Siphonophorae

Siphonophorae (from Ancient Greek σίφων (siphōn), meaning "tube" and -φόρος (-phóros), meaning "bearing"<ref>Template:Cite encyclopedia</ref>) is an order within Hydrozoa, 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.<ref name="WoRMS" />

Siphonophores are highly polymorphic and complex organisms.<ref>Template:Citation</ref> Although they may appear to be individual organisms, each specimen is in fact a colonial organism composed of medusoid and polypoid zooids that are morphologically and functionally specialized.<ref name=":7"/> Zooids are multicellular units that develop from a single fertilized egg and combine to create functional colonies able to reproduce, digest, float, maintain body positioning, and use jet propulsion to move.<ref name=":2" /> Most colonies are long, thin, transparent floaters living in the pelagic zone.<ref name=":0">Template:Cite journal</ref>

Like other hydrozoans, some siphonophores emit light to attract and attack prey. While many sea animals produce blue and green bioluminescence, a siphonophore in the genus Erenna was only the second life form found to produce a red light (the first one being the scaleless dragonfish Chirostomias pliopterus).<ref name=":4" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Anatomy and morphology

Colony characteristics

Siphonophores are colonial hydrozoans that do not exhibit alternation of generations but instead reproduce asexually through a budding process.<ref>Template:Cite journal</ref> 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.<ref name=":0" /> Each zooid is produced to be genetically identical; however, mutations can alter their functions and increase diversity of the zooids within the colony.<ref name=":0" /> Siphonophores are unique in that the pro-bud initiates the production of diverse zooids with specific functions.<ref name=":0" /> 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.<ref name=":0" /> The stem is the vertical branch in the center of the colony to which the zooids attach.<ref name=":0" /> Zooids typically have special functions, and thus assume specific spatial patterns along the stem.<ref name=":0" />

General morphology

Siphonophores typically exhibit one of three standard body plans matching the suborders: Cystonectae, Physonectae, and Calycophorae.<ref name=":6">Template:Cite journal</ref> Cystonects have a long stem with the attached zooids.<ref name=":6" /> Each group of zooids has a gastrozooid.<ref name=":6" /> The gastrozooid has a tentacle used for capturing and digesting food.<ref name=":6" /> The groups also have gonophores, which are specialized for reproduction.<ref name=":6" /> 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.<ref name=":6" /> Physonects have a pneumatophore and nectosome, which harbors the nectophores used for jet propulsion.<ref name=":6" /> The nectophores pump water backwards in order to move forward.<ref name=":6" /> Calycophorans differ from cystonects and physonects in that they have two nectophores and no pneumatophore.<ref name=":6" /> Instead they often possess oil-filled glands which likely help with buoyancy.<ref>Jellyfish: A Natural History</ref>

Siphonophores possess multiple types of zooids.<ref name=":11">Template:Cite journal</ref> Scientists have determined two possible evolutionary hypotheses for this observation: 1. As time has gone on, the amount of zooid types has increased.<ref name=":11" /> 2. The last common ancestor had many types of zooids and the diversity seen today is due to loss of zooid types.<ref name=":11" /> Research shows no evidence supporting the first hypothesis, and has seen some evidence in support of the second.<ref name=":11" />

Zooids

A zooid is a single part of an organism that makes up the greater whole. Zooids are able to move rapidly and reconfigure themselves quickly, something that is useful for free-floating siphonophores.<ref>Le Goc, M., Kim, L. H., Parsaei, A., Fekete, J.-D., Dragicevic, P., & Follmer, S. (2016). Zooids: Building Blocks for Swarm User Interfaces. Proceedings of the 29th Annual Symposium on User Interface Software and Technology, 97–109. https://doi.org/10.1145/2984511.2984547 </ref> In general, siphonophore colonies have a modular body plan, with many different zooids making up the overall structure. These types can include: feeding gastrozooids, movement zooids, and sensory zooids.<ref>Damian-Serrano, A., Haddock, S. H., & Dunn, C. W. (2021). The evolution of siphonophore tentilla for specialized prey capture in the open ocean. Proceedings of the National Academy of Sciences, 118(8). https://doi.org/10.1073/pnas.2005063118 </ref>

