Juergen Freund / Horsehoe crab (Tachypleus gigas) swimming on its back with gills creating the propulsion, Palawan, Philippines. Did you know? Horseshoe crabs are more closely related to spiders than crabs.

Juergen Freund / Horsehoe crab (Tachypleus gigas) underside, Palawan, Philippines

Cyril Ruoso / Horseshoe crabs (Tachypleus gigas) mating among pollution washed up in mangroves, Sungei Buloh Natural Park, Singapore.

Cyril Ruoso / Chinese Horseshoe Crab (Tachypleus gigas) pair mating showing smaller male attached to female, Singapore

Juergen Freund / Pacific horsehoe crabs (Tachypleus gigas) on sandy seabed, Palawan, Philippines.

Juergen Freund / Horsehoe crabs (Tachypleus gigas) in defensive posture on sandy seabed, Palawan, Philippines.

Juergen Freund / Two Horsehoe crabs (Tachypleus gigas) on sandy seabed, one in defensive posture, Palawan, Philippines.

Edwin Giesbers / Two Long-tailed / Crab-eating macaques (Macaca fascicularis) in mangrove habitat feeding on Coastal horseshoe crab (Tachypleus gigas) Bako National Park, Sarawak, Borneo, Malaysia

Tony Heald / Horseshoe crab [Tachypleus gigas] pair spawning, Bako National Park, Sarawak, Borneo, September

Sirachai Arunrugstichai / Indo-Pacific horseshoe crabs (Tachypleus gigas) mate in the sandy substrate, Satun province, Thailand.