Final answer:
A very large protist in a jar of ocean water is likely to be kelp, which is an enormous, multicellular protist part of the Chromalveolata group, distinguishing them from the typically microscopic diatoms and dinoflagellates or various-sized amoebas.
Step-by-step explanation:
If you see a very large protist in a jar of ocean water, you could be looking at kelp. Kelps are enormous, multicellular protists in the group Chromalveolata that can extend for hundreds of feet in underwater "forests." Diatoms and dinoflagellates are generally microscopic, while amoebas, though they can vary in size, are typically not as large as kelps and are not multicellular.
Dinoflagellates vary in size and shape, but many are described as spherical with 'horns' encased in a complex cellulose armor, distinct for their characteristic 'whirling' motion due to their two flagella. Diatoms, on the other hand, are unicellular photosynthetic protists that have intricately patterned, glassy cell walls.