow crab, Stenorhynchus seticornis, acting as a cleaner of longjaw squirrelfish, Holocentrus adscencionis moray eels Gymnothorax funebris, G. moringa, G. vicinus and squirrelfish Holocentrus adscencionis at two reefs in the Baía de Todos os Santos, Bahia State, eastern Brazil.
Biologists believe that other species of reef fish are potential ‘clients’ of Arrow crab and that this activity occurs throughout its distribution.
Arrow Crab and Urchin
Researchers studied the intraspecific host selection (association) of the Arrow crabs with the long-spined urchin, Diadema antillarum, in the shallow coastal waters of Tobago. Because these crabs did not pay any attention to the shorter-spined urchins, they concluded that the longer and mildly toxic spines of D. antillarum afforded more protection from potential predators than the spines of other available urchin hosts.
Whether Arrow crabs identify individuals of Diadema antillarum by visual, tactile, or olfactory cues (or a combination of such cues) remains uncertain.
This association is not a symbioses. First of all, Arrow crabs do not require urchins all the time. Second, the urchin apparently derives no benefit from the crab.
Arrow Crab and Molting
Like all crustaceans, Arrow crabs need to molt (shed their shells off) to grow. Molting is a key stage in the life cycle of any crab, shrimp, crayfish, lobster, etc. This process indicates the healthy growth of the animal.
Although the molting process itself usually lasts only a few minutes. It takes weeks for a crab to prepare for molting. A few days before molting you can notice that Arrow crabs move even slower than usual and do not eat as much. During this time they start pumping up the water to break the old shell and remove itself from it.
Arrow crabs are extremely vulnerable in this soft-shell phase of their life and need a place to hide safely while their shells harden. This is another reason to maintain a sufficient amount of rocks and other hiding spots in your tank.
Tip: Do not throw away old shells. They contain lots of minerals and crabs will gladly eat them after some time.
Note: Like all invertebrates, Arrow crabs need calcium to mineralize (harden) the shell. Calcium is vital for good shell growth. I highly recommend reading my articles “How to Supplement Shrimp and Snails with Calcium” and“Crabs and Molting Process”.
Arrow Crab and Bristworms, Feather Dusters, Vermetids
A lot of marine aquarists complain about brisleworms in the sandbed or algae clumps that are growing on the rocks. These worms are pretty small and almost impossible to notice in the beginning until you see bunches of them swarming. If it is your case, then the Arrow crab can be your solution to the problem. Arrow crabs are very effective against brisleworms.
However, before you decide to introduce these crabs to the tank you need to understand the temper and feeding preferences of this species.
In addition, I would like to point out that most of these worms are beneficial scavengers and efficient detritus consumers that cannot do any harm to your marine system. Do not be surprised!
All brisleworms were jinxed by the one notorious species (Hermodice carunculata or ‘The bearded fireworms’). As a result, most hobbyists consider them all together as pests and try to remove them immediately. Well, this is not so.
You need to calculate all the pros and cons before adding Arrow crab. Otherwise, your cure can become even worse than the disease.
Note: If you have lots of brisleworms, it is a sign of excessive detritus and dissolved nutrients in the tank. Revise your feeding and stocking, do maintenance, clean the substrate, do a water change.
You can read more about it in my article “Bristleworms Profile: Stay of Go?”.
Regarding Vermetid snails (‘Tubeworms’), there is a rumor that Arrow crabs can control the population of these snails. This is an exaggeration. If possible Arrow crabs prefer to eat something tastier and easier to catch.
In addition, Arrow crabs will also eat all kinds of worms including feather dusters. So, be careful with that.
You can read more about it in my article “Vermetid snails Profile. How to remove them”
Feeding Arrow Crab
One of the best (and unfortunately the worst) things about Arrow crabs is that they are opportunistic omnivores. It means that they will eat anything they can catch including other motile invertebrates, mollusks, small fish, and other living organisms smaller than themselves.
