gastrozooids

Gastrozooids are specialized feeding zooids found in colonial hydrozoans. In many hydrozoan species, the colony consists of different types of zooids that perform distinct tasks, and gastrozooids are the feeding polyps responsible for capturing prey and supplying nutrients to the colony.

Morphology and function: A gastrozooid is typically polyp-shaped, bearing a mouth at one end and a crown of tentacles around or near the mouth. The tentacles carry cnidocytes for prey capture, and the gastrozooid contains the gastrovascular cavity through which ingested food is distributed to the rest of the colony, coordinating digestion among connected zooids. Unlike gonozooids, gastrozooids usually lack reproductive organs. They are connected to other zooids by a shared set of canals and tissues within the colonial skeleton, enabling nutrient sharing.

Ecology and variation: Gastrozooids coexist with other zooid types in many colonies, such as gonozooids (involved in reproduction) and dactylozooids (defense and prey capture). The presence and arrangement of gastrozooids can vary among species and developmental stages, reflecting the ecological needs of the colony. In some well-known hydrozoan genera, such as Obelia, the distinction between feeding and reproductive zooids is clear and functionally important for colony efficiency.

Development: Gastrozooids arise during colony growth as buds differentiating from the hydroid stem or from existing polyps, guided by genetic and environmental cues. Their formation allows the colony to allocate specialized roles to different parts of the organism, enhancing feeding efficiency and resource distribution across the zooid network.