Siphonophore ( sī-ˈfä-nə-ˌfȯr ) fall under the Phylum of Cnidaria and can be likened to jellyfish and other hydrozoans, except with some fundamental differences. In fact, instead of singular organisms, siphonophores are called complex aggregate colonial organisms, which isn't even an accurate explanation!
Colonial organisms can mostly be found in the form of Corals, Pterobranchs and Pyrosomes. Such colonies are known to consist of Polyps or Medusae, both of which are described as tiny individual organisms that coexist in a singular body and share nutrients amongst each other. Each polyp is a clone of another, and as such they all share the same functions. If a polyp is separated from the collective body, it will be able to survive on its own indefinitely.
Siphonophores, meanwhile, have individual organisms that each serve their own unique purpose known as Zooids. You would think that those are the same as organs in a single multicellular organism, but they are not. The most prominent difference there is that each zooid is capable of reproducing asexually.
It's uncertain as to how long Siphonophores have existed, as there are no fossils on record, but the first to be reported. But the first to be recorded, Portuguese Man 'o War, Physalia physalis, was described by Carl Linnaeus in 1758 in his Systema Naturae. In a quote from a publication on Siphonophore morphology and terminology by Steven H.D. Haddock, Casey W. Dunn & Philip Pugh says:
" There are multiple implicit terminologies currently in use, and it is often not clear which one is employed in any particular publication. " --HADDOCK et al., 2005; DUNN & WAGNER, 2006
In short, even the experts find Siphonophores are freaking weird and confusing. Let's explore them more!
Below is the taxonomy of the siphonophore as we know it today.
- [ Kingdom ] Animalia
- [ Phylum ] Cnidaria
- [ Class ] Hydrozoa
- [ Subclass ] Hydroidolina
- [ Order ] Siphonophorae
Reportedly there are 175 siphonophores that can be found in the open ocean worldwide. They are most commonly found in the Pelagic Zone, or rather the part of a large body of water such as the sea or ocean which is not associated with the shore or seafloor. Some can however be also found in the Benthic Zone (near the sea floor).
The Epipelagic Zone is host to smaller, warm-water dwelling siphonophore species. Meanwhile, the larger species inhabit the Mesopelagic Zone, as the more turbulent surface-level waters are likely to tear their fragile bodies apart.
Below are the depth levels of the Pelagic Zones.
- [ Epipelagic ] up to 200m
- [ Mesopelagic ] 200m - 1000m
- [ Bathypelagic ] 1000m - 4000m
- [ Abyssopelagic ] 4000m - 6000m
- [ Hadopelagic ] over 6000m
It is necessary to note that the habitat of the Siphonophore largely affects their diet. There are other factors that go into their dietary habits, covered in the Diet Section, but much of these factors remain unknown. As stated in a recent publication on Integrating siphonophores into marine food-web ecology, a study by Purcell in 1981 showed that:
"the number of prey items in siphonophore guts was typically correlated with the available prey community and was also influenced by time of day and the amount of sunlight. It is unclear whether these relationships also exist for deep pelagic or specialized species." --HADDOCK, DUNN & CHOY, 2022
The take-away is that the Diet section will be vague at best. I tried.
As stated in the Introduction, siphonophores are a hazy middle-man between singular organisms and colonial organisms. Each Zooid has a specialized purpose much like organs do. But if you remove one, it will be able to survive for some time, which is dissimilar to organs. It is only the isolation from its mutual zooids that will lead to the zooid's depravation and inevitable death.
There are 5 Siphonophore Zooid types, listed below, not to be found in all species.
- [ Gonophores ] reproductive zooid, releases eggs and sperm into the water
- [ Nectophores ] responsible for mobility. A collective of nectophores is called a Nectosome.
- [ Gastrozooids ] responsible for food consumption and digestion
- [ Palpons ] responsible for circulating fluids which assist in digestion
- [ Bracts ] protects other zooids and keep the siphonophore neutrally buoyant
There are some non-zooid parts to the siphonophore, such as the Stem (the central stalk that all the zooids are connected to) and the Pneumatophore (a gas-filled float at the anterior end which provides buoyancy).
