Because the oceans are so vast—taking up around 70 percent of the Earth’s “surface”— the number of marine animals is not truly known. What is known is that the numbers are immense; for example, of living species, to date, about 800 species of cephalopods (including squids and octopi), around 28,000 species of fish, and estimates of 50,000 to 120,000 species of mollusks exist.