DUNN, CW*; WAGNER, GP; University of Hawaii; Yale University: The evolution and development of colony-level organization in the Siphonophora (Cnidaria, Hydrozoa)

The siphonophores (Cnidaria, Hydrozoa), like other colonial animals, are made up of many multicellular zooids, each of which is homologous to a free-living solitary animal. Each colony arises from a single zygote, but the zooids are produced asexually and remain physiologically integrated. Siphonophore zooids are functionally specialized to a greater degree than are those of any other colonial animal, and are arranged in precise, species-specific patterns. Little is known about the diversity of colony-level organization and development of these �superorganisms�, even though they include the longest animals in the world and are among the most abundant predators in the open ocean. This is because they are extremely fragile and difficult to collect intact. Colony level development has previously been described in detail for only two species. We have described the colony-level organization and development of 6 additional siphonophores that have been strategically sampled across the phylogeny. Intact specimens in pristine condition were collected for 3 of these species by remotely operated underwater vehicles and by SCUBA diving from ships in the open ocean; observations for the remaining taxa were made using preserved material that had been previously collected by manned submersible. These new data indicate that �pro-bud subdivision�, a colony level developmental process whereby all the zooids in a single reiterated sequence arise from a single bud, is a synapomorphy of the Codonophora. The feeding zooids and reproductive elements of the examined Cystonectae arise from independent buds, as in other Hydrozoans. We hypothesize that the origin of pro-bud subdivision established a new organizational framework that allowed for the diversification of colony form and extreme division of labor in the Codonophora.