zooid

{{Short description|Single animal that is part of a colonial animal}}

{{Distinguish|zoid (disambiguation)}}

File:Pyrosoma atlanticum.JPG, a tunicate, is a colony of zooids]]

A zooid or zoöid {{IPAc-en|ˈ|z|oʊ|.|ɔɪ|d}} is an animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooids can either be directly connected by tissue (e.g. corals, Catenulida, Siphonophorae, Pyrosome or Ectoprocta) or share a common exoskeleton (e.g. Bryozoa or Pterobranchia).{{cite book |last1=Thorp |first1=James H. |last2=Wood |first2=Timothy S. |title=Thorp and Covich's Freshwater Invertebrates: Volume 4: Keys to Palaearctic Fauna |date=2019 |isbn=978-0-12-385024-9 |pages=519–529 |edition=4th |language=English |chapter=Chapter 13 - Phylum Ectoprocta}} The colonial organism as a whole is called a zoon {{IPAc-en|ˈ|z|oʊ|.|ɒ|n}}, plural zoa (from Ancient Greek {{grc-tr|ζῷον}} {{wikt-lang|grc|ζῷον}} meaning animal; plural {{grc-tr|ζῷα}}, {{lang|grc|ζῷα}}).

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.{{cite book |last1=Fish |first1=J. D. |title=A Student's Guide to the Seashore |date=1989 |isbn=978-94-011-5888-6 |pages=356–366 |chapter=Bryozoa|publisher=Springer }} However, fossil bryozoans are only known by the colony structures that the zooids formed during life.{{Cite book|last=Prothero|first=Donald R.|title=Bringing fossils to life : an introduction to paleobiology|publisher=Columbia University Press|year=2013|isbn=978-0-231-53690-5|edition=Third|location=New York|pages=339–340|oclc=863683957}}

There are correlations between the size of some zooids and temperature.{{cite journal |last1=Amui-Vedel |first1=Ann-Margret |last2=Hayward |first2=Peter J. |last3=Porter |first3=Joanne S. |title=Zooid size and growth rate of the bryozoan Cryptosula pallasiana Moll in relation to temperature, in culture and in its natural environment |journal=Journal of Experimental Marine Biology and Ecology |date=21 December 2007 |volume=353 |issue=1 |pages=1–12 |doi=10.1016/j.jembe.2007.02.020}} 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.{{cite journal |last1=Okamura |first1=Beth |last2=O'Deaa |first2=Aaron |title=Intracolony variation in zooid size in cheilostome bryozoans as a new technique for investigating palaeoseasonality |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=October 2000 |volume=162 |issue=3–4 |pages=319–332 |doi=10.1016/S0031-0182(00)00136-X|bibcode=2000PPP...162..319O }}{{cite journal |last1=Okamura |first1=Beth |last2=Bishop |first2=John D. D. |title=Zooid size in cheilostome bryozoans as an indicator of relative palaeotemperature |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=August 1988 |volume=66 |issue=3–4 |pages=145–152 |doi=10.1016/0031-0182(88)90197-6|bibcode=1988PPP....66..145O }}

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 motile gamete such as a spermatozoon (in the case of algae now zoid), or an independent animal-like organism produced asexually, as by budding or fission.