Although these crabs are not fussy eaters, it will be wrong to think that they do not require a specific diet. I would highly recommend keeping them fed all the time. Do not let them get hungry.
It is not guaranteed but when well-fed, in general, Arrow crabs can become more tolerant of tank mates. That is why, some aquarists prefer to supplement their diet about 3 – 4 times a week (for example, mysis shrimp, brine shrimp, squid, phytoplankton, zooplankton, enriched flakes, seaweed, etc.).
Are Arrow Crabs Coral Safe?
Generally, Arrow crabs are safe with hardy native corals and anemones. They will not bulldoze through the corals like big crabs often do. However, do not forget that they are omnivores. So, they may start picking at your corals if they’re not given enough food.
To be safe, I would keep Arrow crabs under suspicion until proven innocent. In addition, stick to these rules and it will help you to improve the odds:
- Keep an eye on it for the first week or so.
- Do not let it get hungry.
Keeping and Caring for Arrow Crab
Because of their hardiness, it is very easy to care for the Arrow crabs. The only problem you can have is the tank size.
Tank Size:
Being somewhat large for a saltwater crab, the Arrow crabs require a substantial amount of space in the aquarium. Nano reef tanks are out of the question. They will be like an elephant in a china shop there. Arrow crabs need at least a 20-gallon (80-liter) tank per individual.
Water Requirements:
If you are planning to keep two of them, potentially a 50-gallon (200 liters) tank might do for two of them, provided they are not both male, as males will fight to the death.
While Arrow crabs do not need any additional supplements to survive in captivity, you should take care to ensure that tank water remains at a warm temperature – between 72 to 82 F (22 to 28 C) with around 75 to 79 F (24 – 27C) being optimal.
The concentration of calcium, the PH, and the alkalinity of the aquarium’s water must be maintained at acceptable levels as well. For example, keep calcium concentration in the range of 400 to 450 ppm is optimal, but it can be a bit lower or higher. The pH should be kept around 8.1 to 8.4 and alkalinity should be kept in the range of 7 to 12 dKH, although these often go a little higher or lower.
Hiding places:
Your tank should have plenty of rocks and caves for your crab to hide in. This is crucial for their well-being.
Note: Consistent water parameters are the cornerstone of any successful aquarium. Make sure that they are all kept within a suitable range (at all times) and do not jump around.
Important: Before putting the Arrow crab in your tank do not forget to acclimate it (read more about it) as all invertebrates.
Be careful with chemicals like copper (read more). Crabs, shrimp, and crayfish do not tolerate copper-based medications or fluctuating water parameters.
Basic Tank Equipment (links to check the price on Amazon)
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Sexing Arrow Crab
It is rather easy to tell the difference between a male crab and a female Arrow crab.
- In males, the abdomen has a triangular shape and wider/rounder in females.
- Also, males are a little bit bigger and have larger claws compared to females.
Breeding Arrow Crab
The species Stenorhynchus seticornis is heavily harvested for ornamental purposes. That is why it is extremely important to develop largescale rearing methods for Arrow crabs. I found several studies and researches about breeding this species. This is the compilation of different facts.
Under good conditions, Arrow crabs can breed all year round. After mating, females of this species produce large eggs (0.48 – 0.57mm) and small clutches. Berried female crabs carry eggs on their stomach until they are ready to hatch in a few weeks. In general, every female has about 300 – 900 eggs. There is a significant correlation between the number of eggs per brood and the size of ovigerous females.
In fact, the size of the breeding female has been pointed out as a key variable determining the number
of eggs extruded per batch. For example, according to the study, the weight of the egg mass is usually constrained to about 10% of the female’s weight.
The egg increase during development, this is a result of water uptake. The increase in egg volume caused by water uptake limits the availability of egg carrying space, resulting in smaller broods.
