Siphonophore

Distribution and habitat

Currently, the World Register of Marine Species (WoRMS) identifies 175 species of siphonophores. They can differ greatly in terms of size and shape, which largely reflects the environment that they inhabit. Siphonophores are most often pelagic organisms, yet level species are benthic. Smaller, warm water siphonophores typically live in the epipelagic zone and use their tentacles to capture zooplankton and copepods. 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, such as small fishes or crustaceans. The majority of siphonophores live in the deep sea and can be found in all of the oceans. Siphonophore species rarely only inhabit one location. Some species, however, can be confined to a specific range of depths or an area of the ocean.

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

Predation and feeding

Siphonophores are predatory carnivores. Their diets consist of a variety of copepods, other small crustaceans, cnidarians, ctenophores, and small fish. Some siphonophores, such as Praya dubia, have been observed to feed on other species in the same order. Generally, the diets of strong swimming siphonophores consist of smaller prey, and the diets of weak swimming siphonophores consist of larger prey. Research has shown that specific diets vary even between different individuals of the same species depending on their particular environment. 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. Species with large gastrozooids are capable of consuming a broad range of prey sizes. In one species of siphonophore, Rosacea cymbiformis, digestive experiments showed that ingested prey remained in gastrozooids for at least 8 hours before egestion. Like other Cnidaria, many siphonophore species exhibit nematocyst stinging capsules on branches of their tentacles called tentilla. The nematocysts are arranged in dense batteries on the side of the tentilla. When the siphonophore encounters potential prey, their tentillum react to where the 30–50 cm (12–20 in) tentacles create a net by transforming their shape around the prey. The nematocysts then shoot millions of paralyzing, and sometimes fatal, toxin molecules at the trapped prey which is then transferred to the proper location for digestion. Some species of siphonophores use aggressive mimicry by using bioluminescent light so the prey cannot properly identify the predator.

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

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. The gelatinous body plan allows for flexibility when catching prey, but the gelatinous adaptations are based on habitat. 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. These actions entice the prey to move closer to the siphonophore, allowing it to trap and digest it. The Siphonophore Apolemia uvaria has the ability to break into small fragments to break into fish cages for food.

Predators of Siphonophores include narcomedusae, gastropods, other siphonophores, and large fish such as Mola mola. Although lacking brains, certain species such as Hippopodius have been observed to react to tactile stimulation by contracting their gonophores into a central space protected by their nectophores, in a seemingly defensive maneuver called "crumpling." This species then proceeds to undergo "blanching," and emit a startlingly bright light. Nanomia, among other siphonophore species, can move both forward and backwards to escape a stimulus from the opposite side, using their nectophores for propulsion. While there are few observations of defensive behavior in situ, it is argued that these strategies help siphonophores evade predators.

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

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

Some siphonophore species within the Calycophorae clade release eudoxids, which are zooid clusters, instead of reproduction through gonophore organs. 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. Recent research has identified eudoxid tissue remodeling before release by a specified muscle, as well as a dispersal mechanism that temporarily alters siphonophore buoyancy.

Bioluminescence

Nearly all siphonophores have bioluminescent capabilities. Since these organisms are extremely fragile, they are rarely observed alive. Bioluminescence in siphonophores has been thought to have evolved as a defense mechanism. Siphonophores of the deep-sea genus Erenna (found at depths between 1,600–2,300 metres or 5,200–7,500 feet) are thought to use their bioluminescent capability for offense too, as a lure to attract fish. This genus is one of the few to prey on fish rather than crustaceans. 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. 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.

Bioluminescent lures are found in many different species of siphonophores, and are used for a variety of reasons. Species such as Agalma okenii, Athorybia rosacea, Athorybia lucida, and Lychnafalma utricularia use their lures as a mimicry device to attract prey. A. rosacea mimic fish larvae, A. lucida are thought to mimic larvacean houses, and L. utricularia mimic hydromedusa. 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. Erenna sirena uses bioluminescent lures surrounded by red fluorescence to attract prey and possibly mimic a fish from the Cyclothone genus. Their prey is lured in through a unique flicking behavior associated with the tentilla. 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.

Taxonomy

History

Carl Linnaeus described the first siphonophore, the Portuguese man o' war, in 1758. The discovery rate of siphonophore species was slow in the 18th century, as only four additional species were found. During the 19th century, 56 new species were observed due to research voyages conducted by European powers. The majority of new species found during this time period were collected in coastal, surface waters. During the HMS Challenger expedition in the early to mid 1870s, 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. Nonetheless, some of his descriptions and figures are considered useful by modern biologists. A rate of about 10 new species discoveries per decade was observed during the 20th century. Considered the most important researcher of siphonophores, A. K. Totton introduced 23 new species of siphonophores during the mid-20th century.

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. Another notable recent discovery occurred in early 2025, when a team led by researchers at Yale University, the University of New South Wales, and Griffith University determined that the Portuguese man o'war is actually at least four distinct species. The discovery has implications for scientists' understanding of open-ocean biodiversity, and encourages further research into understanding what caused and maintained this genetic variation.

Modern taxonomy

Siphonophores are classified into the phylum Cnidaria and the class Hydrozoa. The phylogenetic relationships of siphonophores have been of great interest due to the high variability of the organization of their polyp colonies and medusae. 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. 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). The subgroups consisted of Cystonectae, Physonectae, and Calycorphores. Cystonectae had pneumatophores, Calycophores had nectophores, and Physonectae had both. 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.

• 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

Research challenges

For siphonophores, research has been limited. The organisms are gelatinous and fragile, making it difficult for researchers to obtain for sampling. Many siphonophores reside in the mesopelagic and bathypelagic zones, where many remotely operated vehicles (ROVs) and submersibles search to observe and sample these creatures. However, most colonies are destroyed during collections and because of this, there is a lack of preserved specimens for study. Most species of siphonophores are sampled through in situ observations, contributing to the gaps in knowledge about siphonophores and underrepresentation in species records because of sampling bias.

Molecular phylogenetics has helped address some of the inequalities by using nuclear and mitochondrial gene sequences. Most of these scientific studies have provided more insight into the evolution of specialized zooids within colonies. Additionally, genomic analyses have used transcriptome datasets to improve phylogenetic resolution, allowing scientists and researchers to reconstruct patterns in evolutionary traits like the organization of colonies and the tentilla feeding structure.

Using direct observation from submersibles and underwater imaging have also greatly helped knowledge of siphonophore ecology and behavior. The Bathyphysa conifera has been directly observed to have active locomotion though contracting and expanding their stem, a habit uniquely different from common methods of drifting passively with ocean currents. The manefish has been directly observed to associate with the Bathyphysa conifera, gaining shelter and access to food leftovers from the colony.