Invertebrates
Arrow Crab
Stenorhynchus seticornis · also called arrow crabs
After dark, an arrow crab emerges from a reef crevice on impossibly thin legs, its sharply pointed head leading the way toward a bristle worm that, in an aquarium tank, would otherwise have multiplied well beyond what any owner wanted to deal with.
Overview
The arrow crab, Stenorhynchus seticornis, is a delicate, long-legged crab native to coral reef habitat across the Caribbean and western Atlantic. Its spider-like build, arrow-shaped head, and genuine usefulness as a bristle worm predator make it one of the more visually unusual and practically valued small crabs kept by reef aquarium hobbyists.
A crab that barely looks like one
With legs this thin and a body this small, the arrow crab challenges most people’s mental image of a crab entirely, trading the stocky, armoured build of its relatives for a delicate precision suited to hunting in tight reef crevices.
Status
Least Concern — widespread and common across suitable coral reef habitat throughout the Caribbean and western Atlantic.
Arrow Crab: common questions
Why do people keep arrow crabs in reef aquariums?
An arrow crab is often deliberately kept in reef aquariums specifically for its effectiveness at hunting and controlling bristle worm populations, which can otherwise multiply into a nuisance within an aquarium, making the arrow crab a genuinely useful biological pest-control option for reef hobbyists.
Why does the arrow crab have such thin legs?
An arrow crab has extremely long, thin legs relative to its small body, giving it a delicate, spider-like appearance that allows it to move precisely through tight crevices and coral structure while hunting, a build quite different from the stockier legs typical of most other crab species.
Can an arrow crab regrow a lost leg?
Yes, like many crabs, an arrow crab can regenerate a lost leg over successive moults, gradually restoring the limb to something close to its original length and function, allowing the crab to recover from an injury or a deliberate defensive leg loss.
Where does the arrow crab live?
The arrow crab is native to coral reef habitat across the Caribbean Sea and adjacent western Atlantic waters, generally remaining hidden in crevices during the day and becoming active mainly at night.