Yellowline Arrow Crab

The body of S. seticornis is triangular, and the rostrum is drawn out into a long point with serrate edges. The legs are also long and thin, up to 10 cm (3.9 in) across, and the animal's carapace may be up to 6 cm (2.4 in) long. Colouration is variable in this species; the body may be golden, yellow or cream, marked with brown, black or iridescent-blue lines; the legs are reddish or yellow, and the claws are blue or violet. This species presents as diagnostic features: triangular body with long pointed snout (rostrum), carapace decorated with fine dark lines and claws often have violet tips. S. seticornis is most commonly associated with anemones, in groups ranging from one to six individuals in south-eastern Brazil.

Stenorhynchus seticornis is commonly found in the shallow sub-tidal on rock bottoms, corals, calcareous algae, and also on soft sediments, such as shelly gravel and sand. Its geographical distribution is restricted to the Occidental Atlantic, from North Carolina to Argentina. Stenorhynchus seticornis is found in the western Atlantic Ocean, from North Carolina and Bermuda to Brazil, including throughout the Caribbean Sea. It lives on coral reefs at depths of 10–30 feet (3.0–9.1 m).

S. seticornis is nocturnal and territorial. The species has displayed notable negative phototaxis, meaning the organism prefers to avoid sunlight, and changes location during the day and nighttime. It eats small feather duster worms and other coral reef invertebrates. This crab is commonly kept in reef aquariums to control bristle worm populations.

During mating, the male places a spermatophore on the female, which she uses to fertilise her eggs. These fertilised eggs are then carried on the female's pleopods until they are ready to hatch into zoea larvae. These swim towards the ocean surface and feed on plankton. They grow through a series of moults, and eventually metamorphose into the adult form. When mature, the spermatozoa of S. seticornis show five lateral arms, similar to the sperm morphology of Inachus phalangium.