hydrotheca

Hydrotheca is a cup-like protective enclosure that surrounds the feeding polyp, or hydranth, in many colonial hydrozoans. It is part of the polyp’s thecal envelope and is typically formed by the epidermis of the colony. The hydrotheca is usually transparent or pale and may be chitinous or membranous in composition.

It takes a cup- or urn-shaped form, often with a single opening at the apex called the hydrothecal aperture, through which the hydranth’s mouth and the bases of its tentacles protrude during feeding. The shape, size, and ornamentation of hydrothecae vary among taxa and are commonly used in identifying species.

Hydrothecae arise as feeding polyps bud from the hydrorhizal stalk, and the hydranth sits inside the hydrotheca while feeding; when not feeding it remains retracted. The hydrotheca provides mechanical protection against abrasion and predation and helps define the colony’s polyp mosaic.

Hydrothecae are characteristic of many colonial hydrozoans, particularly within groups that bear distinct feeding zooids. The presence, absence, or specific morphology of hydrothecae is used as a diagnostic character in hydrozoan taxonomy. In some lineages, hydrothecae may be reduced or completely absent, and polyps are exposed or enclosed by other coverings.

Overall, hydrothecae reflect the morphological diversity of feeding structures in hydrozoan colonies and are a key feature in the description and classification of many species.