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Polymorphism in hydroids: the extensible polyp of Halecium halecinum (Cnidaria: Hydrozoa: Haleciidae)

Published online by Cambridge University Press:  18 December 2008

Nicole Gravier-Bonnet
Nicole Gravier-Bonnet*
Affiliation:
Laboratoire d'Ecologie Marine, Faculté des Sciences et Technologies, Université de La Réunion, 15 Avenue René Cassin-BP 7151, 97715 Saint-Denis Messag Cedex 9-La Réunion, France
*
Correspondence should be addressed to: Nicole Gravier-Bonnet, Laboratoire d'Ecologie Marine, Faculté des Sciences et Technologies, Université de La Réunion, 15 Avenue René Cassin-BP 7151, Saint-Denis Messag Cedex 9, La Réunion, France email: [email protected]
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Abstract

The study of living Halecium halecinum colonies revealed a new case of zooid polymorphism. Besides the ordinary hydranth, the polyp devoted to feeding, this species is provided with a second kind of polyp, differing only slightly in structure and morphology but most conspicuously in its behaviour. It is named ‘extensible polyp’ in reference to its great extensibility and the resulting filiform shape. There are slight differences in the tentacles: lower number, shorter length, thicker diameter, and the tip slightly swollen and rounded instead of tapering. Their large microbasic mastigophores are abundant and evenly distributed, while the hydranth has a few large ones only on the oral side but has otherwise numerous small ones. When extended and at rest, the tubular column is much longer than that of the hydranth and not delimited from the head of the polyp by a bulge followed by a constriction. Behavioural differences are its capacity to coil and bend during extension and thus being able to move in all directions and exploring a large volume of seawater, and also its ability to produce regional swellings (peristalsis) and to contract by folding and bulging though still extended. Besides a probable role in defence, the extensible polyp exhibits an excretory function and it could also have sensory functions. The extensible polyp type is not classified as a nematophore because it has a functional gastrovascular cavity and a mouth. Polyp dimorphism (hydranth/extensible polyp) is reported in one more halecid and two sertularids.

Keywords

polymorphismhydroidsextensible polypHalecium halecinum
Type
Research Article
Information
Journal of the Marine Biological Association of the United Kingdom , Volume 88 , Special Issue 8: Hydrozoans , December 2008 , pp. 1731 - 1736
Copyright
Copyright © Marine Biological Association of the United Kingdom 2008

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