The growth and degeneration of the hydroid Sertularia perpusilla, an obligate epiphyte of leaves of the seagrass Posidonia oceanica

Abstract

The hydrorhizae of Sertularia perpusilla colonies that originate from stolon attachments initially grow up and down Posidonia leaves. After a short time the distal hydroid tissue degenerates concurrently with the downward growth of the proximal hydrorhiza onto younger sections of the leaves. Consequently the hydroids ‘move’ down the leaves which grow upward from a basal meristem. In situ inversion of leaves had no effect on either the orientation or the rates of growth and degeneration. This indicates that orientation may be in response to age related features of leaf tissue rather than to light, gravity or water movement, the directions of which were reversed by inverting the leaves. However, orientation of growth of new hydroids from attached stolons is immediate, probably before the hydroid colony is long enough to be able to detect an age gradient. This paradox could be explained if initial orientation were based on the direction of gravity or light but subsequent growth were directed along the age gradient. For hydroids in the size range studied there was a relation between growth rate and number of hydranths. Thus growth rate may be limited by food rather than by there being only one growing tip of the hydrorhizae.