A fathom is a unit of measure equal to six feet.
One fathom is equal to 6 feet, which is commonly used as a unit of measurement in nautical contexts to describe the depth of water. It can also be converted to approximately 1.83 meters. The term originates from the Old English word "fæðm," meaning to embrace or encircle, reflecting its historical use in measuring the span of a person's outstretched arms.
I think you are talking about tiny animals without skeletons. If so, these are a/the critical component to the entire food chain. For example we are talking about the ocean's tiniest creatures such as plankton that in massive abundance feed to many species of sealife to even fatham. The greatest mammals on earth - whales - depend on this tiny creature by the ton - filtering them from the ocean waters as a rich source of nutritition.