Specifically, feeding zooids in siphonophores have undergone many unique adaptations to service the deep. Gastrozooids are uniquely specialized organisms that have feeding polyps (similar to a mouth) along with a long tentacle with side branches which is used to capture the prey. This adaptation is unique to zooids living in siphonophore colonies.<ref>Hetherington, E. D., Damian‐Serrano, A., Haddock, S. H., Dunn, C. W., & Choy, C. A. (2022). Integrating siphonophores into Marine Food‐Web Ecology. Limnology and Oceanography Letters, 7(2), 81–95. https://doi.org/10.1002/lol2.10235 </ref>

Nectophores

Nectophores are medusae that assist in the propulsion and movement of some siphonophores in water.<ref name=":2">Template:Cite journal</ref> 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.<ref name=":2" /> The nectophores have also been observed in working in conjunction with reproductive structures in order to provide propulsion during colony detachment.<ref name=":2" />

Bracts

Bracts are zooids that are unique to the siphonophorae order. They function in protection and maintaining a neutral buoyancy.<ref name=":2" /> However, bracts are not present in all species of siphonophore.<ref name=":2" />

Gastrozooids

Gastrozooids are polyps that have evolved a function to assist in the feeding of siphonophores.<ref name=":11" />

Palpons

Palpons are modified gastrozooids that function in digestion by regulating the circulation of gastrovascular fluids.<ref name=":2" />

Gonophores

Gonophores are zooids that are involved in the reproductive processes of the siphonophores.<ref name=":2" />

Pneumatophores

The presence of pneumatophores characterizes the subgroups Cystonectae and Physonectae.<ref name="Phylogeny">Template:Cite journal</ref> They are gas-filled floats that are located at the anterior end of the colonies in these species.<ref name=":2" /> They function to help the colonies maintain their orientation in water.<ref name=":2" /> In the Cystonectae subgroup, the pneumatophores have an additional function of assisting with flotation of the organisms.<ref name=":2" /> The siphonophores exhibiting the feature develop the structure in early larval development via invaginations of the flattened planula structure.<ref name=":2" /> 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.<ref>Template:Cite journal</ref>

Distribution and habitat

Currently, the World Register of Marine Species (WoRMS) identifies 175 species of siphonophores.<ref name=":6" /> They can differ greatly in terms of size and shape, which largely reflects the environment that they inhabit.<ref name=":6" /> Siphonophores are most often pelagic organisms, yet level species are benthic.<ref name=":6" /> Smaller, warm-water siphonophores typically live in the epipelagic zone and use their tentacles to capture zooplankton and copepods.<ref name=":6" /> Larger siphonophores live in deeper waters, as they are generally longer and more fragile and must avoid strong currents. They mostly feed on larger prey.<ref name=":6" /> The majority of siphonophores live in the deep sea and can be found in all of the oceans.<ref name=":6" /> Siphonophore species rarely only inhabit one location.<ref name=":6" /> Some species, however, can be confined to a specific range of depths and/or an area of the ocean.<ref name=":6" />

Behavior

Movement

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.<ref name=":8">Template:Cite journal</ref> Depending on where each individual nectophore is positioned within the siphonophore, their function differs.<ref name=":8" /> 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.<ref name=":8" /> 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.<ref name=":8" /> These larger individuals are important in attaining the maximum speed of the colony.<ref name=":8" /> 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.<ref name=":8" /> The colonial organization of siphonophores, particularly in Nanomia bijuga confers evolutionary advantages.<ref name=":8" /> A large number of concentrated individuals allows for redundancy.<ref name=":8" /> 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.<ref name=":8" /> 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.<ref name=":9">Template:Cite journal</ref> The velum becomes smaller and more circular during times of forward propulsion compared to a large velum that is seen during refill periods.<ref name=":9" /> 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.<ref name=":9" /> The siphonophore Namonia bijuga also practices diel vertical migration, as it remains in the deep-sea during the day but rises during the night.<ref name=":8" />

