Morphological adaptations of the perisarc of the intertidal hydroid Dynamena pumila to reduce damage and enhance feeding efficiency
R.G. Hughes

The structure of the perisarc of Dynamena pumila was examined by light and electron (SEM and TEM) microscopy. The operculum of the hydrotheca consists of two valves of thin perisarc which are bent at their proximal ends, like a spring. This bending may place the valves under tension when the hydranth is extended and cause the operculum to shut as the hydranth withdraws into the hydrotheca. Distally the opercular valves are convex (from the inside) which may prevent the tentacles of a retracting hydranth being "pinched" by the closing valves. On the hydrocaulus (stem) between each adjacent pair of hydranths, there is an annulation of thin perisarc, mostly composed of flexible protein lamellae. The annulations allow the hydrocaulus to bend without damage. Close to the base of the hydrocaulus is an articulating joint which allows the upper part of the hydrocaulus to rotate up to 180' in either direction. The advantage of this is that the hydranths always face downcurrent where they may feed in attached vortices. In strong currents the hydrocaulus also bends at the articulating joint to lie on or near the substratum. Thus the forces which might damage the hydrocaulus and tear the hydrorhiza (stolon) from the substratum are ameliorated by the bending of the hydrocaulus at the annulations, and bending of the articulating joint, both of which lessen the forces transmitted to the hydrorhiza.

Keywords: Dynamena pumila, hydroid, perisarc, operculum, annulations, articulating joint, drag forces.
Contents of this volume Sci. Mar. 56(2-3) : 269-277 Back PDF
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