Tui De Roy / Swallow-tailed Gull (Creagrus furcatus) endemic, guarding chick in pebble nest, Genovesa Tower Island, Galapagos Islands, Ecuador

Tui De Roy / Swallow-tailed Gull (Creagrus furcatus) world's only nocturnal Gull, at pebble nest with chick, Tower Island, Galapagos Islands, Ecuador

Tui De Roy / Swallow-tailed Gull (Creagrus furcatus) incubating eggs in pebble nest, Tower Island, Galapagos Islands, Ecuador

Tui De Roy / Gentoo Penguin (Pygoscelis papua) nesting adults rebuild pebble nest, Aitcho Island, South Shetland Islands, Antarctica

Fred Bavendam / Splendid Pebble Crab (Etisus splendidus) photographed at night, Bali, Indonesia

Martin Dohrn / Ammonite fossil on pebble beach. Dorset, England, UK, Europe

Hiroya Minakuchi / Adelie Penguin (Pygoscelis adeliae) picking rock for nest, Weddell Sea, Antarctic Peninsula, Antarctica

Paul D Stewart / 1837 portrait of Gideon Mantell (3 February 1790 - 10 November 1852). Engraved from a painting by Samuel Stepney, later hand colouring. Published in G. Mantell's 'Thoughts on a Pebble' 1849. Gideon Ma...

Onne van der Wal / Loca men leaning on a wooden fishing boat on a pebble beach with a smaller boat moored near shore, Cuba.

Onne van der Wal / A fisherman in swimming trunks holding up his catch of fish on a pebble beach, Cuba.

Onne van der Wal / Wooden fishing boat on a pebble beach, Cuba.

Robert Thompson / Pebble prominent moth (Notodonta ziczac) resting on tree bark, Annagarriff Wood NNR, Peatlands, County Armagh, Northern Ireland, UK, June

Robert Thompson / Pebble hook-tip moth (Drepana falcataria) resting on tree bark, Nottingham, UK, April

Robert Thompson / Pebble hook-tip moth (Drepana falcataria) resting on tree bark, Lackan Bog, County Down, Northern Ireland, UK, July

Kim Taylor / Migratory locust (Schistocerca / Locusta migratoria) hopper on quartz pebble, captive, Surrey, England

Pete Oxford / White-bellied Shag (Phalacrocorax atriceps albiventer) flying, Pebble Island, Falkland Islands

Premaphotos / Red-shanked grasshopper (Xanthippus corallipes) nymph mimicking a pebble in desert, Arizona

Yva Momatiuk & John Eastcott / Grizzly Bear (Ursus arctos horribilis) 2 year old blonde male playing with small stone he rolls with his nose along his raised front paw, Katmai National Park, Alaska