Predation and feeding

Siphonophores are predatory carnivores.<ref name=":7">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Their diets consist of a variety of copepods, other small crustaceans, cnidarians, ctenophores, and small fish.<ref name=":7"/><ref name=":1">Template:Cite journal</ref> Some siphonophores, such as Praya dubia, have been observed to feed on other species in the same order.<ref name=":5">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Generally, the diets of strong swimming siphonophores consist of smaller prey, and the diets of weak swimming siphonophores consist of larger prey.<ref name=":10">Cite: Purcell, Jennifer E. (1980). Influence of Siphonophore Behavior upon Their Natural Diets: Evidence for Aggressive Mimicry. Science, vol. 209, pp. 1045-1047. DOI: 10.1126/science.209.4460.1045</ref> A majority of siphonophores have gastrozooids that have a characteristic tentacle attached to the base of the zooid. This structural feature functions in assisting the organisms in catching prey.<ref name=":2" /> Species with large gastrozooids are capable of consuming a broad range of prey sizes.<ref name=":10" /> Like other Cnidaria, many siphonophore species exhibit nematocyst stinging capsules on branches of their tentacles called tentilla.<ref name=":2" /> The nematocysts are arranged in dense batteries on the side of the tentilla.<ref name=":2" /> When the siphonophore encounters potential prey, their tentillum react to where the Template:Cvt tentacles create a net by transforming their shape around the prey.<ref name=":7"/><ref>Template:Cite journal</ref><ref name="biorxiv.org">Template:Cite journal</ref> The nematocysts then shoot millions<ref name=":10" /> of paralyzing, and sometimes fatal, toxin molecules at the trapped prey which is then transferred to the proper location for digestion.<ref name=":7"/> Some species of siphonophores use aggressive mimicry by using bioluminescent light so the prey cannot properly identify the predator.<ref name="biorxiv.org"/>

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

Due to the scarcity of food in the deep sea environment, a majority of siphonophore species function in a sit-and-wait tactic for food.<ref name="Dunn">Dunn, Casey (2005). "Siphonophores". Retrieved 2008-07-08.</ref> The gelatinous body plan allows for flexibility when catching prey, but the gelatinous adaptations are based on habitat.<ref>Template:Citation</ref> 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.<ref name=":4">Template:Cite journal</ref> These actions entice the prey to move closer to the siphonophore, allowing it to trap and digest it.<ref name=":4" />

Predators of Siphonophores include narcomedusae, gastropods, other siphonophores, and large fish such as Mola mola.<ref name=":1" /><ref name=":5" />

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.<ref name=":7"/> The fertilized egg matures into a protozooid, which initiates the budding process and creation of a new zooid.<ref name=":7" /> This process repeats until a colony of zooids forms around the central stalk.<ref name=":7" /> 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.<ref name=":7" /> The polyps may then be fertilized outside of the organism.<ref name=":7" />

Siphonophores use the gonophore organ to make the reproductive gametes.<ref name=":11"/> Gonophores are either male or female; however, the types of gonophores in a colony can vary among species.<ref name=":11"/> Species are characterized as monoecious or dioecious based on their gonophores.<ref name=":11"/> 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.<ref name=":11"/>

Some siphonophore species within the Calycophorae clade release eudoxids, which are zooid clusters, instead of reproduction through gonophore organs.<ref name=":23">Template:Citation</ref> There is limited research on the mechanistic release of eudoxid fragments for reproduction, and studies are determining whether to consider them as clustered communities or individuals.<ref name=":23" /> Recent research has identified eudoxid tissue remodeling before release by a specified muscle, as well as a dispersal mechanism that temporarily alters siphonophore buoyancy.<ref name=":23" />

Zooids come together out of a necessity for survival. They reproduce via fission. This process happens inside a colony. In the colony, there is a main linear chain of a main zooid which produces secondary zooids. Fission occurs on the outermost chain.<ref>Falconi, R., Gugnali, A., & Zaccanti, F. (2015). Quantitative observations on asexual reproduction of aeolosoma viride (Annelida, Aphanoneura). Invertebrate Biology, 134(2), 151–161. https://doi.org/10.1111/ivb.12087 </ref>

