A striking spider-like crab with an arrow-shaped body and impossibly long, banded legs. Hardy and entertaining, it scavenges detritus and bristleworms but is only conditionally reef-safe and may hunt ornamental shrimp and small fish.
ℹ️
Educational only. KinStation content is reviewed by licensed veterinarians but cannot replace an in-person exam. Always consult a licensed veterinarian or board-certified specialist for diagnosis, treatment, or any decision affecting your pet's health.
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Body up to about 0.6 in (1.5 cm), but spindly legs span up to 6 in (15 cm).
Lifespan
2–6 years
Social needs
solo
Native region
Western Atlantic and Caribbean, from the southeastern US and Gulf of Mexico through the West Indies to South America
Origin
New World
Climate
🌴 Tropical
Water type
🌊 Marine
Family
Inachidae
Genus
Stenorhynchus
Part of the Marine Crabs
Reef-dwelling crabs kept mostly as algae-grazing and scavenging cleanup crew. Many are useful and largely reef-safe when well fed, but most are opportunistic and can turn on snails, corals, or small fish if hungry — so stock conservatively and watch behavior.
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Photo coming soon
Minimum
Reef aquarium
30 gal (≈ 114 L) reef
Stenorhynchus seticornis is a reef inhabitant with a long leg span (~15 cm). Keep singly in a 30+ gal reef with rockwork. They eat bristleworms and feather dusters — semi-reef-safe; will pick at corals and small inverts when hungry.
Photo coming soon
Recommended
Established reef aquarium
55 gal reef, complex rockwork
A 55+ gal reef with extensive rockwork and varied tankmates. One arrow crab per tank — territorial with conspecifics. Feed regularly to prevent coral picking.
Photo coming soon
Ideal
Larger mature reef
75+ gal mature reef
A larger mature reef with diverse rockwork and a peaceful community. Generous footprint supports the arrow crab's wide leg span and ambush hunting behaviour.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Larva
Most marine invertebrates hatch into microscopic planktonic larvae (such as the zoea of crustaceans or the bipinnaria/veliger of echinoderms and mollusks) that drift and feed in the water column. The larva looks nothing like the adult and undergoes major reorganization.
Photo coming soon
Juvenile
After settling out of the plankton, the juvenile takes on a recognizable miniature of the adult body plan — a tiny shell, a small star, or a translucent shrimp. Crustaceans grow by molting, shedding the exoskeleton to enlarge.
Adult
Adults reach full size and reproductive maturity with the species' mature shell, shape, or coloration. Many continue to molt or grow throughout life, and some show sex differences in size or claw/appendage shape.
Habitat & enclosure
Needs a tank of at least 75-115 L (20-30 gal) to accommodate its long-legged span and territorial nature. Keep standard reef water: temperature 24-27 C (75-80 F), pH 8.1-8.4, specific gravity 1.023-1.026, with copper at zero and stable salinity.
Provide abundant live rock with caves and overhangs, which it uses as a daytime base and ambush point. Lighting is unimportant to the crab; moderate flow and a mature tank with a healthy detritus and worm population suit it best. Keep one per tank, as they are aggressive toward their own kind.
Substrate
Mature live rock with caves is the key aquascape feature for shelter and bristleworm hunting. Any sand or crushed-coral bed is acceptable, as the crab lives primarily among the rocks.
Equipment & setup
A typical reef system supplies what it needs: live-rock biofiltration, a stable heater, protein skimmer, and moderate powerhead flow. No special lighting is required.
Diet
An opportunistic carnivore and scavenger that eats detritus, leftover food, and especially bristleworms, making it a popular worm-control animal. It readily takes meaty foods such as mysis, brine, chopped seafood, and pellets. A well-fed arrow crab is less likely to hunt tankmates, so feed it directly several times a week.
Behavior & temperament
Conditionally reef-safe: it leaves most corals alone but will catch and eat small ornamental shrimp (including cleaner and sexy shrimp), tiny fish, and snails, particularly when hungry. It is territorial toward other arrow crabs and some inverts. House one per tank with robust, non-tiny tankmates, and avoid pairing with prized small shrimp.
Health
Hardy and long-lived if salinity and copper are controlled; main losses come from acclimation shock and copper exposure. Molting is normal and produces a complete, eerily lifelike shed — do not assume the crab has died. Legs can be lost to fights or rough handling but usually regenerate over successive molts.
Tips, DIY & hacks
Add one to a tank plagued by bristleworms, but never alongside small shrimp you want to keep. Drip-acclimate slowly over 30-60 minutes, since this species is sensitive to rapid salinity changes. If it starts ambushing fish or shrimp, trap and rehome it rather than starving it.
