Ingo Arndt / Christmas Island Red Crabs (Gecarcoidea natalis) massing on coastal rocks, the morning after spawning, Christmas Island, Indian Ocean, Australian Territory

Ingo Arndt / Christmas Island Red Crabs (Gecarcoidea natalis) shortly before sunrise sitting on rock ready for spawning, Christmas Island, Indian Ocean, Australian Territory

Linda Pitkin / 2020VISION / Spiny spider crab (Maja brachydactyla / squinado) pair on rock covered with red algae, Lundy Island Marine Conservation Zone, Devon, England, UK, May.

Nick Upton / Velvet swimming crab (Necora puber) in a rock pool, hiding among Coralweed (Corallina officinalis), Cornwall, UK, March.

Nick Upton / Common shore crab (Carcinus maenas) hiding among Coralweed (Corallina officinalis) in a rock pool, Cornwall, UK, March.

Heather Angel / Land crab crawling on red sandstone after overnight storm. Shunan Zhuhai National Park, Sichuan Province, China.

Duncan Murrell / Red-eyed rock crab (Eriphia sebana) on the Forgotten Coast, Lagoons of New Caledonia: Reef Diversity and Associated Ecosystems UNESCO World Heritage Site. New Caledonia.

Franco Banfi / Red rock crab (Grapsus grapsus), Los Islotes, Sea of Cortez, Baja California peninsula, Mexico, East Pacific Ocean.

Nick Upton / Marbled rock crab (Pachygrapsus marmoratus) foraging on intertidal rock face among barnacles, mussels, anemones and encrusting red algae, exposed at low tide, Asturias, Spain, August.

Kim Taylor / Red rock crab (Grapsus adscensionis) on rock, La Palma, Canary Islands.

Kim Taylor / Red rock crab (Grapsus adscensionis) on rock, La Palma, Canary Islands.

Linda Pitkin / 2020VISION / Spiny spider crab (Maja brachydactyla / squinado) pair on rock covered with red algae, Lundy Island Marine Conservation Zone, Devon, England, UK, May.

Linda Pitkin / 2020VISION / Spiny spider crab (Maja brachydactyla / squinado) pair on rock covered with red algae, Lundy Island Marine Conservation Zone, Devon, England, UK, May.

Linda Pitkin / 2020VISION / Spiny spider crab (Maja brachydactyla / squinado) pair on rock covered with red algae, Lundy Island Marine Conservation Zone, Devon, England, UK, May.

Linda Pitkin / 2020VISION / Spiny spider crab (Maja brachydactyla / squinado) pair on rock covered with red algae, Lundy Island Marine Conservation Zone, Devon, England, UK, May.

Linda Pitkin / 2020VISION / Spiny spider crab (Maja brachydactyla / squinado) pair on rock covered with red algae, Lundy Island Marine Conservation Zone, Devon, England, UK, May.

Linda Pitkin / 2020VISION / Spiny spider crab (Maja brachydactyla / squinado) pair on rock covered with red algae, Lundy Island Marine Conservation Zone, Devon, England, UK, May.

Sue Daly / Red rock crab (Cancer productus) Pacific coast, Canada, August

Ingo Arndt / Christmas Island Red Crab (Gecarcoidea natalis) mass resting in rock pool close to coast before spawning, Christmas Island, Indian Ocean, Territory of Australia

Stephen Belcher / Christmas Island Red Crab (Gecarcoidea natalis) group gathered into a rock crevice, Christmas Island, Australia

Luis Quinta / Lightfoot crab {Grapsus adscensionis} on rock, Madeira, Portugal

Luis Quinta / Lightfoot crab {Grapsus adscensionis} on rock, Madeira, Portugal

Luis Quinta / Lightfoot crab {Grapsus adscensionis} on rock, Madeira, Portugal

Fred Bavendam / Bat Star (Asterina miniata) group scavenging on dead Pacific Rock Crab (Cancer antennarius), Monterey Bay, California

Norbert Wu / Red Rock Crab (Cancer productus) pair mating with the female holding the male, Vancouver Island, British Columbia, Canada

Scott Leslie / Atlantic Rock Crab (Cancer irroratus) on algae, Nova Scotia, Canada

Pete Oxford / Sally Lightfoot Crab (Grapsus grapsus) on volcanic rock, Santiago Island, Galapagos Islands, Ecuador

Juergen Freund / Christmas Island red crab {Gecarcoidea natalis} on rock, Christmas Is, Indian Ocean

Ingo Arndt / Sally Lightfoot Crab (Grapsus grapsus) on rocks, Santiago Island, Galapagos Islands, Ecuador

Pete Oxford / Sally Lightfoot Crab (Grapsus grapsus) in rock crevice near tideline, Santiago Island, Galapagos Islands, Ecuador

Pete Oxford / Sally Lightfoot Crab (Grapsus grapsus) in rock crevice near tideline, Santiago Island, Galapagos Islands, Ecuador

Fred Bavendam / Red Rock Crab (Cancer productus) eating a Lion Nudibranch (Melibe leonina), Vancouver Island, British Columbia, Canada

Jeff Rotman / Convex Rock crab {Carpilius convexus} on Alcyonarian coral at night, Red Sea.

Tui De Roy / Marine iguana (Amblyrhynchus cristatus) sharing volcanic rock with two Sally lightfoot crabs (Grapsus grapsus). Mosquera Island, Galapagos Islands, Ecuador.

Fernando Faciole / Sally lightfoot crab (Grapsus grapsus) on rock. Isabela Island, Galapagos Islands, Ecuador.

Nick Upton / Red rock crab (Grapsus adscensionis) grazing on algae from rock, Tenerife, Canary Islands, October.

Lucas Bustamante / Sally Lightfoot crabs (Grapsus grapsus) on coastal volcanic rocks, Santiago Island, Galapagos National Park, Galapagos Islands.

Lucas Bustamante / Sally Lightfoot crabs (Grapsus grapsus) on rocks caught in the swell, Espanola Island, Galapagos National Park, Galapagos Islands.

Lucas Bustamante / Sally Lightfoot crabs (Grapsus grapsus) climbing over volcanic rocks with Galapagos sea lion (Zalophus wollebaeki) resting above, Santiago Island, Galapagos National Park, Galapagos Islands. Endangere...

David Fleetham / Wave washing over Red rock crab (Grapsus grapsus) searching for algae to feed on in intertidal zone, Santa Cruz Island, Galapagos Islands.

Alex Mustard / Edible crab (Cancer pagurus) foraging on a Maerl (Phymatolithon calcareum) bed, Fal River estuary, Cornwall, UK.

David Hall / Red Irish lord fish (Hemilepidotus hemilepidotus) with Pygmy rock crab (Cancer oregonensis). Seven-Tree Island, Browning Pass, British Columbia. September.

David Hall / Red Rock Crab (Cancer productus) female with a mass of brown eggs attached to her abdomen, which she will carry until the eggs have hatched. Gilford Island, British Columbia, Canada. March.