Siphonophorae

Siphonophorae
(A) Rhizophysa eysenhardtii scale bar = 1 cm, (B) Bathyphysa conifera 2 cm, (C) Hippopodius hippopus 5 mm, (D) Kephyes hiulcus 2 mm (E) Desmophyes haematogaster 5 mm (F) Sphaeronectes christiansonae 2 mm, (G) Praya dubia 40 m (130 ft), (H) Apolemia sp. 1 cm, (I) Lychnagalma utricularia 1 cm, (J) Nanomia sp. 1 cm, (K) Physophora hydrostatica 5 mm
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Hydrozoa
Subclass:	Hydroidolina
Order:	Siphonophorae Eschscholtz, 1829
Suborders[1]
Calycophorae Cystonectae Physonectae
Synonyms
Siphonophora Eschscholtz, 1829

Siphonophorae (from Greek siphōn 'tube' + pherein 'to bear'^[2]) is an order within Hydrozoa, which is a class of marine organisms within the phylum Cnidaria. According to the World Register of Marine Species, the order contains 175 species described thus far.^[3]

Siphonophores are highly polymorphic and complex organisms.

Siphonophores are colonial hydrozoans that do not exhibit alternation of generations but instead reproduce asexually through a budding process.^[10] 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.^[7] Each zooid is produced to be genetically identical; however, mutations can alter their functions and increase diversity of the zooids within the colony.^[7] Siphonophores are unique in that the pro-bud initiates the production of diverse zooids with specific functions.^[7] 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.^[7] The stem is the vertical branch in the center of the colony to which the zooids attach.^[7] Zooids typically have special functions, and thus assume specific spatial patterns along the stem.^[7]

General morphology

Siphonophores typically exhibit one of three standard body plans matching the suborders: Cystonectae, Physonectae, and Calycophorae.^[11] Cystonects have a long stem with the attached zooids.^[11] Each group of zooids has a gastrozooid.^[11] The gastrozooid has a tentacle used for capturing and digesting food.^[11] The groups also have gonophores, which are specialized for reproduction.^[11] They use a pneumatophore, a gas-filled float, on their anterior end and drift at the surface of the water or stay afloat in the deep sea.^[11] Physonects have a pneumatophore and nectosome, which harbors the nectophores used for jet propulsion.^[11] The nectophores pump water backwards in order to move forward.^[11] Calycophorans differ from cystonects and physonects in that they have two nectophores and no pneumatophore.^[11] Instead they often possess oil-filled glands which likely help with buoyancy.^[12]

Siphonophores posssess multiple types of zooid.^[13] Scientists have determined two possible evolutionary hypothesis for this observation: 1. As time has gone on, the amount of zooid types has increased.^[13] 2. The last common ancestor had many types of zooids and the diversity seen today is due to loss of zooid types.^[13] Research shows no evidence supporting the first hypothesis, and has seen some evidence in support of the second.^[13]

Zooids

Siphonophores can have zooids that are either polyps or medusae.^[14] However, zooids are unique and can develop to have different functions.^[14]

Nectophores

Nectophores are medusae that assist in the propulsion and movement of some siphonophores in water.^[6] 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.^[6] The nectophores have also been observed in working in conjunction with reproductive structures in order to provide propulsion during colony detachment.^[6]

Bracts

Bracts are zooids that are unique to the siphonophorae order. They function in protection and maintaining a neutral buoyancy.^[6] However, bracts are not present in all species of siphonophore.^[6]

Gastrozooids

Gastrozooids are polyps that have evolved a function to assist in the feeding of siphonophores.^[13]

Palpons

Palpons are modified gastrozooids that function in digestion by regulating the circulation of gastrovascular fluids.^[6]

Gonophores

Gonophores are zooids that are involved in the reproductive processes of the siphonophores.^[6]

Pneumatophores

The presence of pneumatophores characterizes the subgroups Cystonectae and Physonectae.^[15] They are gas-filled floats that are located at the anterior end of the colonies in these species.^[6] They function to help the colonies maintain their orientation in water.^[6] In the cystonectae subgroup, the pneumatophores have an additional function of assisting with flotation of the organisms.^[6] The siphonophores exhibiting the feature develop the structure in early larval development via invaginations of the flattened planula structure.^[6] 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.^[16]

Distribution and habitat

Currently, the World Register of Marine Species (WoRMS) identifies 175 species of siphonophores.

Siphonophores use a method of locomotion similar to jet propulsion. A siphonophore is a complex aggregate colony made up of many nectophores, which are clonal individuals that form by budding and are genetically identical.^[17] Depending on where each individual nectophore is positioned within the siphonophore, their function differs.^[17] Colonial movement is determined by individual nectophores of all developmental stages. The smaller individuals are concentrated towards the top of the siphonophore, and their function is turning and adjusting the orientation of the colony.^[17] Individuals will get larger the older they are. The larger individuals are located at the base of the colony, and their main function is thrust propulsion.^[17] These larger individuals are important in attaining the maximum speed of the colony.^[17] Every individual is key to the movement of the aggregate colony, and understanding their organization may allow us to make advances in our own multi-jet propulsion vehicles.^[17] The colonial organization of siphonophores, particularly in Nanomia bijuga confers evolutionary advantages.^[17] A large number of concentrated individuals allows for redundancy.^[17] This means that even if some individual nectophores become functionally compromised, their role is bypassed so the colony as a whole is not negatively affected.^[17] The velum, a thin band of tissue surrounding the opening of the jet, also plays a role in swimming patterns, shown specifically through research done on the previous mentioned species N. bijuga.^[18] The velum becomes smaller and more circular during times of forward propulsion compared to a large velum that is seen during refill periods.^[18] Additionally, the position of the velum changes with swimming behaviors; the velum is curved downward in times of jetting, but during refill, the velum is moved back into the nectophore.^[18] The siphonophore Namonia bijuga also practices diel vertical migration, as it remains in the deep-sea during the day but rises during the night.^[17]

