Arrow Crab Care Guide: The Bristle Worm Hunter of the Reef

The arrow crab (Stenorhynchus seticornis) is one of the most distinctive invertebrates in the saltwater hobby. With a pointed, arrow-shaped body, a long forward-jutting rostrum, and spindly legs that can reach across nearly a foot of rockwork, it looks more like a deep-sea spider than a crab. Reef keepers buy them for one job: hunting bristle worms out of live rock. They are good at it — arguably the best biological control on the market — but they come with a list of compatibility caveats that anyone keeping shrimp, small gobies, or LPS corals needs to read before pulling the trigger.

Species Overview#

Arrow crabs are decapod crustaceans native to the western Atlantic, ranging from North Carolina down through the Caribbean and into the coastal waters of Brazil. In the wild they live in reef crevices, rubble fields, and around sponge colonies at depths of 10 to 100 feet, using their long legs to pick through tight spaces for worms, small crustaceans, and detritus. Hobbyists often call them "yellowline arrow crabs" because of the yellow or golden lines that run along the body and legs of healthy specimens, set against a tan or cream background.

Leg span

5-6 in (13-15 cm)

Carapace

1.5 in (3.8 cm)

Min tank

30 gallons (reef)

Temperament

Semi-aggressive

Difficulty

Intermediate

Diet

Carnivore / scavenger

The "Spider Crab" of the Caribbean: Physical Traits and the Rostrum#

The body of an arrow crab is shaped like a triangular arrowhead, tapering forward into a long, sharp rostrum that extends well past the eyes. The rostrum is one of the most fragile parts of the animal — it can snap during shipping or rough handling and will only regenerate slowly across multiple molts. The legs are extraordinarily long relative to the body, often three to four times the length of the carapace, ending in small dark claws tipped with purple or blue. Healthy adults display crisp yellow or gold striping; faded or grayish coloration usually signals stress, poor nutrition, or an upcoming molt.

Adult Size Expectations#

The carapace itself stays small — typically around 1.5 inches at full adult size — but the leg span tells a different story. Mature arrow crabs commonly reach a 5 to 6 inch leg span in captivity, with exceptional specimens documented at 10 inches across in the wild. Plan tank layout around the leg span, not the carapace. A crab that physically fits in a tight rockwork crevice may not be able to reach across an open span to a feeding station, and a cramped scape leads to repeated leg loss during molts.

Natural Habitat: Reef Crevices and Coral Rubble#

Wild arrow crabs spend most of their time wedged into reef crevices, picking at the substrate with their forelimbs. They are nocturnal foragers in the wild but adapt readily to daytime activity in captivity, especially around feeding times. Mimic this in the home tank by providing vertical rock structures with overhangs, caves, and ledges. A flat layout with a shallow rock pile gives them nowhere to retreat during molts and makes them easy targets for any aggressive tankmate.

Water Parameters & Tank Requirements#

Arrow crabs are reasonably hardy once acclimated but extremely sensitive to copper, salinity swings, and ammonia. Stable parameters matter more than chasing perfect numbers.

Ideal Parameters: Temp 72-78 F, pH 8.1-8.4, Specific Gravity 1.023-1.025#

Keep temperature locked between 72 and 78 F. Anything above 80 F accelerates metabolism and pushes the crab into molting cycles before the new shell has properly formed. pH should sit in the standard reef range of 8.1 to 8.4, and specific gravity at 1.023 to 1.025 (or salinity at 33-35 ppt). Arrow crabs are particularly sensitive to salinity shock during acclimation — a sudden drop of even 0.002 in specific gravity can kill a freshly shipped specimen. Always drip acclimate over 60 to 90 minutes.

Minimum Tank Size and Vertical Space Needs#

A 30-gallon reef tank is the practical minimum for a single arrow crab. The carapace is small, but the leg span and territorial behavior demand room. In tanks under 30 gallons the crab dominates the available rockwork and tends to corner shrimp, gobies, and even small wrasses. For displays under 50 gallons, plan rock layout with at least one open vertical span the crab can stretch across without bumping into other invertebrates. Skip the arrow crab entirely in nano tanks under 20 gallons.

Importance of Iodine for Successful Molting#

Iodine is the single most overlooked supplement for arrow crab keepers. Crustaceans require trace iodine to harden the new exoskeleton after each molt — without it, the new shell stays soft and the crab dies during the molt window. Most reef salt mixes provide adequate iodine for the first month, but established tanks with carbon filtration or protein skimmers strip iodine continuously. Test monthly and dose Lugol's solution or a commercial reef iodine supplement to maintain 0.06 to 0.10 ppm. Failed molts are almost always traced back to depleted iodine or an unstable calcium/magnesium ratio.

Diet & Feeding#

Arrow crabs are opportunistic carnivores and scavengers. In a healthy reef they will work the rockwork constantly, picking at worms, pods, leftover food, and detritus.

Biological Control: Hunting Bristle Worms and Flatworms#

The number one reason hobbyists buy arrow crabs is bristleworm control. They probe rock crevices with their long forelimbs, latch onto a worm with the small claws, and tear it apart over several minutes. They will also pick at flatworms, amphipod populations that have grown out of control, and any small dying organism on the rockwork. Effectiveness depends on how hungry the crab is — if you target-feed daily with mysis or krill, the crab will stop working bristleworms because there is easier food available.

