Siphonophorae

Siphonophores are colonial hydrozoans that reproduce asexually through a budding process.[6] Zooids are the multicellular units that build the colonies. A single bud called the pro-bud initiates the growth of a colony by undergoing fission.[6] Each zooid is produced to be genetically identical; however, mutations can alter their functions and increase diversity of the zooids within the colony.[6] Siphonophores are unique in that the pro-bud initiates the production of diverse zooids with specific functions.[6] The functions and organizations of the zooids in colonies widely vary among the different species; however, the majority of colonies are bilaterally arranged with dorsal and ventral sides to the stem.[6] The stem is the vertical branch in the center of the colony to which the zooids attach.[6] Zooids typically have special functions, and thus assume specific spatial patterns along the stem.[6]

Siphonophores typically exhibit one of the three standard body plans. The body plans are named Cystonecta, Physonecta, and Calycophorae.[8] Cystonects have a long stem with the attached zooids.[8] Each group of zooids has a gastrozooid.[8] The gastrozooid has a tentacle used for capturing and digesting food.[8] The groups also have gonophores, which are specialized for reproduction.[8] They use a pneumatophore, a gas-filled float, on their anterior end and mainly drift at the surface of the water.[8] Physonects have a pneumatophore and nectosome, which harbors the nectophores used for jet propulsion.[8] The nectophores pump the water back in order to move forward.[8] Calycophorans differ from cystonects and physonects in that they have two nectophores and no pneumatophore.[8]

Zooids

Siphonophores can have zooids that are either polyps or medusae.[9] However, zooids are unique and can develop to have different functions.[9]

Nectophores

Nectophores are medusae that assist in the propulsion and movement of some siphonophores in water.[5] They are characteristic in physonectae and calycophores. The nectophores of these organisms are located in the nectosome where they can coordinate the swimming of colonies.[5] The nectophores have also been observed in working in conjunction with reproductive structures in order to provide propulsion during colony detachment.[5]

Bracts

Bracts are zooids that are unique to the siphonophorae order. They function in protection and maintaining a neutral buoyancy.[5] However, bracts are not present in all species of siphonophore.[5]

Gastrozooids

Gastrozooids are polyps that have evolved a function to assist in the feeding of siphonophores.[10]

Palpons

Palpons are modified gastrozooids that function in digestion by regulating the circulation of gastrovascular fluids.[5]

Gonophores

Gonophores are zooids that are involved in the reproductive processes of the siphonophores.[5]

Pneumatophores

The presence of pneumatophores characterizes the subgroups Cystonectae and Physonectae.[11] They are gas-filled floats that are located at the anterior end of the colonies in these species.[5] They function to help the colonies maintain their orientation in water.[5] In the cystonectae subgroup, the pneumatophores have an additional function of assisting with flotation of the organisms.[5] The siphonophores exhibiting the feature develop the structure in early larval development via invaginations of the flattened planula structure. [5] Further observations of the siphonophore species Nanomia bijuga indicate that the pneumatophore feature potentially also functions to sense pressure changes and regulate chemotaxis in some species.[12]

Siphonophores are predatory carnivores.[4] Their diets consist of a variety of copepods, small crustaceans, and small fish.[4] A majority of siphonophores have gastrozooids that have a characteristic tentacle attached to the base of the zooid.[5] This structural feature functions in assisting the organisms in catching prey. Similar to many other organisms in the phylum of Cnidaria, many siphonophore species exhibit nematocyst stinging capsules on branches of their tentacles called tentilla.[5] The nematocysts are arranged in dense batteries on the side of the tentilla.[5] When the siphonophore encounters potential prey, they utilize their 30–50 cm (12–20 in) tentacles to create a net around the prey.[4] The nematocysts then shoot paralyzing, and sometimes fatal, toxins at the trapped prey which is then transferred to the proper location for digestion.[4]

