The so called Halloween Crab, Gecarcinus quadratus (Decapoda - Gecarcinidae) is a neotropical land crab with a distinctive patterning; the upper carapace seems completely black (actually dark brown when examined closely), the body and limbs are a bright orange-red, two bright yellow to white triangular spots decorate the front of the upper carapace, and the claws are purple.
The species is distributed along Pacific shorelines from Mexico to Peru. Some authors have treated Gecarcinus quadratus as a subspecies of Gecarcinus lateralis (the Atlantic species), or as synonym with Gecarcinus lateralis, while others have maintained Gecarcinus quadratus as a valid species.
Whatever, these crabs play an important ecological role on its tropical environment and is regarded as an engineering species that controls nutrient cycling in tropical forests. Gecarcinus quadratus and other similar species of land crabs process large quantities of leaf litter, thereby influencing nutrient cycling. They alter the structure of plant communities through selective consumption of seeds and seedlings, and their burrows provide habitat for obligatory and facultative arthropod symbionts. Based on the direct and indirect influences of the land crabs on resource availability, as well as their modification of habitat, they can be considered allogenic ecosystem engineers.