The first time a guide points one out, most divers need a few seconds to understand what they are looking at. A pointed, golden body no bigger than a thumb, a long spear sticking out of the front, and eight legs so thin and long that the whole animal looks like it was drawn with a fine pen. That is the yellowline arrow crab, one of the strangest-looking residents of Punta Cana's reefs.
It is also one of the most rewarding animals to look for. Arrow crabs are harmless, they tend to stay put rather than flee, and once you learn where they hide you start seeing them on dive after dive. They are a perfect introduction to the small life that most people swim straight past.
Quick Facts
- Name: yellowline arrow crab, Stenorhynchus seticornis
- Family: a spider crab (family Inachidae), related to decorator crabs
- Size: the body is usually 3–6 cm (1–2.5 in) long; the legs are more than three times the body length
- Diet: small invertebrates such as feather duster worms and bristle worms, plus scavenged scraps
- Activity: mostly nocturnal; hides in crevices by day
- Depth: usually seen from about 3 to 40 m (10 to 130 ft); recorded much deeper
- Range: western Atlantic, from North Carolina and Bermuda through the Gulf of Mexico and the Caribbean to Brazil
How to Recognize an Arrow Crab
Nothing else on a Caribbean reef has this shape. The body is a sharp triangle that ends in a long, serrated spike called the rostrum, which gives the crab its "arrow" name. The legs are extremely long and spindly, and the two front legs end in slender claws, often tipped in blue or violet.
The color is the second clue. The body is golden, tan or cream, crossed by fine dark lines that run lengthwise, and the legs are usually reddish or yellow. Under a dive light those colors pop; in daylight the crab blends surprisingly well into the shadows of a ledge.
The only real look-alikes are other small spider crabs, which are much stockier and lack the long spear on the front. If it looks like a daddy-longlegs with an arrowhead, it is an arrow crab.
Behavior: Shy Hunter, Part-Time Cleaner
Arrow crabs avoid bright light and move around more at night. By day they wedge themselves under ledges, between sponges and in the crevices of coral heads, often with only the legs and the rostrum visible. After dark they pick their way slowly across the reef looking for food.
They are small predators and scavengers. Feather duster worms and bristle worms are favorite prey, which is why aquarium keepers sometimes use them to control bristle worms. They also pick at detritus and whatever leftovers they find.
The old idea that arrow crabs are "reef cleaners" has some truth in it, but it needs context. Researchers in Brazil have documented arrow crabs picking parasites and dead tissue off reef fish, including moray eels and squirrelfish, which is remarkable because both of those fish eat crabs. That cleaning has so far been reported mainly from Brazil, and arrow crabs are not full-time cleaners like the cleaner shrimp you see waving their antennae from coral heads. They are opportunists that clean sometimes.
They also have a well-known link with sea anemones. Arrow crabs are often found living among the tentacles of the branching anemone (Lebrunia danae), sometimes sharing the same anemone with Pederson's cleaner shrimp or spotted cleaner shrimp. The crab seems to gain shelter, and the arrangement makes a busy little micro-habitat worth a long look. Arrow crabs can also be territorial toward their own kind, so you usually see them alone or in small, spaced-out groups.
Like other crabs, they grow by molting. Females carry their fertilized eggs under the abdomen until they hatch into tiny larvae that drift in the plankton before settling on the reef.
Where to See Arrow Crabs in Punta Cana
Arrow crabs are a regular find on our local reefs for divers who slow down and look closely, though they are not something you will see on every dive. The best places are anywhere with lots of holes, overhangs and shade. At Cuevitas, at about 10 m (33 ft), the small caves and swim-throughs are full of the crevices they like, and the same site is known for cleaner shrimp. Rock City has canyons and cracks where you can find them too, and the artificial structure of the Submarine site is worth checking as well.
Wrecks are classic arrow crab habitat all over the Caribbean, so keep an eye out on Wreck Monica and on the Bayahibe wrecks if you join that trip. Look under ledges, around the bases of sponges and next to anemones. A small dive light helps even in daytime, because it shows the golden body inside dark holes.
They live alongside many of the animals on our species guide, and you will often find one near a moray eel's hiding place or next to a coral crab wedged deeper in the same crevice.
Photography Tips 📸
- Get low and patient. Arrow crabs usually stay still, so you can take your time framing. Settle in a sandy spot or hover; never kneel on coral to get the shot.
- Use a macro lens or close-focus mode. The rostrum and the eyes are the sharpest details, so focus there.
- Mind your light. Arrow crabs avoid bright light. A few frames with a strobe are fine, but do not keep a powerful video light on one for minutes.
- Never poke or herd it. The legs are fragile. If it walks away, let it go and look for the next one.
- Look for the bonus shot. An arrow crab inside an anemone with a cleaner shrimp is one of the best small-life pictures in the Caribbean.
For more small, odd animals worth hunting with a camera, see our field guide to weird reef things.
Frequently Asked Questions
Are arrow crabs dangerous?
No. They are harmless to divers and too small to give a meaningful pinch. The animal is far more at risk from you than the other way round, so look but do not touch.
How big do arrow crabs get?
The body usually measures about 3 to 6 cm (1 to 2.5 in), but the legs make the crab look much larger, often spanning around 10 cm (4 in).
Do arrow crabs really clean fish?
Sometimes. Scientists have recorded them cleaning reef fish, including morays, in Brazil. Most of their food, though, is small worms and scraps they find on the reef.
Can you see arrow crabs while snorkeling?
Occasionally in very shallow, rocky areas, but they hide in shaded holes and are hard to spot from the surface. Scuba gives you much better odds.
Want to try spotting one yourself? Join us for a two-tank reef dive in Punta Cana, tell your guide you are into small critters, and we will slow down and look into the crevices together.
