What you see in the video is not one large animal, but a colony of individual animals. Each animal of the colony is specialized to do certain functions so that the colony acts like an entire body. So, they may shed light on evolution of complex multicellular body plans. More from wiki -
Siphonophores are of special scientific interest because they are composed of medusoid and polypoid zooids that are morphologically and functionally specialized. Each zooid is an individual, but their integration with each other is so strong, the colony attains the character of one large organism. Indeed, most of the zooids are so specialized, they lack the ability to survive on their own. Siphonophorae thus exist at the boundary between colonial and complex multicellular organisms. Also, because multicellular organisms have cells which, like zooids, are specialized and interdependent, siphonophores may provide clues regarding their evolution.
Like other hydrozoans, certain siphonophores can emit light. A siphonophore of the genus Erenna has been discovered at a depth of around 1,600 m (5,200 ft) off the coast of Monterey, California. The individuals from these colonies are strung together like a feather boa. They prey on small animals using stinging cells. Among the stinging cells are stalks with red glowing ends. The tips twitch back and forth, creating a twinkling effect. Twinkling red lights are thought to attract the small fish eaten by these siphonophores. While many sea animals produce blue and green bioluminescence, this siphonophore was only the second lifeform found to produce a red light (the first being the scaleless dragonfish Chirostomias pliopterus).
There is not much molecular data available on them, and we found only one researcher at Brown university (Casey Dunn) actively looking into gene expression of siphonophores.
Siphonophores belong to the Cnidaria, a group of animals that includes the corals, hydroids, and true jellyfish. There are about 175 described species. Some siphonophores are the longest animals in the world, and specimens as long as 40 meters have been found. The majority of siphonophores are long and thin, consisting mostly of a clear gelatinous material. Some deep water species have dark orange or red digestive systems that can be seen inside their transparent tissues. Siphonophores are exceedingly fragile and break into many pieces under even the slightest forces. Many siphonophores are bioluminescent, glowing green or blue when disturbed. All siphonophores are predators, and use their many tentacles to capture crustaceans and small fish.
While one species of siphonophore lives at the surface of the ocean (the familiar Portuguese Man O’ War, Physalia physalis), and members of another group (the Rhodaliids) tethered themselves to the bottom with their tentacles, the vast majority of siphonophores are active swimmers and live in the water column of the open ocean. A few hardy species are sometimes found near the shore, but these are the exception.
Here is the abstract of his PhD thesis, which gives you a perspective about how much (or how little) was known about these species in 2005. All that will change, thanks to cheap sequencing.
The colony-level evolution and development of the Siphonophora (Cnidaria, Hydrozoa)
Animal colonies are made up of multicellular zooids. Each zooid is homologous to a solitary animal, but all of the zooids of a given colony remain attached to each other, are physiologically integrated, and arise from a single zygote. The siphonophores, a group of about 170 species of pelagic colonial hydrozoans (Cnidaria), are the most complex colonial animals in that they have a high degree of functional specialization between zooids and the zooids are arranged in precise, species specific patterns. Siphonophores are extremely fragile, however, and most live only in the open. As a result, their colony-level organization and development have remained poorly understood.
I have collected intact siphonophore specimens from oceanic research vessels using remotely operated underwater vehicles and blue water SCUBA diving. Phylogenetic analyses of 16S and 18S ribosomal sequence data indicate that the
Cystonectae are monophyletic and sister to a group that includes the Physonectae and Calycophorae. The Calycophorae are monophyletic and nested within the Physonectae, which are paraphyletic. The monophyletic group that includes both the Physonectae and the Calycophorae is here named the Codonophora. Parsimony reconstructions indicate that functionally specialized zooids have been gained and lost across the siphonophore
phylogeny, and maximum likelihood and Bayesian analyses indicate that the transition rate for gaining zooid types is not greater than that for losing zooid types. New observations on the colony-level organization of three species of longstemmed. Cystonectae verify that gonodendra and gastrozooids arise as separate buds in these taxa, and suggest that their siphosomal zooids are not organized into cormidia. New observations on three species of Physonectae, in conjunction with previous data on two Calycophorae, indicate that each reiterated sequence of zooids in the siphosome of the Codonophora arises as a single pro-bud that later subdivides into multiple zooids. The phylogenetic relationship of these taxa and the morphology of the outgroup taxa indicate that pro-bud subdivision is a synapomorphy of the Codonophora.
Three new species of siphonophores are described, all of which are so fragile that their existence was entirely unknown until they were observed with submersibles.