Arrow Crab

Arrow Crab (Stenorhynchus seticornis): A striking, long‑legged reef invertebrate known for its triangular body, elongated rostrum, and vivid blue‑violet claws. Commonly used in reef aquaria for worm control. 🦀 Identification & Appearance Body shape: Triangular carapace with an extremely long, serrated rostrum (pointed “nose”). Size: Carapace up to 6 cm; leg span up to 10 cm. Coloration: Body: golden, yellow, or cream with brown/black/iridescent blue lines. Legs: reddish or yellow. Claws: blue or violet, often with violet tips. General look: Spider‑like, with legs more than 3× body length. 🌍 Distribution & Habitat Range: Western Atlantic — North Carolina & Bermuda through the Caribbean to Brazil. Depth: Typically 10–30 ft on coral reefs. Microhabitats: Rock bottoms, coral heads, calcareous algae, shelly gravel, sand. Often found living among sea anemones (especially Lebrunia danae). 🌙 Behavior Nocturnal: Actively forages at night; hides under ledges, sponges, sea fans during the day. Territorial: Known to be aggressive toward other small invertebrates when defending space. Negative phototaxis: Avoids bright light and shifts location between day and night. Aquarium role: Commonly kept to help control bristle worms. 🍽 Diet Primarily small reef invertebrates: Feather duster worms Bristle worms Other tiny benthic invertebrates Some algae and detritus (general scavenging) 🧬 Reproduction & Life Cycle Mating: Male deposits a sperm packet while holding the female against his abdomen. Egg brooding: Female carries eggs under her abdomen until hatching. Larval stages: Zoea: Transparent, planktonic, rounded body; swims toward surface. Megalops: Begins to resemble a crab; abdomen not yet folded. Adult: After multiple molts, reaches full form. 🪸 Ecological Associations Frequently found living among pseudotentacles of Lebrunia danae anemones, often in small groups (1–6 individuals). Shares habitat with cleaner shrimp species such as Ancylomenes pedersoni and Periclimenes yucatanicus.