Siphonophores are arranged into 3 different body plans.
- [ Custonecta ] have pneumatophore but no nectosome
- [ Calyphoraw ] have a nectosome but no pneumatophore
- [ Physonecta] have both nectosome and pneumatophore
Siphonophores can be Monoecious, meaning they have both male and female reproductive zooids in a single colony, or Dioecious, meaning the entire colony is either male or female.
FUN FACT! Some kinds of nectosomes are called "Swimming Bells" due to their bell-like appearance!
A siphonophore's diet depends on a variety of conditions, including their size, depth of habitat, time of day, amount of sunlight, and many other factors. As stated in the publication mentioned previously in the Habitat section,
"In pelagic ecosystems, community density and composition—and therefore available prey—varies horizontally, vertically, and temporally and is driven by various physical and biological factors (Sommer et al. 2017; Netburn and Koslow 2018; Thompson et al. 2019)" --HADDOCK, DUNN & CHOY, 2022
Smaller Epipelagic warm-water siphonophores such as the Man 'o War and Velella Velella, are reported to feed on Zooplankton and Copepods. Meanwhile larger Mesopelagic siphonophores like Apolemia Uvaria can consume Plankton, Copepods, small fish, Crustaceans, and even sometimes other siphonophores.
The siphonophore catches its prey using its tentacles, similarly to jellyfish. Every Gastrozooid has a tentacle which branches off into smaller Tentilla (Tentillum plural), each of which contain Nematocysts, also similar to jellyfish. The number of variations in Nematocysts is insurmountable especially for this small documentation, but thanks to a helpful Marine Biology Youtube Channel, The Octopus Lady, we now have a simplified guide for us hobbyists.
According to The Octopus Lady, we now know that the classification of Nematocysts depends on their functions and their placement are two functions of Nematocysts: Adhesives, and Adhesive-Penetrants. Apparently there are no Nematocysts that are only penetrants. As for their placement, the Nematocysts are either in the Cnidoband or the Terminal Filament. The Tentillum has its own anatomy, which can be seen below.
- [ Pedicle ] provides the connection to the tentacle
- [ Cnidoband ] contains the adhesive and penetrant Nematocysts
- [ Elastic Strand ] (cited as "not very elastic", their words not mine) has a vague and confusing definition
- [ Terminal Filament ] coated in only adhesive nematocysts
When a tentillum touches its prey, it extends and wraps the terminal filament around to restrain it. It then subdues the captured prey by, as quoted from The Evolutionary History of Siphonophore Tentilla,
"rapidly unfolding the nematocyst battery to slap around the prey." --HADDOCK, DUNN & DAMIAN-SERRANO, 2021
Then, if the Nematocyst contains Adhesive-Penetrants, it stabs the prey with the Cnidoband. After that, the Gastrozooid will pull up the prey, partially digest it if it is too big to eat all at once, and consume it. The nutrients from the food are shared with the rest of the Zooids within the siphonophore colony via a pumping process performed by the Gastrozooid, the Palpons, and occasionally also the stem.
There is a lot more that can be said about Siphonophores, but as a hobbyist I am prone to error and inconsistency. If you want some more detailed, more dedicated publications from professionals in the field of Marine Biology, check any of the links listed below! I will leave you with this final statement which accurately summarizes the very nature of Siphonophores.
"Siphonophores do not convey the message a favorite theme of unthinking romanticism that nature is but one gigantic whole, all its parts intimately connected and interacting in some higher, ineffable harmony. Nature revels in boundaries and distinctions; we inhabit a universe of structure. But since our universe of structure has evolved historically, it must present us with fuzzy boundaries, where one kind of thing grades into another." --STEPHEN JAY GOULD, 2010
- The Evolutionary History of Siphonophore Tentilla: Novelties, Convergence, and Integration
- Complex colony-level organization of the deep-sea siphonophore Bargmannia elongata (Cnidaria, Hydrozoa) is directionally asymmetric and arises by the subdivision of pro-buds
- Integrating siphonophores into marine food-web ecology
- Siphonophore Wikipedia Article
- How Researching Siphonophores Almost Made Me Have A Nervous Breakdown | Alien Ocean, The Octopus Lady