Predation and feeding

Siphonophores are predatory

There are four types of nematocysts in siphonophore tentilla: heteronemes, haplonemes, desmonemes, and rhopalonemes.[21] Heteronemes are the largest nematocysts and are spines on a shaft close to tubules attached to the center of the siphonophore.^[21] Haplonemes have open-tipped tubules with spines, but no distinct shaft.^[21] This is the most common nematocyst among siphonophores.^[21] Desmonemes do not have spines but instead there are adhesive properties on the tubules to hold onto prey.^[21] Rhopalonemes are nematocysts with wide tubules for prey.^[21]

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.^[22] The gelatinous body plan allows for flexibility when catching prey, but the gelatinous adaptations are based on habitat.^[23] They swim around waiting for their long tentacles to encounter prey. In addition, siphonophores in a group denoted Erenna have the ability to generate bioluminescence and red fluorescence while its tentilla twitches in a way to mimic motions of small crustaceans and copepods.^[8] These actions entice the prey to move closer to the siphonophore, allowing it to trap and digest it.^[8]

Reproduction

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.^[5] The fertilized egg matures into a protozooid, which initiates the budding process and creation of a new zooid.^[5] This process repeats until a colony of zooids forms around the central stalk.^[5] 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.^[5] The polyps may then be fertilized outside of the organism.^[5]

Siphonophores use

Bioluminescent lures are found in many different species of siphonophores, and are used for a variety of reasons. Species such as Agalma okeni, Athorybia rosacea, Athorybia lucida, and Lychnafalma utricularia use their lures as a mimicry device to attract prey.^[11] A. rosacea mimic fish larvae, A. lucida are thought to mimic larvacean houses, and L. utricularia mimic hydromedusa.^[11] The species Resomia ornicephala uses their green and blue fluorescing tentilla to attract krill, helping them to outcompete other organisms that are hunting for the same prey.^[11] Siphonophores from the genus Erenna use bioluminescent lures surrounded by red fluorescence to attract prey and possibly mimic a fish from the Cyclothone genus.^[11] Their prey is lured in through a unique flicking behavior associated with the tentilla.^[8] When young, the tentilla of organisms in the Erenna genus contain only bioluminescent tissue, but, as the organism ages, red fluorescent material is also present in these tissues.^[8]

Taxonomy

Organisms in the order of Siphonophorae have been classified into the phylum Cnidaria and the class Hydrozoa.^[3] The phylogenetic relationships of siphonophores have been of great interest due to the high variability of the organization of their polyp colonies and medusae.^[24][13] Once believed to be a highly distinct group, larval similarities and morphological features have led researchers to believe that siphonophores had evolved from simpler colonial hydrozoans similar to those in the orders Anthoathecata and Leptothecata.^[15] Consequently, they are now united with these in the subclass Hydroidolina.

Early analysis divided siphonophores into three main subgroups based on the presence or the absence of two different traits: swimming bells (nectophores) and floats (pneumatophores).^[15] The subgroups consisted of Cystonectae, Physonectae, and Calycorphores. Cystonectae had pneumatophores, Calycophores had nectophores, and Physonectae had both.^[15]

Eukaryotic nuclear small subunit ribosomal gene 18S, eukaryotic mitochondrial large subunit ribosomal gene 16S, and transcriptome analyses further support the phylogenetic division of Siphonophorae into two main clades: Cystonectae and Codonophora. Suborders within Codonophora include Physonectae (consisting of the clades Calycophorae and Euphysonectae), Pyrostephidae, and Apolemiidae.^[6][13]

• Suborder Calycophorae
  • Abylidae Agassiz, 1862
  • Clausophyidae Totton, 1965
  • Diphyidae Quoy & Gaimard, 1827
  • Hippopodiidae Kölliker, 1853
  • Prayidae Kölliker, 1853
  • Sphaeronectidae Huxley, 1859
  • Tottonophyidae Pugh, Dunn & Haddock, 2018
• Suborder Cystonectae
  • Physaliidae
    Brandt, 1835
  • Rhizophysidae Brandt, 1835
• Suborder Physonectae
  • Agalmatidae Brandt, 1834
  • Apolemiidae
    Huxley, 1859
  • Cordagalmatidae Pugh, 2016
  • Erennidae Pugh, 2001
  • Forskaliidae
    Haeckel, 1888
  • Physophoridae Eschscholtz, 1829
  • Pyrostephidae Moser, 1925
  • Resomiidae Pugh, 2006
  • Rhodaliidae Haeckel, 1888
  • Stephanomiidae Huxley, 1859

History

Discovery

Carl Linnaeus described the first siphonophore, the Portuguese man o' war, in 1758.^[11] The discovery rate of siphonophore species was slow in the 18th century, as only four additional species were found.^[11] During the 19th century, 56 new species were observed due to research voyages conducted by European powers.^[11] The majority of new species found during this time period were collected in coastal, surface waters.^[11] 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.^[11] 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.^[11] Considered the most important researcher of siphonophores, A. K. Totton introduced 23 new species of siphonophores during the mid-20th century.^[11]

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, and longest animal, ever recorded.^[25][26]

There is no

Ernst Haeckel described numerous siphonophores, and several plates from his Kunstformen der Natur (1904) depict members of the taxon:^[27]

• 
  Plate 7
• 
  Plate 37
• 
  Plate 59
• 
  Plate 77

References