Zooids can exhibit polymorphism. For instance, extant bryozoans may have zooids adapted for different functions, such as feeding, anchoring the colony to the substratum and for brooding embryos. However, fossil bryozoans are only known by the colony structures that the zooids formed during life.
There are correlations between the size of some zooids and temperature. Variations in zooid size within colonies of fossils can be used as an indicator of the temperature and the seasonality of seas in the geological past.
The term zooid has historically also been used for an organic cell or organized body that has independent movement within a living organism, especially a motilegamete such as a spermatozoon (in the case of algae now zoid), or an independent animal-like organism produced asexually, as by budding or fission.
^Prothero, Donald R. (2013). Bringing fossils to life : an introduction to paleobiology (Third ed.). New York: Columbia University Press. pp. 339–340. ISBN978-0-231-53690-5. OCLC863683957.
^Amui-Vedel, Ann-Margret; Hayward, Peter J.; Porter, Joanne S. (21 December 2007). "Zooid size and growth rate of the bryozoan Cryptosula pallasiana Moll in relation to temperature, in culture and in its natural environment". Journal of Experimental Marine Biology and Ecology. 353 (1): 1–12. doi:10.1016/j.jembe.2007.02.020.
^Okamura, Beth; O'Deaa, Aaron (October 2000). "Intracolony variation in zooid size in cheilostome bryozoans as a new technique for investigating palaeoseasonality". Palaeogeography, Palaeoclimatology, Palaeoecology. 162 (3–4): 319–332. Bibcode:2000PPP...162..319O. doi:10.1016/S0031-0182(00)00136-X.