siphosome

The siphosome is the central axis of a siphonophore colony, a colonial hydrozoan within the order Siphonophorae. The siphosome bears most of the feeding, digestive, and reproductive zooids of the colony and forms the stalk-like portion behind the gas-filled float, the pneumatophore. Along the siphosome are specialized zooids, including gastrozooids (feeding and digestion), palpons (sensory and digestive roles), nectophores (swimming bells that propel the colony), and gonophores (reproductive units).

All zooids on the siphosome are physiologically integrated, sharing a common gastrovascular system and functioning together as a single organism despite being composed of many individual units. The colony grows by budding along the siphosome, producing new gastrozooids, nectophores, gonophores, and other specialized forms in a species-specific arrangement. The siphosome’s length and organization influence the colony’s feeding strategy and locomotion; propulsion is primarily provided by the nectophores along the siphosome, while the pneumatophore provides buoyancy.

In distinguishing anatomical regions, the siphosome is separated from the pneumatophore (the float) and from nectophores, which may be distributed along the siphosome to drive movement. Siphonophores exhibit a high degree of division of labor among their zooids, making the siphosome a central component in the study of colonial life and modular organization within cnidarians.