Defining Optimal Conditions for breeding Arrow Crabs
Larval development in Arrow crabs occurs within a well-defined range of environmental conditions characteristic to a species. Of the environmental factors that affect crustacean development, temperature, and salinity. They significantly affect survival and the extent of larval life.
- Larvae need a separate rearing tank. Light attracts Arrow crab larvae. So, you can lure them to a spot with a flashlight or adjustable spot desk lamp to the corner of the tank. Then use a turkey baster or a big syringe to catch them.
- If you already have a separate tank, after hatching of the eggs, the females should be removed from the rearing tank.
- According to the study, the highest mean survival from hatching to the first crab stage (4.5%) occurred at 25 C (77 F). Salinity – 30%. Photoperiod of 10 h light/14 h dark. pH of 8.1 – 8.4. The rate of survival shows a steeper decline at 28 C at all salinities. At 22 C (71 F) larvae required longer times (27–35 days) to complete development to the first crab.
- If you are moving larvae from the main to the rearing tank, as a precaution against possible thermal shock, larvae require slow acclimatization by gradually reducing or increasing temperatures (approximately 1 degree/hour).
- Feeding is extremely important.
- Keep in mind that larvae cannot recover from nutritional stress caused by previous starvation even though they are subsequently fed (‘point of no return’). In addition, the lack of food of proper size and nutritional value during the period when larvae first begin feeding cause extensive mortalities.
- Arrow crab larvae are Feed them daily (ideally twice) on newly hatched Artemia nauplii. (approximately 5 nauplii per ml).
- To avoid salinity changes in the rearing setup, screen and rinse nauplii with filtered seawater at the necessary
- Water parameters should be stable. Remove any detritus in the tank.
Arrow crab Larvae Development
The early postembryonic development of Stenorhynchus seticornis consists of two zoeal stages before attaining the megalopa stage. The babies of Arrow crabs are called zoea, and once they are born they swim towards the surface and feed on small plankton.
First zoea stage
Mean survival at this stage is less than 61% under all experimental conditions.
Depending on the temperature, the stage lasts from 3 – 5 days (28 C – 22 C or 82 – 71 F).
Second zoea stage
Mean survival at this stage varied from:
- 26% at 28 C, 35‰ to
- 6% at 25 C, 30‰.
Depending on the temperature, the stage lasts from 3 – 6 days (28 C – 22 C).
Megalopa stage
This stage shows the highest mortality. Percentage survival is lower than 12% under optimal conditions (25 C, 30‰).
Depending on the temperature, the stage lasts from 18 – 24 days (22 C – 28 C).
Cumulative survival from hatching to the first crab stage is very low (only 3 – 4 %!). However, in nature, larval survival is generally even lower, often, less than 1%. The reasons for the high mortality of megalopae of Arrow crabs remain unsolved.
Arrow Crab and Suitable Tank mates
Let me start off by saying that Arrow crabs are not completely safe and peaceful with other tank mates. Unfortunately, they can become pretty aggressive the older and bigger they got. So, it is risky to house Arrow crabs with small and peaceful fish, shrimp, and snails. As they will likely begin to prey on them eventually.
I know that some aquarists keep them with relatively large shrimp like Peppermint shrimp, Red Fire shrimp, Skunk Cleaner Shrimp, Camel shrimp, etc. Well, this is not a good idea. Arrow crabs have very sharp claws, which can easily damage antennae or legs of the shrimp, in the best-case scenario.
Snails (Cerith Snails, Bumble Bee Snails, Conch snails, Nassarius snails, etc.) and small hermit crabs (Halloween hermit crabs, Blue Leg Hermit Crabs, etc.) can be also on their menu. They move very slowly making them ideal prey for the Arrow crab.
Regarding Coral banded shrimp, I would still not recommend keeping them together. Coral banded shrimp are also territorial and aggressive. Their size lets them fight back but eventually, Coral banded shrimp will lose the battle if things get serious.
