Arrow Crab

Arrow crabs are rewarding subjects for underwater photographers and macro enthusiasts, but finding them requires a keen eye and respectful dive practices. Because they exhibit negative phototaxis, avoid shining bright dive lights directly onto them for extended periods, as intense light can cause stress and prompt them to scurry into inaccessible crevices. During daytime dives, search along fore-reef slopes, shipwrecks, and overhangs. Look closely into tube sponges, under the skirts of anemones like *Lebrunia danae*, and around long-spined sea urchins.

Night dives offer the best opportunity to see arrow crabs out in the open. As dusk falls, they emerge from their refuges to position themselves on elevated coral heads or suspended substrates. When approaching, maintain neutral buoyancy and move smoothly to avoid disturbing the surrounding habitat. Although small, arrow crabs are territorial and fast; avoid poking around their shelter with reef hooks or dive lights, as they may withdraw rapidly or raise their violet claws defensively.

Arrow crabs can be observed by scuba divers year-round across their western Atlantic range, as adults maintain stable home ranges in reef ecosystems. However, their local activity and reproductive cycles are significantly influenced by environmental factors such as seasonal water temperature changes and varying lengths of daily sunlight.

The breeding period of *Stenorhynchus seticornis* is majorly regulated by these seasonal climatic variations. During peak breeding periods, which correlate with optimal temperature and daylight conditions, females are frequently seen carrying masses of fertilised eggs on their pleopods beneath their abdomens. Larger females produce a higher output of eggs per batch. Once hatched, the zoea larvae ascend to surface waters to feed on plankton before metamorphosing and settling into the reef. Divers visiting locations from North Carolina and Bermuda down through the Caribbean and Brazil will encounter active populations regardless of the calendar month, provided water conditions remain warm and stable.

The yellowline arrow crab is restricted geographically to the Occidental Atlantic Ocean. Its distribution spans from North Carolina and Bermuda in the north, extending southwards throughout the Gulf of Mexico and the Caribbean Sea, and continuing down the South American coast to Brazil and Argentina.

Ecologically, *Stenorhynchus seticornis* primarily inhabits shallow sub-tidal environments and fore-reef slopes. While commonly found on coral reefs at typical diving depths between 3 and 15 metres (10 to 30 feet), it adapts to a wide variety of marine substrates. These include rocky bottoms, calcareous algae beds, and soft sediments such as sand and shelly gravel. Within these ecosystems, the crab relies on complex structural refugia—such as deep rock fissures, coral heads, shipwreck frames, and living hosts like anemones and sponges—to shelter from light and predators during the day.

The arrow crab functions as both an active predator and an opportunistic scavenger within reef ecosystems. Nocturnal by nature, it emerges after dark to forage across coral heads and rock faces. Its diet primarily consists of small coral reef invertebrates, including feather duster worms (sabellids) and bristle worms, as well as small crustaceans such as shrimp and various types of reef algae.

During nocturnal feeding, the crab utilises its exceptionally long chelipeds (claws) to pluck prey items from crevices or perform suspension feeding, capturing planktonic organisms from passing water currents while perched on elevated substrates. Its speed and precise claws allow it to quickly seize small prey and scurry away from potential food competitors. In marine aquariums, this voracious diet is often exploited to manage unwanted bristle worm populations, reflecting its natural role as a controller of benthic invertebrate numbers on the reef.