Here is the number of crustaceans on the planet:
|Copepods||Crustacean||1 x 10^18||2017||Weborus estimation|
|Shellfishes||Crustacean||1,9 x 10^16||2017||Weborus estimation|
|Sea urchins||Crustacean||9 x 10^14||2017||Weborus estimation|
|Antarctic krills||Crustacean||5 x 10^14||2011||NPR|
|Crabs||Crustacean||1,25 x 10^14||2006||FAO|
|Spider crabs||Crustacean||100,000,000||2017||Weborus estimation|
|Horseshoe crabs||Crustacean||7,000||2011||Schaller and Dorsey|
The crab breathes via gills. It keeps water in it when it emerges so that it can continue to breathe. The female can lay thousands of eggs (up to 3 million depending on the species) that are attached to its belly. A crab lives between 8 and 15 years depending on the conditions of its environment. There are over 4,000 crab species in the world. The crab is omnivorous, it can even eat congeners.
There is at least 1 quintillion (billion of billion) of crustaceans on Earth.