[Animals] Siphonophorae Wiki

Aronus · November 5

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.[4] 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.[5] 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.[6] Most colonies are long, thin, transparent floaters living in the pelagic zone.[7]

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).[8][9]

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]

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 possess 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]