Full-grown Emerald crabs should not worry about Arrow crabs. The size of the crab will play a huge role here.
Do not keep Arrow crabs with aggressive fish such as Groupers, Lionfish, Eels, and Triggerfish as they will prey on them. The extreme length of the legs makes the crabs difficult to eat at first. However, once the crab loses most of its legs, it becomes easy prey.
In essence, I am saying you’d be doing no harm by keeping Arrow crab alone.
In Conclusion
The Arrow crab is indeed an interesting addition to your home aquarium. They are great scavengers and will control bristleworms in the tank. They are very hardy and easy to care for.
However, people love getting Arrow crabs for their saltwater aquariums because of how cool these alien creatures can be. For this reason, people make impulse purchases all the time on inverts without realizing the potential consequences. They do not understand that their semi-aggressive nature limits options if they want to keep them in a community tank as they may attack other crabs, fish, snails, and shrimp.
References:
- The effect of temperature and salinity on the larval development of Stenorhynchus seticornis (Brachyura: Inachidae) reared in the laboratory. Article in Journal of the Marine Biological Association of the UK. May 2010. DOI: 10.1017/S0025315410000809.
- Majoid crabs community (Crustacea: Decapoda) from infralittoral rocky/sandy bottom of Anchieta Island, Ubatuba. Article in Brazilian Archives of Biology and Technology · June 2004. DOI: 10.1590/S1516-89132004000200015.
- Morphological analysis of the female reproductive system of Stenorhynchus seticornis (Brachyura: Inachoididae) and comparisons with other Majoidea. Article in Invertebrate Biology. April 2016. DOI: 10.1111/ivb.12118.
- Interspecific selection of three potential urchin host species by the arrow crab Stenorhynchus seticornis (Crustacea, Decapoda, Brachyura). Article. January 1998.
- Nutritional vulnerability in zoeal stages of the yellowline arrow crab Stenorhynchus seticornis (Brachyura: Majoidea). Article in Marine and Freshwater Research. March 2018. DOI: 10.1071/MF17145.
- Breeding period of the arrow crab Stenorhynchus seticornis from Couves Island, south-eastern Brazilian coast. Article in Journal of the Marine Biological Association of the UK · December 2002. DOI: 10.1017/S0025315402006598.
- Selection by two Decapod Crabs (Percnon gibbesi and Stenorhynchus seticornis) Associating with an Urchin (Diadema antillarum) at Tobago, West Indies. Article in Bulletin of Marine Science –Miami. July 1998.
- Schriever, G. (1978). In situ observations on the behaviour and biology of the tropical spider crab Stenorhynchus seticornis Herbst (Crustacea, Decapoda, Brachyura), in: McLusky, D.S. et al. (Ed.) Physiology and Behaviour of Marine Organisms: Proceedings of the 12th European Symposium on Marine Biology Stirling, Scotland, September 1977. pp. 297-302 DOI: 10.1016/B978-0-08-021548-8.50041-3.
- Bezerra, L.E.A & Pachelle, P. 2016. Avaliação do Caranguejo Stenorhynchus seticornis (Herbst, 1788) (Decapoda: Inachoididae). Cap. 17: p. 212-220. In: Pinheiro, M. & Boos, H. (Org.). Livro Vermelho dos Crustáceos do Brasil: Avaliação 2010-2014. Porto Alegre, RS, Sociedade Brasileira de Carcinologia – SBC, 466 p.
- Yellowline arrow crab Stenorhynchus seticornis (Brachyura: Majidae) acting as a cleaner of reef fish, eastern Brazil. Article in Marine Biodiversity RecordsDecember 2011. DOI: 10.1017/S1755267211000637.
- Fecundity of the arrow crab Stenorhynchus seticornis in the southern Brazilian coast. Journal of the Marine Biological Association of the United Kingdom (2003). J. Mar. Biol. Ass. U.K.
Main Foto from flowergarden.noaa.gov