Due to the lack of food in the deep sea environment, a majority of siphonophore species function in a sit and wait tactic for food.[13] They swim around waiting for their long tentacles to encounter prey. In addition, siphonophores in a group denoted Erenna have the ability to generate red bioluminescence while its tentilla twitches in a way to mimic motions of small crustaceans and copepods.[7] These actions entice the prey to move closer to the siphonophore, allowing it to trap and digest it.[7]

The modes of reproduction for siphonophores vary among the different species, and to this day, several modes remain unknown. Generally, a single zygote begins the formation of a colony of zooids.[14] The fertilized egg matures into a protozooid, which initiates the budding process and creation of a new zooid.[14] This process repeats until a colony of zooids forms around the central stalk.[14] In contrast, several species reproduce using polyps. Polyps can hold eggs and/or sperm and can be released into the water from the posterior end of the siphonophore.[14] The polyps may then be fertilized outside of the organism.[14]

Siphonophores use gonophores to make the reproductive gametes.[10] Gonophores are either male or female; however, the types of gonophores in a colony can vary among species.[10] Species are characterized as monoecious or dioecious based on their gonophores.[10] Monoecious species contain male and female gonophores in a single zooid colony, whereas dioecious species harbor male and female gonophores separately in different colonies of zooids.[10]

Nearly all siphonophores have bioluminescent capabilities. Since these organisms are extremely fragile, they are rarely observed alive.[7] Bioluminescence in siphonophores has been thought to have evolved as a defense mechanism. Siphonophores in the genus Erenna are thought to use their bioluminescent capability as a lure to attract fish.[7] This genus is one of the few to prey on fish rather than crustaceans.[7] The bioluminescent organs on these individuals flicker, and thus it has been concluded that they use bioluminescence to attract prey.[7]

Siphonophores use a method of locomotion similar to jet propulsion. A nectophore is a gathering of many siphonophores, and depending on where each individual siphonophore is positioned within the nectophore, their function differs.[15] Colonial movement is determined by individual siphonophores of all developmental stages. The smaller individuals are concentrated towards the top of the nectophore, and their function is turning and adjusting the orientation of the colony.[15] The smaller the individual, the younger it is. The larger the individual, the older it is. The larger individuals are located at the base of the colony, and their main function is thrust propulsion.[15] These larger individuals are important in attaining the maximum speed of the colony.[15] The colonial organization of siphonophores, particularly in Nanomia bijuga confers evolutionary advantages.[15] A large amount of concentrated individuals allows for redundancy.[15] This means that even if some individual siphonophores become functionally compromised, the colony as a whole is not negatively affected.

Carl Linnaeus discovered and described the first siphonophore, the Portuguese man o' war, in 1758.[8] The discovery rate of siphonophore species was slow in the 18th century, as only four additional species were found.[8] During the 19th century, 56 new species were observed due to research voyages conducted by European powers.[8] The majority of new species found during this time period were collected in coastal, surface waters.[8] During the HMS Challenger expedition, various species of siphonophores were collected. Ernst Haeckel attempted to conduct a write up of all of the species of siphonophores collected on this expedition. He introduced 46 "new species"; however, his work was heavily critiqued because some of the species that he identified were eventually found not to be siphonophores.[8] Nonetheless, some of his descriptions and figures (pictured below) are considered useful by modern biologists. A rate of about 10 new species discoveries per decade was observed during the 20th century.[8] Considered the most important researcher of siphonophores, A. K. Totton introduced 23 new species of siphonophores during the mid 20th century.[8]

On April 6, 2020 the Schmidt Ocean Institute announced the discovery of a giant Apolemia siphonophore in submarine canyons near Ningaloo Coast, measuring 15 m (49 ft) diameter with a ring approximately 47 m (154 ft) long, possibly the largest siphonophore ever recorded.[17][18]

Ernst Haeckel described a number of siphonophores, and several plates from his Kunstformen der Natur (1904) depict members of the taxon:[19]

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  Plate 7
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  Plate 37
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  Plate 59
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  Plate 77