Biologists estimate that there are approximately 8.8 million species of eukaryotes (animals with complex cell structure) currently alive on Earth. So far, humankind has only cataloged 1.9 million species and entire biomes remain largely unknown to us.
To illustrate this point, here is a photograph of a completely unknown genus of nudibranch mollusk photographed 1 mile beneath the surface of the ocean near Davidson seamount (which is an extinct underwater volcano just off the coast of Monterey). I wish I could tell you more about the strange mollusk, but this photograph, taken from a robotic deep sea submersible in 2002 is pretty much all that humankind knows about this species. The mission photographed a huge number of other gelatinous creatures in the middle depths of the ocean, and in fact caused scientists to rethink the importance of such animals in the oceanic ecosystem. The Monterey Bay Aquarium Research Institute (MBARI) worked on the mission with the National Oceanic and Atmospheric Administration. Their website explains the robotic study by paraphrasing Bruce Robinson, an ecologist who pioneered the use of robot submersibles:
One of the most important discoveries has been the realization that gelatinous animals are important as grazers and predators that comprise a large percentage of the open ocean animal biomass. Robison estimates that gelatinous animals make up about 40 percent of the biomass in the deep sea water column.
Nudibranch mollusks are largely thought of as colorful predators of the tropical reef, so it is a big deal if they (together with other floating mollusks, cnidarians, and siphonophores) constitute such a substantial percentage of the biomass of the largest portion of the ocean. As an unscientific postscript I think the delicate translucent nudibranch is very beautiful with its alien and ghostlike (and, yes, gelatinous) features.