Organisms in the suborder Physonectae follow the classic Siphonophore body plan. They are almost all pelagic, and are composed of a colony of specialized zooids that originate from the same fertilized egg. While species of Physonectae are not generally well-known by the public, a famous example of the order Siphonophorae is the Portuguese man o' war, well-known for its painful sting.
The majority of physonect siphonophores are pelagic, with the exception of Rhodallidae, which are a family of benthic physonects first collected during the Challenger expedition and described by Ernst Haekel in his Challenger monograph.[4]
Physonects, and siphonophores in general, are known to be widely distributed globally, but are understudied. Few individuals have been collected and are often misidentified. As a result, their exact global distributions are unclear.[5]
All physonect siphonophores have an aboral, apical pneumatophore filled with gas, which is formed by invagination of the superficial cell layers of the apical side of the larva.[4] In some species in the suborder Physonectae, the pneumatophore has a pore located either on the apical or basal pole of the organism and plays a role in controlling gas volume and pressure for buoyancy control.[4] Gas in the pneumatophore is high in carbon monoxide and is secreted by the pneumadenia, or gas gland.[4]
Below the pneumatophore, an organism has a long stem with two distinct regions. The nectosome is more apical relative to the siphosome and is composed of asexual nectophores, or swimming bells.[4] These swimming bells are the animal's zooids specialized for swimming. The siphophore includes the gastrozooids (feeding polyps) and sexual medusoids.[4]
Each physonect colony is composed of individual organisms originating from the same fertilized egg, with specialized functions including locomotion, feeding, and reproduction.[6] The ventral budding zone of the nectophores in the animal lies immediately below pneumatophore. As new buds are produced asexually, the swimming bells are displaced downwards.[4]