Siphonophores

Siphonophorae (from Greek ''siphōn'' 'tube' + ''pherein'' 'to bear') 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 thus far.

Although a siphonophore may appear to be an individual organism, each specimen is in fact a colonial organism composed of medusoid and polypoid zooids that are morphologically and functionally specialized. 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. Most colonies are long, thin, transparent floaters living in the pelagic zone.

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'').

Anatomy and morphology

Colony characteristics

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

General morphology

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

Since their origin, an increase in the number of zooid types has been observed in siphonophores. Scientists have determined two possible evolutionary hypothesis for this observation: 1. As time has gone on, the amount of zooid types has increased. 2. The last common ancestor had many types of zooids and the diversity seen today is due to loss of zooid types. Research shows no evidence supporting the first hypothesis, and has seen some evidence in support of the second.

;Zooids
:Siphonophores can have zooids that are either polyps or medusae. However, zooids are unique and can develop to have different functions.
;Nectophores
:Nectophores are medusae that assist in the propulsion and movement of some siphonophores in water. 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. The nectophores have also been observed in working in conjunction with reproductive structures in order to provide propulsion during colony detachment.
;Bracts
:Bracts are zooids that are unique to the siphonophorae order. They function in protection and maintaining a neutral buoyancy. However, bracts are not present in all species of siphonophore.
;Gastrozooids
:Gastrozooids are polyps that have evolved a function to assist in the feeding of siphonophores.
;Palpons
:Palpons are modified gastrozooids that function in digestion by regulating the circulation of gastrovascular fluids.
;Gonophores
:Gonophores are zooids that are involved in the reproductive processes of the siphonophores.
;Pneumatophores
:The presence of pneumatophores characterizes the subgroups Cystonectae and Physonectae. They are gas-filled floats that are located at the anterior end of the colonies in these species. They function to help the colonies maintain their orientation in water. In the cystonectae subgroup, the pneumatophores have an additional function of assisting with flotation of the organisms. The siphonophores exhibiting the feature develop the structure in early larval development via invaginations of the flattened planula structure. 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.

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. 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 and/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 nectophore