Supplemental Feeding: Mysis Shrimp, Silver Sides, and Pellet Foods#

Once the visible bristleworm population is gone, supplemental feeding becomes necessary. Offer frozen mysis shrimp, chopped silver sides, krill, or sinking carnivore pellets two to three times per week. Drop food directly in front of the crab using long feeding tongs to avoid drawing it out into open water where it might harass other tankmates. A well-fed arrow crab is a calmer, less predatory arrow crab.

Target Feeding Techniques to Prevent Aggression#

Target feeding is the single best behavior intervention for arrow crab keepers. Hungry crabs roam the tank, climb on corals to steal food, and stalk shrimp. Fed crabs sit in their preferred crevice and only leave during the next feeding cycle. Use a turkey baster or feeding tongs to deliver food directly to the crab's territory. If the crab leaves its spot to grab food at the front of the tank during fish feedings, scale back the supplemental schedule until it stays put.

Tank Mates & Compatibility#

Arrow crab compatibility is a sliding scale. The crab is not aggressive in the way a mantis shrimp or hawkfish is aggressive, but it is opportunistic, and over months of cohabitation it will pick off the slowest, smallest, or sickest animals in the tank.

Are Arrow Crabs Reef Safe? (Caution with Small Polyps and Crustaceans)#

Arrow crabs do not eat coral tissue, but adults have been documented picking at zoanthid and palythoa colonies, ripping food out of LPS feeder tentacles, and occasionally damaging soft coral polyps. The risk is highest with feather dusters and Christmas tree worms, both of which look like oversized bristleworms to a hungry arrow crab. SPS-only reefs tolerate arrow crabs better than mixed reefs.

Dangerous Pairings: Small Gobies, Ornamental Shrimp, and Slow-Moving Fish#

Avoid pairing arrow crabs with small bottom-dwelling fish like neon gobies, clown gobies, or jawfish. Sleeping fish are the easiest target, and a crab that has tasted fish once will hunt actively from then on. Sexy shrimp, anemone shrimp, and most small ornamental shrimp under 1 inch are at risk as well. Even peppermint shrimp, which are normally robust, can be picked off during molts.

Safe Companions: Large Tangs, Angels, and Robust Invertebrates#

Large active fish — yellow tangs, tomini tangs, larger wrasses, dwarf angels, and clownfish — coexist well with arrow crabs because they are too fast and too alert to be ambushed. Robust invertebrates like turbo snails, larger hermit crabs, and skunk cleaner shrimp generally hold their own. Pair an arrow crab with a skunk cleaner shrimp only in tanks 75 gallons or larger, where the shrimp has multiple territories to retreat to.

For a broader look at compatible inverts, see the peppermint shrimp care guide and the general saltwater fish overview.

Molting & Growth#

Molting is the most dangerous time in an arrow crab's life. Every few weeks to a few months, the crab sheds its entire exoskeleton in a single piece, including the linings of its gills, mouth, and legs. The new shell underneath is soft and vulnerable for 24 to 48 hours.

Signs of an Impending Molt and Post-Molt Vulnerability#

A crab about to molt becomes sluggish, hides for several days, and stops feeding. The carapace may look slightly cloudy or dull. After the molt, the crab is white and rubbery, often hiding in its deepest crevice. Do not move rocks, run aggressive maintenance, or add new tankmates during this window. Many losses are blamed on tankmate aggression when the real cause is a stressed molt that produced a weak new shell.

Calcium and Magnesium Requirements for Exoskeleton Health#

Calcium should sit at 400-450 ppm and magnesium at 1280-1350 ppm. These are standard reef numbers, but arrow crabs draw down both elements rapidly during heavy molting cycles. Test weekly if you keep multiple molting invertebrates. Keep a discarded molt skin in the tank for 24 hours after the molt — the crab often eats it to recover the minerals.

Common Health Issues#

Iodine Deficiency and Failed Molts#

The most common cause of arrow crab death is a failed molt driven by depleted iodine. The crab gets stuck halfway out of the old shell and dies within hours. Test iodine monthly and supplement to maintain 0.06-0.10 ppm. Activated carbon and protein skimmers strip iodine continuously, so well-filtered tanks need more frequent dosing than lightly filtered systems.

Sensitivity to Copper-Based Medications and Nitrate Spikes#

Copper is lethal to all crustaceans at trace concentrations. Never dose copper-based ich treatments, parasite medications, or algaecides in a tank holding an arrow crab. If a tankmate needs copper treatment, move it to a quarantine tank — moving the crab is rarely worth the stress. Sustained nitrate above 40 ppm also drives failed molts and shortens lifespan.

Where to Buy & What to Look For#

Arrow crabs are widely available in saltwater specialty stores and online, but quality varies dramatically. A damaged specimen often dies during the first or second molt regardless of how well you care for it.

Inspecting the Rostrum and Legs for Damage#

Always inspect the crab in person before buying. The rostrum should be straight, sharp, and unbroken. Count the legs — eight walking legs plus the two cheliped (clawed) front limbs. Stubby or regenerating legs are not deal breakers but indicate past stress. Ask the seller how long the crab has been in the tank; specimens that have made it past two weeks at the store have already survived the most dangerous post-shipping window.

Acclimation Protocols: Drip Method for Salinity Sensitivity#

Arrow crabs are extremely sensitive to salinity shock. Always drip acclimate over 60 to 90 minutes, equalizing the bag water with tank water gradually before transferring the crab into the display. Skip the bag-floating step — temperature alone is not the issue. Use airline tubing with a gang valve set to 2-3 drops per second. Never pour bag water into the display.

For setup details on the broader system, see our guides on setting up a saltwater aquarium and stocking saltwater fish.

Quick Reference#