Zooid Explained

A zooid or zoöid is a single 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).^[1] The colonial organism as a whole is called a zoon, plural zoa (from Ancient Greek meaning animal; plural, Greek, Ancient (to 1453);: 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.^[2] However, fossil bryozoans are only known by the colony structures that the zooids formed during life.^[3]

There are correlations between the size of some zooids and temperature.^[4] 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.^{[5] [6]}

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.

See also

• Siphonophorae for colonial Hydrozoa which superficially resemble the other Cnidaria colloquially referred to as "jellyfish"
• Pyrosome for colonial chordates in Tunicata

Notes and References

1. Book: Thorp . James H. . Wood . Timothy S. . Thorp and Covich's Freshwater Invertebrates: Volume 4: Keys to Palaearctic Fauna . 2019 . 978-0-12-385024-9 . 519–529 . 4th . English . Chapter 13 - Phylum Ectoprocta.
2. Book: Fish . J. D. . A Student's Guide to the Seashore . 1989 . 978-94-011-5888-6 . 356–366 . Bryozoa.
3. Book: Prothero, Donald R.. Bringing fossils to life : an introduction to paleobiology. Columbia University Press. 2013. 978-0-231-53690-5. Third. New York. 339–340. 863683957.
4. Amui-Vedel . Ann-Margret . Hayward . Peter J. . Porter . Joanne S. . 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 . 21 December 2007 . 353 . 1 . 1–12 . 10.1016/j.jembe.2007.02.020.
5. Okamura . Beth . O'Deaa . Aaron . Intracolony variation in zooid size in cheilostome bryozoans as a new technique for investigating palaeoseasonality . Palaeogeography, Palaeoclimatology, Palaeoecology . October 2000 . 162 . 3–4 . 319–332 . 10.1016/S0031-0182(00)00136-X. 2000PPP...162..319O .
6. Okamura . Beth . Bishop . John D. D. . Zooid size in cheilostome bryozoans as an indicator of relative palaeotemperature . Palaeogeography, Palaeoclimatology, Palaeoecology . August 1988 . 66 . 3–4 . 145–152 . 10.1016/0031-0182(88)90197-6. 1988PPP....66..145O .