Bioluminescence

Nearly all siphonophores have bioluminescent capabilities. Since these organisms are extremely fragile, they are rarely observed alive.<ref name=":4" /> Bioluminescence in siphonophores has been thought to have evolved as a defense mechanism.<ref name=":4" /> Siphonophores of the deep-sea genus Erenna (found at depths between Template:Convert) are thought to use their bioluminescent capability for offense too, as a lure to attract fish.<ref name=":4" /> This genus is one of the few to prey on fish rather than crustaceans.<ref name=":4" /> The bioluminescent organs, called tentilla, on these non-visual individuals emit red fluorescence along with a rhythmic flicking pattern, which attracts prey as it resembles smaller organisms such as zooplankton and copepods. Thus, it has been concluded that they use luminescence as a lure to attract prey.<ref name=":4" /> Some research indicates that deep-sea organisms can not detect long wavelengths, and red light has a long wavelength of 680 nm. If this is the case, then fish are not lured by Erenna, and there must be another explanation. However, the deep-sea remains largely unexplored and red light sensitivity in fish such as Cyclothone and the deep myctophid fish should not be discarded.<ref name=":4" />

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.<ref name=":6" /> A. rosacea mimic fish larvae, A. lucida are thought to mimic larvacean houses, and L. utricularia mimic hydromedusa.<ref name=":6" /> 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.<ref name=":6" /> Siphonophores from the genus Erenna use bioluminescent lures surrounded by red fluorescence to attract prey and possibly mimic a fish from the Cyclothone genus.<ref name=":6" /> Their prey is lured in through a unique flicking behavior associated with the tentilla.<ref name=":4" /> 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.<ref name=":4" />

Taxonomy

File:Portuguese Man-O-War (Physalia physalis).jpg

Portuguese Man o' War (Physalia physalis)

Organisms in the order of Siphonophorae have been classified into the phylum Cnidaria and the class Hydrozoa.<ref name="WoRMS">"Siphonophorae". World Register of Marine Species (2018). Retrieved 8 January 2018.</ref> The phylogenetic relationships of siphonophores have been of great interest due to the high variability of the organization of their polyp colonies and medusae.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=":11"/> 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.<ref name="Phylogeny" /> 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).<ref name="Phylogeny" /> The subgroups consisted of Cystonectae, Physonectae, and Calycorphores. Cystonectae had pneumatophores, Calycophores had nectophores, and Physonectae had both.<ref name="Phylogeny" />

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.<ref name=":2" /><ref name=":11"/>

• 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

File:Differential-Gene-Expression-in-the-Siphonophore-Nanomia-bijuga-(Cnidaria)-Assessed-with-Multiple-pone.0022953.s013.ogv

style }}

Discovery

Carl Linnaeus described the first siphonophore, the Portuguese Man o' War, in 1758.<ref name=":6" /> The discovery rate of siphonophore species was slow in the 18th century, as only four additional species were found.<ref name=":6" /> During the 19th century, 56 new species were observed due to research voyages conducted by European powers.<ref name=":6" /> The majority of new species found during this time period were collected in coastal, surface waters.<ref name=":6" /> 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.<ref name=":6" /> 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.<ref name=":6" /> Considered the most important researcher of siphonophores, A. K. Totton introduced 23 new species of siphonophores during the mid-20th century.<ref name=":6" />

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="EA-20200409">Template:Cite news</ref>

There is no fossil record of siphonophores, though they have evolved and adapted for an extensive time period. Their phylum, Cnidaria, is an ancient lineage that dates back to c. 640 million years ago.<ref name=":6" />

Haeckel's siphonophores

Ernst Haeckel described numerous siphonophores, and several plates from his Kunstformen der Natur (1904) depict members of the taxon:<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

• Haeckel Siphonophorae 7.jpg

  Plate 7

• Haeckel Siphonophorae 37.jpg

  Plate 37

• Haeckel Siphonophorae 59.jpg

  Plate 59

• Haeckel Siphonophorae 77.jpg

  Plate 77

Further reading

• Template:Cite book
• PinkTentacle.com (2008): Siphonophore: Deep-sea superorganism (video). Retrieved 2009-MAY-23.

External links

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• ''Deep sea siphonophore'' (10 April 2017) YouTube. Imaged by the NOAA Okeanos Explorer on March 14, 2017, at 1,560 meters west of Winslow Reef complex. Retrieved 28 January 2018.