Final answer:
Organisms attached to the seafloor and unable to move are known as Benthos, which includes sponges, clams, and certain fish such as the anglerfish.
Step-by-step explanation:
The organisms that are attached to the seafloor and cannot move are classified as Benthos. Benthic organisms include various forms of life such as sponges, clams, and certain types of fish like the anglerfish. They often crawl in the sediments at the bottom of aquatic environments, and many serve as decomposers within their ecosystems. Unlike benthos, plankton are tiny, mostly microscopic organisms that drift in the water column and include both phytoplankton and zooplankton. Nekton are able to move independently through the water and include organisms such as fish and shrimp. It is essential to note that while plankton, including phytoplankton, are unable to swim against currents, they are not necessarily attached to the seafloor like benthos.