zo·o·chlor·el·la (z???-kl?-r?l??)n. pl. zo·o·chlor·el·lae (-kl?-r?l??) Any of various one-celled green algae that live symbiotically within the cells of aquatic invertebrate organisms such as sponges and sea anemones or within certain protists.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.