zo·o·xan·thel·la (z???-z?n-th?l??)n. pl. zo·o·xan·thel·lae (-th?l??) Any of various yellow-brown photosynthetic dinoflagellates that live symbiotically within the cells of other organisms, especially certain corals and other marine invertebrates.[New Latin : zoo- + xanth(o)- + -ella, diminutive suff.]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.zooxanthella (?z???zæn???l?) n (Biology) any of several yellow-green algae that inhabit other organisms such as marine invertebratesCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014