Arrow Crab – Stenorhynchus seticornis

Overview:
Arrow Crab (Stenorhynchus seticornis) is a long-legged, reef-associated crab best kept in a stable, mature saltwater aquarium with abundant live rock and crevices. Provide strong biological filtration, consistent salinity, and high oxygenation with moderate flow; sudden parameter swings are a common cause of decline. Keep temperature and pH steady (typical reef ranges), and prioritize low ammonia/nitrite with controlled nitrate. The crab is largely nocturnal and will spend much of its time wedged into rockwork; design the aquascape with tight shelters and overhangs so it can rest and molt safely. Because it can be delicate during molts and may be harassed, avoid overcrowding and maintain a calm environment. Ensure all pump intakes are screened—its long legs can be vulnerable.

Compatibility:
Despite the “cleaner” reputation, Arrow Crabs can be predatory. They may pick at feather dusters, small shrimp, tiny crabs, and other mobile inverts, especially if underfed. Snails are usually safer than hermits, but small or slow tankmates can still be tested. With fish, they are generally reef-safe in larger, peaceful communities, though very small sleeping fish could be at risk in cramped tanks. Avoid housing with aggressive crabs, large predatory fish (triggers, puffers, large wrasses), and any species likely to nip legs. Keeping more than one Arrow Crab is risky unless the tank is large with ample hiding; they may fight, particularly during molts.

Health and Quarantine:
Molting is the main vulnerability period: provide iodine through regular water changes (don’t dose blindly), stable alkalinity/calcium typical of reef systems, and plentiful hiding. Signs of trouble include repeated failed molts, missing limbs without regrowth over time, lethargy, inability to right itself, and refusal to eat for extended periods. Quarantine is recommended in a separate, fully cycled saltwater tank with PVC shelters; avoid copper-based medications (toxic to inverts). Observe for 2–4 weeks for parasites on fish introduced later, and to confirm feeding response. Use drip acclimation when moving the crab, and keep handling to a minimum to prevent leg damage and stress.

Carnivore

Meaty Foods Worms Small Shrimp
Arrow Crabs are long-legged, inquisitive scavengers that move deliberately across rockwork and crevices while searching for food. Much of their natural behavior revolves around hunting small invertebrates such as bristleworms, which makes them useful in worm-heavy systems but also capable of preying on tiny shrimp or very small fish if given the opportunity. They become more active and territorial at night, patrolling their chosen areas and defending them from other bottom-dwellers. When kept in appropriately sized tanks with suitable tank mates, they function as efficient scavengers with a strong predatory edge that reef keepers should account for.

FAQ

Frequently Asked Questions

About the Arrow Crab

  • My Arrow Crab is picking at rock crevices all night. Is it hunting something?
    Yes. Arrow Crabs are natural nighttime hunters and use their long legs to probe deep into rockwork for small prey. They often feed on bristleworms, amphipods, and other tiny invertebrates that emerge after lights go out. This behavior is normal and even beneficial in tanks where bristleworm populations are high. As long as your crab is well-fed and tank mates are appropriately sized, this hunting activity is expected and healthy.
  • Are Arrow Crabs safe with shrimp?
    It depends on the shrimp and the size of the Arrow Crab. Juvenile Arrow Crabs are generally safe with larger ornamental shrimp like cleaner shrimp or peppermint shrimp. However, as Arrow Crabs grow, they may view very small or freshly molted shrimp as prey. If you keep nano shrimp species or young shrimp, introduce the Arrow Crab only after ensuring there are plenty of hiding spaces and consistent supplemental feeding.
  • My Arrow Crab is standing on top of a coral and waving its legs. Is it damaging the coral?
    Arrow Crabs do not intentionally harm corals, but their sharp, thin legs can irritate polyps if they stand on them frequently. The waving motion is a natural territorial or foraging behavior. If the coral retracts occasionally, this is not usually a concern. If the crab routinely chooses high-traffic coral locations, rearranging rock structures can encourage it to perch elsewhere.
  • Why does my Arrow Crab fight with hermit crabs or small snails?
    Arrow Crabs are territorial and will defend their preferred feeding areas. If another scavenger repeatedly enters the same crevice or rock zone, the Arrow Crab may chase or pinch them to maintain control of the area. Ensuring there are multiple feeding zones and distributing food around the tank reduces these interactions. In most cases, the conflicts are brief and do not result in injury.
  • My Arrow Crab suddenly looks much larger and there is a hollow shell on the sand. Is this normal?
    Yes, this is a molt. Arrow Crabs shed their exoskeletons as they grow, often leaving behind a ghost-like hollow body. After molting, they hide for several hours or days until their new exoskeleton hardens. During this time they are vulnerable and may not appear in the open. Avoid disturbing them or moving rocks during this period, as hiding is essential to their safety.