Tagasi otsingusse
Taylor, 2020j

Zooid Morphology and Function

Taylor, P. D.
DOI
DOI10.1002/9781118454961.ch3
Aasta2020
KirjastusWiley
Kuulub kogumikkuTaylor, 2020
Leheküljed41-90
Tüüppeatükk raamatus
Keelinglise
Id24358

Abstrakt

Zooids are the fundamental modules constituting bryozoan colonies. They are homologous with the individuals of unitary organisms (e.g. Mackie 1986, p. 177), such as the closely related brachiopods and molluscs. In the most primitive bryozoans, the zooids within a colony are clearly distinct, identical or nearly so, and capable of sustaining themselves in all life functions (feeding, respiration, reproduction, defence etc.). At the other end of the spectrum are species in which the physical boundaries between the zooids are vague, different zooids in the same colony differ in appearance and function, and some of the zooids are incapable of surviving independently. Functional differences between zooids are reflected in differences in their morphologies, resulting in polymorphism. However, not all variations in zooid morphology within colonies reflect polymorphism: some are due to differences in zooid ages (ontogeny), the age of the colony at the time the zooid was budded (astogeny), or small‐scale, microenvironmental variations (Boardman and Cheetham 1973). Primitively, each zooid in a bryozoan colony is capable of undertaking all of the life functions required of a suspension‐feeding animal: in addition to acquiring food and disposing of solid waste (faeces), these include respiration, excretion, defence, and reproduction, both as a male and as a female. These functions, and the organs associated with them, were inherited from the solitary (non‐colonial) ancestor of the colonial bryozoans. Through evolutionary time, however, bryozoans have partitioned life functions between the constituent zooids in the colony, resulting in colonies that contain zooids of differing morphologies, i.e. polymorphism. Zooid polymorphs retaining the capability to feed are called autozooids, while non‐feeding polymorphs are termed heterozooids. There may also be polymorphism within autozooids according to their ability to perform other functions; for instance, some autozooids in cheilostome bryozoans not only feed but also perform a female reproductive function, producing eggs and brooding the embryos that result from fertilization, whereas other autozooids in the same colony feed but do not reproduce as females. A wide range of heterozooids can be distinguished in recent bryozoans. Their functions are not always well established and even in some living bryozoans they are unknown as, for example, the heterozooids of the cyclostome Jullienipora with striated exterior walls (Reverter‐Gil and Fernández‐Pulpeiro 2005). In fossil bryozoans, the functions of polymorphic zooids can often be inferred by comparison with living species, but for some extinct groups their functions remain uncertain in the absence of soft part preservation.

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