[Solved] A primary disadvantage of mail surveys is no control over who fills out the questionnaire. Tropical Waters: Warm ocean water found near the equator, characterized by high temperatures and supporting diverse marine life. Zooxanthellae: Photosynthetic algae found in close symbiotic relationships with marine invertebrates such as corals and sea anemones. Microscopic Algae: Tiny, often unicellular algae, not visible to the naked eye, that live in water and can perform photosynthesis. Phytoplankton: Microscopic floating algae and cyanobacteria that are the base of most aquatic food webs. Compare with zooplankton. See plankton and nanoplankton.