From Australia: A Link Between Salps and Climate Change?
The jellyfish-like animals are known as salps and their main food is phytoplankton (marine algae) which absorbs the greenhouse gas carbon dioxide in the top level of the ocean. This in turn comes from the atmosphere. ...
By eating the algae, the salps turn the algae and their carbon dioxide into faeces which drops to the ocean floor. They also take carbon to the floor with them when they die after a life cycle as short as only a couple of weeks.
This is thought to be a natural form of carbon sequestration similar to what scientists are trying to do with carbon capture from emission sources such as power stations.
This story also offers an explanation about why they can be so numerous:
"They are interesting because they are the fastest reproducing multi-celled animal on the planet and can double their numbers several times a day."