The arrow crab looks like a spider built from drinking straws. Its body is a small triangle ending in a long, pointed snout, balanced on legs up to 10 cm long. Stenorhynchus seticornis is a fascinating, hardy crab that is often bought to control bristleworms.
It is also a predator that grows bolder with age. In the right tank it is a great oddball; in the wrong one it hunts small fish and shrimp.
Where arrow crabs come from
Arrow crabs live in the western Atlantic, from North Carolina through the Caribbean to Brazil, and in the eastern Atlantic around the Canary Islands and West Africa. They are found on reefs and rocky areas, often sheltering among sea urchin spines and in anemones.
Appearance and behaviour
The body is golden to brown with thin dark lines and is only 2–6 cm long, but the legs give it a span of up to 15 cm. The claws are small and purple-tipped. Arrow crabs are slow, careful walkers that pick through the rock for food.
Two males will fight, often to the death, so keep one per tank unless the tank is very large.
Tank setup
One arrow crab needs at least 120 litres with plenty of live rock to climb and pick through. Moderate flow and normal reef lighting are fine. Acclimate slowly, as with all crustaceans; see our drip acclimation guide.
Water parameters
Keep the temperature at 22–27°C, salinity at 1.024–1.026 and pH 8.1–8.4. Like all crustaceans, arrow crabs cannot tolerate copper, and they need stable calcium and magnesium to moult well.
Feeding
Arrow crabs are scavengers and hunters. They eat bristleworms, leftovers and small invertebrates. Feed them two or three times a week with frozen mysis, krill and pieces of shrimp or fish, placed in front of them. A well-fed arrow crab is much less likely to hunt your other animals.
Tank mates
Arrow crabs suit tanks with medium to large fish that are not crab eaters: tangs, larger clownfish, angels and wrasses that do not hunt crabs. Avoid small fish that sleep on the bottom, such as gobies, firefish and blennies, and small ornamental shrimp, which may be caught at night. They may also pick at feather duster worms. Most corals are safe, though they may climb over them.
Breeding
Arrow crabs rarely breed in aquariums, and the larvae are not usually raised.
Common problems and lifespan
The main problems are predation on tank mates, failed moults and copper exposure. With good care arrow crabs live 1–3 years.
Explore other topics
Dive into more aquarium guides across the site.