Crab

Crabs were a type of crustacean found on several planets, including Earth. These crabs sometimes grew or evolved into giant crabs, such as those that lived on the island Salutua. (PROSE: The Eye of the Giant)

Of Earth-origin
On Earth, crabs were a food source. Fishermen such as Charlie Johns fished for them along the coastline such as the Cornish coast. (PROSE: Rip Tide)

During a visit to Keelmouth in the 20th century, a small orange crab nipped at the First Doctor's big toe. (PROSE: Bide-a-Wee)

Crabs were listed in the Avalonian Bestiary of all animal life on the Earth colony Avalon. (PROSE: The Sorcerer's Apprentice)

In the 40th century, crabs were among the sea creatures mutated to giant size by an unexploded H-bomb lost off the coast of Spain two thousand years earlier. Pete Nesbit and Hal of the Marine Rescue Brigade discovered a crab that was at least twenty feet in diameter. (COMIC: The Sea Monsters)

Elsewhere
On one planet, crabs were, as the Sixth Doctor and Peri Brown discovered, genetically-engineered to become more intelligent. (PROSE: Shell Shock)

Giant hermit crabs existed on a planet the Fourth Doctor and Romana II once visited. (PROSE: The Little Things)

On at least two different planets the Doctor encountered violent crabs. The First Doctor, John and Gillian encountered a large crab in a sea on a desolate planet. It attacked John, but the Doctor was able to drive it off with his electroliser. (COMIC: Shark Bait) On the planet Praxos 9 the Tenth Doctor and Rose Tyler encountered and had to fight off luminous sucker crabs. (COMIC: Blood and Tears)

In the Divergent Universe, the Eighth Doctor, Charley Pollard and C'rizz encountered giant crabs on an island. (AUDIO: The Next Life)