nectophores

Nectophores, commonly known as swimming bells, are a specialized type of zooid found in certain siphonophores, a colonial group within Hydrozoa. They function as the primary propulsion units of the colony, enabling locomotion through the coordinated contraction of their muscular bell to eject water and generate thrust. In a siphonophore colony, the nectophores typically occur along a distinct region called the nectosome, which is separate from feeding and reproductive zooids.

Structurally, a nectophore resembles a small bell and is equipped with muscular layers and a central canal system. When contracted, the bell expels water through exhalant openings, and elastic tissues plus retractor muscles restore the resting shape, allowing a cycle of jet propulsion. The contractions can be rhythmic and, in some species, synchronized across nectophores to produce forward movement.

Functional role: nectophores provide locomotion for the entire colony, enabling ascent, descent, and forward progression, as well as rapid repositioning to capture prey, avoid predators, or respond to currents. They are not involved in feeding or reproduction; those tasks are performed by gastrozooids and gonophores, respectively.

Ecology and diversity: many siphonophore colonies bear multiple nectophores, each functioning as a propulsion module. The arrangement and number vary among taxa, reflecting different locomotor needs and life histories. Taxonomically, nectophores are a defining feature of siphonophores and illustrate the modular, highly integrated organization of these colonies, where distinct zooids specialize for feeding, reproduction, or locomotion.